srcu: Expedited grace periods with reduced memory contention
Commit f60d231a87 ("srcu: Crude control of expedited grace periods")
introduced a per-srcu_struct atomic counter to track outstanding
requests for grace periods. This works, but represents a memory-contention
bottleneck. This commit therefore uses the srcu_node combining tree
to remove this bottleneck.
This commit adds new ->srcu_gp_seq_needed_exp fields to the
srcu_data, srcu_node, and srcu_struct structures, which track the
farthest-in-the-future grace period that must be expedited, which in
turn requires that all nearer-term grace periods also be expedited.
Requests for expediting start with the srcu_data structure, run up
through the srcu_node tree, and end at the srcu_struct structure.
Note that it may be necessary to expedite a grace period that just
now started, and this is handled by a new srcu_funnel_exp_start()
function, which is invoked when the grace period itself is already
in its way, but when that grace period was not marked as expedited.
A new srcu_get_delay() function returns zero if there is at least one
expedited SRCU grace period in flight, or SRCU_INTERVAL otherwise.
This function is used to calculate delays: Normal grace periods
are allowed to extend in order to cover more requests with a given
grace-period computation, which decreases per-request overhead.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>
This commit is contained in:
Paul E. McKenney
parent
7f6733c3c6
commit
1e9a038b7f
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@ -43,6 +43,7 @@ struct srcu_data {
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spinlock_t lock ____cacheline_internodealigned_in_smp;
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struct rcu_segcblist srcu_cblist; /* List of callbacks.*/
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unsigned long srcu_gp_seq_needed; /* Furthest future GP needed. */
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unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
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bool srcu_cblist_invoking; /* Invoking these CBs? */
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struct delayed_work work; /* Context for CB invoking. */
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struct rcu_head srcu_barrier_head; /* For srcu_barrier() use. */
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@ -63,6 +64,7 @@ struct srcu_node {
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/* is > ->srcu_gq_seq. */
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unsigned long srcu_data_have_cbs[4]; /* Which srcu_data structs */
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/* have CBs for given GP? */
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unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
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struct srcu_node *srcu_parent; /* Next up in tree. */
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int grplo; /* Least CPU for node. */
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int grphi; /* Biggest CPU for node. */
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@ -81,7 +83,7 @@ struct srcu_struct {
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unsigned int srcu_idx; /* Current rdr array element. */
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unsigned long srcu_gp_seq; /* Grace-period seq #. */
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unsigned long srcu_gp_seq_needed; /* Latest gp_seq needed. */
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atomic_t srcu_exp_cnt; /* # ongoing expedited GPs. */
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unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */
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struct srcu_data __percpu *sda; /* Per-CPU srcu_data array. */
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unsigned long srcu_barrier_seq; /* srcu_barrier seq #. */
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struct mutex srcu_barrier_mutex; /* Serialize barrier ops. */
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@ -72,6 +72,7 @@ static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
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snp->srcu_have_cbs[i] = 0;
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snp->srcu_data_have_cbs[i] = 0;
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}
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snp->srcu_gp_seq_needed_exp = 0;
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snp->grplo = -1;
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snp->grphi = -1;
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if (snp == &sp->node[0]) {
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@ -102,6 +103,7 @@ static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
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rcu_segcblist_init(&sdp->srcu_cblist);
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sdp->srcu_cblist_invoking = false;
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sdp->srcu_gp_seq_needed = sp->srcu_gp_seq;
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sdp->srcu_gp_seq_needed_exp = sp->srcu_gp_seq;
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sdp->mynode = &snp_first[cpu / levelspread[level]];
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for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
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if (snp->grplo < 0)
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@ -135,7 +137,6 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
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mutex_init(&sp->srcu_gp_mutex);
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sp->srcu_idx = 0;
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sp->srcu_gp_seq = 0;
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atomic_set(&sp->srcu_exp_cnt, 0);
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sp->srcu_barrier_seq = 0;
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mutex_init(&sp->srcu_barrier_mutex);
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atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
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@ -143,6 +144,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
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if (!is_static)
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sp->sda = alloc_percpu(struct srcu_data);
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init_srcu_struct_nodes(sp, is_static);
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sp->srcu_gp_seq_needed_exp = 0;
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smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
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return sp->sda ? 0 : -ENOMEM;
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}
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@ -307,6 +309,18 @@ static bool srcu_readers_active(struct srcu_struct *sp)
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#define SRCU_INTERVAL 1
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/*
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* Return grace-period delay, zero if there are expedited grace
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* periods pending, SRCU_INTERVAL otherwise.
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*/
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static unsigned long srcu_get_delay(struct srcu_struct *sp)
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{
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if (ULONG_CMP_LT(READ_ONCE(sp->srcu_gp_seq),
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READ_ONCE(sp->srcu_gp_seq_needed_exp)))
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return 0;
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return SRCU_INTERVAL;
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}
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/**
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* cleanup_srcu_struct - deconstruct a sleep-RCU structure
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* @sp: structure to clean up.
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@ -318,7 +332,8 @@ void cleanup_srcu_struct(struct srcu_struct *sp)
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{
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int cpu;
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WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
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if (WARN_ON(!srcu_get_delay(sp)))
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return; /* Leakage unless caller handles error. */
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if (WARN_ON(srcu_readers_active(sp)))
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return; /* Leakage unless caller handles error. */
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flush_delayed_work(&sp->work);
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@ -444,15 +459,14 @@ static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
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* schedule this invocation on the corresponding CPUs.
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*/
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static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
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unsigned long mask)
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unsigned long mask, unsigned long delay)
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{
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int cpu;
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for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
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if (!(mask & (1 << (cpu - snp->grplo))))
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continue;
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srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu),
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atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
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srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu), delay);
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}
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}
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@ -467,6 +481,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
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*/
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static void srcu_gp_end(struct srcu_struct *sp)
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{
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unsigned long cbdelay;
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bool cbs;
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unsigned long gpseq;
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int idx;
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@ -481,8 +496,11 @@ static void srcu_gp_end(struct srcu_struct *sp)
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spin_lock_irq(&sp->gp_lock);
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idx = rcu_seq_state(sp->srcu_gp_seq);
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WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
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cbdelay = srcu_get_delay(sp);
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rcu_seq_end(&sp->srcu_gp_seq);
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gpseq = rcu_seq_current(&sp->srcu_gp_seq);
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if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
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sp->srcu_gp_seq_needed_exp = gpseq;
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spin_unlock_irq(&sp->gp_lock);
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mutex_unlock(&sp->srcu_gp_mutex);
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/* A new grace period can start at this point. But only one. */
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@ -497,12 +515,14 @@ static void srcu_gp_end(struct srcu_struct *sp)
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cbs = snp->srcu_have_cbs[idx] == gpseq;
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snp->srcu_have_cbs[idx] = gpseq;
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rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
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if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
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snp->srcu_gp_seq_needed_exp = gpseq;
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mask = snp->srcu_data_have_cbs[idx];
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snp->srcu_data_have_cbs[idx] = 0;
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spin_unlock_irq(&snp->lock);
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if (cbs) {
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smp_mb(); /* GP end before CB invocation. */
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srcu_schedule_cbs_snp(sp, snp, mask);
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srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);
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}
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}
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@ -517,15 +537,43 @@ static void srcu_gp_end(struct srcu_struct *sp)
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srcu_gp_start(sp);
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spin_unlock_irq(&sp->gp_lock);
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/* Throttle expedited grace periods: Should be rare! */
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srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) &&
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rcu_seq_ctr(gpseq) & 0xf
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? 0
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: SRCU_INTERVAL);
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srcu_reschedule(sp, rcu_seq_ctr(gpseq) & 0x3ff
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? 0 : SRCU_INTERVAL);
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} else {
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spin_unlock_irq(&sp->gp_lock);
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}
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}
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/*
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* Funnel-locking scheme to scalably mediate many concurrent expedited
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* grace-period requests. This function is invoked for the first known
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* expedited request for a grace period that has already been requested,
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* but without expediting. To start a completely new grace period,
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* whether expedited or not, use srcu_funnel_gp_start() instead.
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*/
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static void srcu_funnel_exp_start(struct srcu_struct *sp, struct srcu_node *snp,
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unsigned long s)
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{
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unsigned long flags;
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for (; snp != NULL; snp = snp->srcu_parent) {
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if (rcu_seq_done(&sp->srcu_gp_seq, s) ||
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ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
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return;
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spin_lock_irqsave(&snp->lock, flags);
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if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
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spin_unlock_irqrestore(&snp->lock, flags);
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return;
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}
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WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
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spin_unlock_irqrestore(&snp->lock, flags);
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}
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spin_lock_irqsave(&sp->gp_lock, flags);
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if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
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sp->srcu_gp_seq_needed_exp = s;
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spin_unlock_irqrestore(&sp->gp_lock, flags);
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}
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/*
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* Funnel-locking scheme to scalably mediate many concurrent grace-period
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* requests. The winner has to do the work of actually starting grace
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@ -533,9 +581,8 @@ static void srcu_gp_end(struct srcu_struct *sp)
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* number is recorded on at least their leaf srcu_node structure, or they
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* must take steps to invoke their own callbacks.
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*/
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static void srcu_funnel_gp_start(struct srcu_struct *sp,
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struct srcu_data *sdp,
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unsigned long s)
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static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
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unsigned long s, bool do_norm)
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{
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unsigned long flags;
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int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
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@ -554,13 +601,20 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
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spin_unlock_irqrestore(&snp->lock, flags);
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if (snp == sdp->mynode && snp_seq != s) {
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smp_mb(); /* CBs after GP! */
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srcu_schedule_cbs_sdp(sdp, 0);
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srcu_schedule_cbs_sdp(sdp, do_norm
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? SRCU_INTERVAL
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: 0);
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return;
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}
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if (!do_norm)
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srcu_funnel_exp_start(sp, snp, s);
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return;
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}
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snp->srcu_have_cbs[idx] = s;
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if (snp == sdp->mynode)
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snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
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if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
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snp->srcu_gp_seq_needed_exp = s;
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spin_unlock_irqrestore(&snp->lock, flags);
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}
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@ -573,6 +627,8 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
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*/
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smp_store_release(&sp->srcu_gp_seq_needed, s); /*^^^*/
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}
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if (!do_norm && ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
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sp->srcu_gp_seq_needed_exp = s;
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/* If grace period not already done and none in progress, start it. */
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if (!rcu_seq_done(&sp->srcu_gp_seq, s) &&
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@ -580,9 +636,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *sp,
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WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
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srcu_gp_start(sp);
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queue_delayed_work(system_power_efficient_wq, &sp->work,
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atomic_read(&sp->srcu_exp_cnt)
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? 0
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: SRCU_INTERVAL);
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srcu_get_delay(sp));
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}
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spin_unlock_irqrestore(&sp->gp_lock, flags);
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}
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@ -597,7 +651,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
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for (;;) {
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if (srcu_readers_active_idx_check(sp, idx))
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return true;
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if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 0)
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if (--trycount + !srcu_get_delay(sp) <= 0)
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return false;
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udelay(SRCU_RETRY_CHECK_DELAY);
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}
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@ -650,10 +704,11 @@ static void srcu_flip(struct srcu_struct *sp)
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* srcu_read_lock(), and srcu_read_unlock() that are all passed the same
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* srcu_struct structure.
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*/
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void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
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rcu_callback_t func)
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void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
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rcu_callback_t func, bool do_norm)
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{
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unsigned long flags;
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bool needexp = false;
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bool needgp = false;
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unsigned long s;
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struct srcu_data *sdp;
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@ -672,16 +727,28 @@ void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
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sdp->srcu_gp_seq_needed = s;
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needgp = true;
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}
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if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
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sdp->srcu_gp_seq_needed_exp = s;
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needexp = true;
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}
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spin_unlock_irqrestore(&sdp->lock, flags);
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if (needgp)
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srcu_funnel_gp_start(sp, sdp, s);
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srcu_funnel_gp_start(sp, sdp, s, do_norm);
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else if (needexp)
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srcu_funnel_exp_start(sp, sdp->mynode, s);
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}
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void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
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rcu_callback_t func)
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{
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__call_srcu(sp, rhp, func, true);
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}
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EXPORT_SYMBOL_GPL(call_srcu);
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/*
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* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
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*/
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static void __synchronize_srcu(struct srcu_struct *sp)
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static void __synchronize_srcu(struct srcu_struct *sp, bool do_norm)
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{
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struct rcu_synchronize rcu;
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@ -697,7 +764,7 @@ static void __synchronize_srcu(struct srcu_struct *sp)
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check_init_srcu_struct(sp);
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init_completion(&rcu.completion);
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init_rcu_head_on_stack(&rcu.head);
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call_srcu(sp, &rcu.head, wakeme_after_rcu);
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__call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
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wait_for_completion(&rcu.completion);
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destroy_rcu_head_on_stack(&rcu.head);
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}
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@ -714,18 +781,7 @@ static void __synchronize_srcu(struct srcu_struct *sp)
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*/
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void synchronize_srcu_expedited(struct srcu_struct *sp)
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{
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bool do_norm = rcu_gp_is_normal();
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check_init_srcu_struct(sp);
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if (!do_norm) {
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atomic_inc(&sp->srcu_exp_cnt);
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smp_mb__after_atomic(); /* increment before GP. */
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}
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__synchronize_srcu(sp);
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if (!do_norm) {
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smp_mb__before_atomic(); /* GP before decrement. */
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WARN_ON_ONCE(atomic_dec_return(&sp->srcu_exp_cnt) < 0);
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}
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__synchronize_srcu(sp, rcu_gp_is_normal());
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
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@ -773,7 +829,7 @@ void synchronize_srcu(struct srcu_struct *sp)
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if (rcu_gp_is_expedited())
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synchronize_srcu_expedited(sp);
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else
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__synchronize_srcu(sp);
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__synchronize_srcu(sp, true);
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}
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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@ -1008,14 +1064,13 @@ void process_srcu(struct work_struct *work)
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sp = container_of(work, struct srcu_struct, work.work);
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srcu_advance_state(sp);
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srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
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srcu_reschedule(sp, srcu_get_delay(sp));
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}
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EXPORT_SYMBOL_GPL(process_srcu);
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void srcutorture_get_gp_data(enum rcutorture_type test_type,
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struct srcu_struct *sp, int *flags,
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unsigned long *gpnum,
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unsigned long *completed)
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struct srcu_struct *sp, int *flags,
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unsigned long *gpnum, unsigned long *completed)
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{
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if (test_type != SRCU_FLAVOR)
|
||||
return;
|
||||
|
|
|
|||
Loading…
Reference in New Issue