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rcu: Make rcu_start_this_gp() check for grace period already started 

In the old days of ->gpnum and ->completed, the code requesting a new
grace period checked to see if that grace period had already started,
bailing early if so.  The new-age ->gp_seq approach instead checks
whether the grace period has already finished.  A compensating change
pushed the requested grace period down to the bottom of the tree, thus
reducing lock contention and even eliminating it in some cases.  But why
not further reduce contention, especially on large systems, by doing both,
especially given that the cost of doing both is extremely small?

This commit therefore adds a new rcu_seq_started() function that checks
whether a specified grace period has already started.  It then uses
this new function in place of rcu_seq_done() in the rcu_start_this_gp()
function's funnel locking code.

Reported-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-05-15 11:53:41 -07:00
parent 5ca0905f67
commit 2e3e5e5501
2 changed files with 10 additions and 1 deletions
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9
kernel/rcu/rcu.h
View File

@ -107,6 +107,15 @@ static inline unsigned long rcu_seq_current(unsigned long *sp)
return READ_ONCE(*sp);
}
/*
* Given a snapshot from rcu_seq_snap(), determine whether or not the
* corresponding update-side operation has started.
*/
static inline bool rcu_seq_started(unsigned long *sp, unsigned long s)
{
return ULONG_CMP_LT((s - 1) & ~RCU_SEQ_STATE_MASK, READ_ONCE(*sp));
}
/*
* Given a snapshot from rcu_seq_snap(), determine whether or not a
* full update-side operation has occurred.

2
kernel/rcu/tree.c
View File

@ -1583,7 +1583,7 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
if (rnp_root != rnp)
raw_spin_lock_rcu_node(rnp_root);
if (ULONG_CMP_GE(rnp_root->gp_seq_needed, c) ||
rcu_seq_done(&rnp_root->gp_seq, c) ||
rcu_seq_started(&rnp_root->gp_seq, c) ||
(rnp != rnp_root &&
rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) {
trace_rcu_this_gp(rnp_root, rdp, c, TPS("Prestarted"));