sched/core: Fix remote wakeups
Commit:b5179ac70d
("sched/fair: Prepare to fix fairness problems on migration") ... introduced a bug: Mike Galbraith found that it introduced a performance regression, while Paul E. McKenney reported lost wakeups and bisected it to this commit. The reason is that I mis-read ttwu_queue() such that I assumed any wakeup that got a remote queue must have had the task migrated. Since this is not so; we need to transfer this information between queueing the wakeup and actually doing the wakeup. Use a new task_struct::sched_flag for this, we already write to sched_contributes_to_load in the wakeup path so this is a hot and modified cacheline. Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com> Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Andrew Hunter <ahh@google.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Ben Segall <bsegall@google.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Matt Fleming <matt@codeblueprint.co.uk> Cc: Morten Rasmussen <morten.rasmussen@arm.com> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Paul Turner <pjt@google.com> Cc: Pavan Kondeti <pkondeti@codeaurora.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: byungchul.park@lge.com Fixes:b5179ac70d
("sched/fair: Prepare to fix fairness problems on migration") Link: http://lkml.kernel.org/r/20160523091907.GD15728@worktop.ger.corp.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -1533,6 +1533,7 @@ struct task_struct {
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unsigned sched_reset_on_fork:1;
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unsigned sched_contributes_to_load:1;
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unsigned sched_migrated:1;
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unsigned sched_remote_wakeup:1;
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unsigned :0; /* force alignment to the next boundary */
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/* unserialized, strictly 'current' */
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@ -1768,13 +1768,15 @@ void sched_ttwu_pending(void)
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cookie = lockdep_pin_lock(&rq->lock);
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while (llist) {
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int wake_flags = 0;
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p = llist_entry(llist, struct task_struct, wake_entry);
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llist = llist_next(llist);
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/*
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* See ttwu_queue(); we only call ttwu_queue_remote() when
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* its a x-cpu wakeup.
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*/
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ttwu_do_activate(rq, p, WF_MIGRATED, cookie);
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if (p->sched_remote_wakeup)
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wake_flags = WF_MIGRATED;
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ttwu_do_activate(rq, p, wake_flags, cookie);
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}
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lockdep_unpin_lock(&rq->lock, cookie);
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@ -1819,10 +1821,12 @@ void scheduler_ipi(void)
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irq_exit();
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}
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static void ttwu_queue_remote(struct task_struct *p, int cpu)
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static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)
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{
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struct rq *rq = cpu_rq(cpu);
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p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);
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if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list)) {
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if (!set_nr_if_polling(rq->idle))
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smp_send_reschedule(cpu);
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@ -1869,7 +1873,7 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
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#if defined(CONFIG_SMP)
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if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {
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sched_clock_cpu(cpu); /* sync clocks x-cpu */
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ttwu_queue_remote(p, cpu);
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ttwu_queue_remote(p, cpu, wake_flags);
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return;
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}
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#endif
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