sched/core: Optimize try_to_wake_up() for local wakeups
Jens reported that significant performance can be had on some block workloads by special casing local wakeups. That is, wakeups on the current task before it schedules out. Given something like the normal wait pattern: for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); if (cond) break; schedule(); } __set_current_state(TASK_RUNNING); Any wakeup (on this CPU) after set_current_state() and before schedule() would benefit from this. Normal wakeups take p->pi_lock, which serializes wakeups to the same task. By eliding that we gain concurrency on: - ttwu_stat(); we already had concurrency on rq stats, this now also brings it to task stats. -ENOCARE - tracepoints; it is now possible to get multiple instances of trace_sched_waking() (and possibly trace_sched_wakeup()) for the same task. Tracers will have to learn to cope. Furthermore, p->pi_lock is used by set_special_state(), to order against TASK_RUNNING stores from other CPUs. But since this is strictly CPU local, we don't need the lock, and set_special_state()'s disabling of IRQs is sufficient. After the normal wakeup takes p->pi_lock it issues smp_mb__after_spinlock(), in order to ensure the woken task must observe prior stores before we observe the p->state. If this is CPU local, this will be satisfied with a compiler barrier, and we rely on try_to_wake_up() being a funcation call, which implies such. Since, when 'p == current', 'p->on_rq' must be true, the normal wakeup would continue into the ttwu_remote() branch, which normally is concerned with exactly this wakeup scenario, except from a remote CPU. IOW we're waking a task that is still running. In this case, we can trivially avoid taking rq->lock, all that's left from this is to set p->state. This then yields an extremely simple and fast path for 'p == current'. Reported-by: Jens Axboe <axboe@kernel.dk> Tested-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Qian Cai <cai@lca.pw> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: akpm@linux-foundation.org Cc: gkohli@codeaurora.org Cc: hch@lst.de Cc: oleg@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
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@ -1991,6 +1991,29 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
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unsigned long flags;
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int cpu, success = 0;
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if (p == current) {
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/*
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* We're waking current, this means 'p->on_rq' and 'task_cpu(p)
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* == smp_processor_id()'. Together this means we can special
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* case the whole 'p->on_rq && ttwu_remote()' case below
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* without taking any locks.
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*
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* In particular:
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* - we rely on Program-Order guarantees for all the ordering,
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* - we're serialized against set_special_state() by virtue of
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* it disabling IRQs (this allows not taking ->pi_lock).
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*/
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if (!(p->state & state))
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return false;
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success = 1;
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cpu = task_cpu(p);
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trace_sched_waking(p);
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p->state = TASK_RUNNING;
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trace_sched_wakeup(p);
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goto out;
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}
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/*
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* If we are going to wake up a thread waiting for CONDITION we
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* need to ensure that CONDITION=1 done by the caller can not be
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@ -2000,7 +2023,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
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raw_spin_lock_irqsave(&p->pi_lock, flags);
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smp_mb__after_spinlock();
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if (!(p->state & state))
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goto out;
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goto unlock;
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trace_sched_waking(p);
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@ -2030,7 +2053,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
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*/
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smp_rmb();
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if (p->on_rq && ttwu_remote(p, wake_flags))
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goto stat;
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goto unlock;
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#ifdef CONFIG_SMP
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/*
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@ -2090,10 +2113,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
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#endif /* CONFIG_SMP */
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ttwu_queue(p, cpu, wake_flags);
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stat:
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ttwu_stat(p, cpu, wake_flags);
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out:
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unlock:
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raw_spin_unlock_irqrestore(&p->pi_lock, flags);
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out:
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if (success)
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ttwu_stat(p, cpu, wake_flags);
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return success;
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}
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