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Commit 9344c92c authored by Rik van Riel's avatar Rik van Riel Committed by Ingo Molnar
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time, acct: Drop irq save & restore from __acct_update_integrals() 



It looks like all the call paths that lead to __acct_update_integrals()
already have irqs disabled, and __acct_update_integrals() does not need
to disable irqs itself.

This is very convenient since about half the CPU time left in this
function was spent in local_irq_save alone.

Performance of a microbenchmark that calls an invalid syscall
ten million times in a row on a nohz_full CPU improves 21% vs.
4.5-rc1 with both the removal of divisions from __acct_update_integrals()
and this patch, with runtime dropping from 3.7 to 2.9 seconds.

With these patches applied, the highest remaining cpu user in
the trace is native_sched_clock, which is addressed in the next
patch.

For testing purposes I stuck a WARN_ON(!irqs_disabled()) test
in __acct_update_integrals(). It did not trigger.

Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarRik van Riel <riel@redhat.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: clark@redhat.com
Cc: eric.dumazet@gmail.com
Cc: fweisbec@gmail.com
Cc: luto@amacapital.net
Link: http://lkml.kernel.org/r/1455152907-18495-4-git-send-email-riel@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent b2add86e

kernel/tsacct.c

+4 −5
Original line number Diff line number Diff line
@@ -126,20 +126,18 @@ static void __acct_update_integrals(struct task_struct *tsk, 
				    cputime_t utime, cputime_t stime)

{

	cputime_t time, dtime;

	unsigned long flags;

	u64 delta;



	if (!likely(tsk->mm))

		return;



	local_irq_save(flags);

	time = stime + utime;

	dtime = time - tsk->acct_timexpd;

	/* Avoid division: cputime_t is often in nanoseconds already. */

	delta = cputime_to_nsecs(dtime);



	if (delta < TICK_NSEC)

		goto out;

		return;



	tsk->acct_timexpd = time;

	/*
@@ -149,8 +147,6 @@ static void __acct_update_integrals(struct task_struct *tsk, 
	 */

	tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm) >> 10;

	tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >> 10;

out:

	local_irq_restore(flags);

}



/**
@@ -160,9 +156,12 @@ static void __acct_update_integrals(struct task_struct *tsk, 
void acct_update_integrals(struct task_struct *tsk)

{

	cputime_t utime, stime;

	unsigned long flags;



	local_irq_save(flags);

	task_cputime(tsk, &utime, &stime);

	__acct_update_integrals(tsk, utime, stime);

	local_irq_restore(flags);

}



/**