Commit 9344c92c authored Feb 10, 2016 by Rik van Riel Committed by Ingo Molnar Feb 29, 2016

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: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas 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: Ingo 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 @@ out:
		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);
		}

		/**

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