On Fri, Jan 29, 2016 at 10:36:04PM -0500, r...@redhat.com wrote:
> From: Rik van Riel <r...@redhat.com>
>
> 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.
>
> Suggested-by: Peter Zijlstra <pet...@infradead.org>
> Signed-off-by: Rik van Riel <r...@redhat.com>
> ---
> kernel/tsacct.c | 6 +-----
> 1 file changed, 1 insertion(+), 5 deletions(-)
>
> diff --git a/kernel/tsacct.c b/kernel/tsacct.c
> index 8908f8b1d26e..b2663d699a72 100644
> --- a/kernel/tsacct.c
> +++ b/kernel/tsacct.c
> @@ -124,26 +124,22 @@ 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 (unlikely(!tsk->mm))
> return;
>
> - local_irq_save(flags);
> time = stime + utime;
> dtime = time - tsk->acct_timexpd;
> delta = cputime_to_nsecs(dtime);
>
> if (delta < TICK_NSEC)
> - goto out;
> + return;
>
> tsk->acct_timexpd = time;
> /* The final unit will be Mbyte-usecs, see xacct_add_tsk */
> tsk->acct_rss_mem1 += delta * get_mm_rss(tsk->mm) / 1024;
> tsk->acct_vm_mem1 += delta * tsk->mm->total_vm / 1024;
> -out:
> - local_irq_restore(flags);
> }
I think you need this as well, because do_exit() probably doesn't have
irqs disabled at that point:
diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 975cb49..12c6047 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -153,9 +153,12 @@ static void __acct_update_integrals(struct task_struct
*tsk,
void acct_update_integrals(struct task_struct *tsk)
{
cputime_t utime, stime;
+ unsigned long flags;
task_cputime(tsk, &utime, &stime);
+ local_irq_save(flags);
__acct_update_integrals(tsk, utime, stime);
+ local_irq_restore(flags);
}
/**
