[tip:sched/core] time, acct: Drop irq save & restore from __acct_update_integrals()

From: tip-bot for Rik van Riel
Date: Mon Feb 29 2016 - 06:19:23 EST

Commit-ID: 9344c92c2e72e495f695caef8364b3dd73af0eab
Gitweb: http://git.kernel.org/tip/9344c92c2e72e495f695caef8364b3dd73af0eab
Author: Rik van Riel <riel@xxxxxxxxxx>
AuthorDate: Wed, 10 Feb 2016 20:08:26 -0500
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Mon, 29 Feb 2016 09:53:09 +0100

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

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

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: clark@xxxxxxxxxx
Cc: eric.dumazet@xxxxxxxxx
Cc: fweisbec@xxxxxxxxx
Cc: luto@xxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1455152907-18495-4-git-send-email-riel@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/tsacct.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index d12e815..f8e26ab 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -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))

- 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;
- 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);