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>

1 files changed, 4 insertions, 5 deletions

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index d12e815b7bcd..f8e26ab963ed 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))
                 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);
 }
 
 /**