[PATCH 3/4] time,acct: drop irq save & restore from __acct_update_integrals

[riel]

From: Rik van Riel <riel@xxxxxxxxxx>

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@xxxxxxxxxxxxx>
Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
---
 kernel/tsacct.c | 9 ++++-----
 1 file 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 @@ 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);
 }
 
 /**
-- 
2.5.0