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From: Johannes Weiner <hannes@cmpxchg.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@kernel.org, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	bristot@redhat.com, bsingharora@gmail.com, pbonzini@redhat.com,
	maz@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, riel@surriel.com
Subject: Re: [PATCH 1/6] delayacct: Use sched_clock()
Date: Thu, 6 May 2021 11:17:54 -0400	[thread overview]
Message-ID: <YJQIopjVwzBjxg4n@cmpxchg.org> (raw)
In-Reply-To: <YJP6fWhwg95JZ1Kg@hirez.programming.kicks-ass.net>

On Thu, May 06, 2021 at 04:17:33PM +0200, Peter Zijlstra wrote:
> On Thu, May 06, 2021 at 09:59:11AM -0400, Johannes Weiner wrote:
> > On Wed, May 05, 2021 at 12:59:41PM +0200, Peter Zijlstra wrote:
> > > @@ -42,10 +42,9 @@ void __delayacct_tsk_init(struct task_st
> > >   * Finish delay accounting for a statistic using its timestamps (@start),
> > >   * accumalator (@total) and @count
> > >   */
> > > -static void delayacct_end(raw_spinlock_t *lock, u64 *start, u64 *total,
> > > -			  u32 *count)
> > > +static void delayacct_end(raw_spinlock_t *lock, u64 *start, u64 *total, u32 *count)
> > >  {
> > > -	s64 ns = ktime_get_ns() - *start;
> > > +	s64 ns = local_clock() - *start;
> > 
> > I don't think this is safe. These time sections that have preemption
> > and migration enabled and so might span multiple CPUs. local_clock()
> > could end up behind *start, AFAICS.
> 
> Only if you have really crummy hardware, and in that case the drift is
> bounded by around 1 tick. Also, this function actually checks: ns > 0.

Oh, I didn't realize it was that close. I just went off the dramatic
warnings on cpu_clock() :-) But yeah, that seems plenty accurate for
this purpose.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

  reply	other threads:[~2021-05-06 15:18 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-05 10:59 [PATCH 0/6] sched,delayacct: Some cleanups Peter Zijlstra
2021-05-05 10:59 ` [PATCH 1/6] delayacct: Use sched_clock() Peter Zijlstra
2021-05-05 14:40   ` Rik van Riel
2021-05-06 13:59   ` Johannes Weiner
2021-05-06 14:17     ` Peter Zijlstra
2021-05-06 15:17       ` Johannes Weiner [this message]
2021-05-07 12:40   ` Balbir Singh
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 10:43   ` [PATCH 1/6] " Mel Gorman
2021-05-05 10:59 ` [PATCH 2/6] sched: Rename sched_info_{queued,dequeued} Peter Zijlstra
2021-05-05 14:39   ` Rik van Riel
2021-05-06 13:59   ` Johannes Weiner
2021-05-10  8:45   ` Balbir Singh
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 10:49   ` [PATCH 2/6] " Mel Gorman
2021-05-05 10:59 ` [PATCH 3/6] sched: Simplify sched_info_on() Peter Zijlstra
2021-05-06 14:03   ` Johannes Weiner
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 11:10   ` [PATCH 3/6] " Mel Gorman
2021-05-12 11:32     ` Peter Zijlstra
2021-05-12 12:51       ` Mel Gorman
2021-05-05 10:59 ` [PATCH 4/6] kvm: Select SCHED_INFO instead of TASK_DELAY_ACCT Peter Zijlstra
2021-05-05 11:37   ` Paolo Bonzini
2021-05-06 14:38   ` Marc Zyngier
2021-05-07 12:42   ` Balbir Singh
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 11:11   ` [PATCH 4/6] " Mel Gorman
2021-05-05 10:59 ` [PATCH 5/6] delayacct: Add static_branch in scheduler hooks Peter Zijlstra
2021-05-06 14:05   ` Johannes Weiner
2021-05-10  8:42   ` Balbir Singh
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 11:13   ` [PATCH 5/6] " Mel Gorman
2021-05-05 10:59 ` [PATCH 6/6] [RFC] delayacct: Default disabled Peter Zijlstra
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 11:35   ` [PATCH 6/6] [RFC] " Mel Gorman
2021-05-05 22:29 ` [PATCH 0/6] sched,delayacct: Some cleanups Balbir Singh
2021-05-06  9:13   ` Peter Zijlstra
2021-05-07 12:38     ` Balbir Singh
2021-05-12 11:34       ` Mel Gorman
2021-05-12 11:38         ` Peter Zijlstra
2021-05-12 12:23         ` Paul Wise
2021-05-12 13:00           ` Mel Gorman
2021-05-13  1:29             ` Paul Wise
2021-06-25  0:50         ` Paul Wise
2021-05-07  9:05 ` Thomas Gleixner
2021-05-10  7:08 ` Ingo Molnar
2021-05-10 12:05 ` [PATCH 7/6] delayacct: Add sysctl to enable at runtime Peter Zijlstra
2021-05-10 12:06   ` Peter Zijlstra
2021-05-12 10:28   ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2021-05-12 11:40   ` [PATCH 7/6] " Mel Gorman
2021-05-19  8:09     ` [tip: sched/core] delayacct: Document task_delayacct sysctl tip-bot2 for Mel Gorman

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