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Date:   Thu, 6 May 2021 16:17:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.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()
Message-ID: <YJP6fWhwg95JZ1Kg@hirez.programming.kicks-ass.net>
References: <20210505105940.190490250@infradead.org>
 <20210505111525.001031466@infradead.org>
 <YJP2L1lUvUrur4pK@cmpxchg.org>
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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.

And if you do have crummy hardware like that, ktime_get_ns() is the very
last thing you want to call at any frequency because it'll be the HPET.