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Date:   Fri, 30 Sep 2022 14:16:34 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     zhengzucheng <zhengzucheng@huawei.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        hucool.lihua@huawei.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] sched/cputime: Fix the time backward issue about
 /proc/stat
Message-ID: <20220930121634.GA266766@lothringen>
References: <20220928033402.181530-1-zhengzucheng@huawei.com>
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 <20220928121134.GA233658@lothringen>
 <5126b2dc-8624-babc-2e1e-58ac27927c31@huawei.com>
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On Fri, Sep 30, 2022 at 10:43:58AM +0800, zhengzucheng wrote:
> 
> 在 2022/9/28 20:11, Frederic Weisbecker 写道:
> > @@ -1024,20 +1045,20 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
> >   		 * add pending nohz time to the right place.
> >   		 */
> >   		if (state == VTIME_SYS) {
> > -			cpustat[CPUTIME_SYSTEM] += vtime->stime + delta;
> > +			cpustat[CPUTIME_SYSTEM] += delta;
> >   		} else if (state == VTIME_USER) {
> >   			if (task_nice(tsk) > 0)
> > -				cpustat[CPUTIME_NICE] += vtime->utime + delta;
> > +				cpustat[CPUTIME_NICE] += delta;
> >   			else
> > -				cpustat[CPUTIME_USER] += vtime->utime + delta;
> > +				cpustat[CPUTIME_USER] += delta;
> “delta” has the same problem as vtime->utime, which varies with different
> tasks. switching between different tasks may cause time statistics to be
> reversed.

I'm a bit confused, can you provide an example?

Thanks.

> >   		} else {
> >   			WARN_ON_ONCE(state != VTIME_GUEST);
> >   			if (task_nice(tsk) > 0) {
> > -				cpustat[CPUTIME_GUEST_NICE] += vtime->gtime + delta;
> > -				cpustat[CPUTIME_NICE] += vtime->gtime + delta;
> > +				cpustat[CPUTIME_GUEST_NICE] += delta;
> > +				cpustat[CPUTIME_NICE] += delta;
> >   			} else {
> > -				cpustat[CPUTIME_GUEST] += vtime->gtime + delta;
> > -				cpustat[CPUTIME_USER] += vtime->gtime + delta;
> > +				cpustat[CPUTIME_GUEST] += delta;
> > +				cpustat[CPUTIME_USER] += delta;
> >   			}
> >   		}
> >   	} while (read_seqcount_retry(&vtime->seqcount, seq));
> > .