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Date: Fri, 30 Sep 2022 10:43:58 +0800
From: zhengzucheng <zhengzucheng@...wei.com>
To: Frederic Weisbecker <frederic@...nel.org>,
Peter Zijlstra <peterz@...radead.org>
CC: <mingo@...hat.com>, <juri.lelli@...hat.com>,
<vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
<rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
<bristot@...hat.com>, <vschneid@...hat.com>,
<hucool.lihua@...wei.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] sched/cputime: Fix the time backward issue about
/proc/stat
在 2022/9/28 20:11, Frederic Weisbecker 写道:
> On Wed, Sep 28, 2022 at 10:12:33AM +0200, Peter Zijlstra wrote:
>> On Wed, Sep 28, 2022 at 11:34:02AM +0800, Zucheng Zheng wrote:
>>> From: Zheng Zucheng <zhengzucheng@...wei.com>
>>>
>>> The cputime of cpuN read from /proc/stat has an issue of cputime descent.
>>> For example, the phenomenon of cputime descent of user is as followed:
>>>
>>> The value read first is 319, and the value read again is 318. As follows:
>>> first:
>>> cat /proc/stat | grep cpu1
>>> cpu1 319 0 496 41665 0 0 0 0 0 0
>>> again:
>>> cat /proc/stat | grep cpu1
>>> cpu1 318 0 497 41674 0 0 0 0 0 0
>>>
>>> The value read from /proc/stat should be monotonically increasing. Otherwise
>>> user may get incorrect CPU usage.
>>>
>>> The root cause of this problem is that, in the implementation of
>>> kcpustat_cpu_fetch_vtime, vtime->utime + delta is added to the stack variable
>>> cpustat instantaneously. If the task is switched between two times, the value
>>> added to cpustat for the second time may be smaller than that for the first time.
>>>
>>> CPU0 CPU1
>>> First:
>>> show_stat()
>>> ->kcpustat_cpu_fetch()
>>> ->kcpustat_cpu_fetch_vtime()
>>> ->cpustat[CPUTIME_USER] = kcpustat_cpu(cpu) + vtime->utime + delta rq->curr is task A
>>> A switch to B，and A->vtime->utime is less than 1 tick
>>> Then:
>>> show_stat()
>>> ->kcpustat_cpu_fetch()
>>> ->kcpustat_cpu_fetch_vtime()
>>> ->cpustat[CPUTIME_USER] = kcpustat_cpu(cpu) + vtime->utime + delta; rq->curr is task B
>> You're still not explaining where the time gets lost. And the patch is a
>> horrible band-aid.
>>
>> What I think you're saying; after staring at this for a while, is that:
>>
>> vtime_task_switch_generic()
>> __vtime_account_kernel(prev, vtime)
>> vtime_account_{guest,system}(tsk, vtime)
>> vtime->*time += get_vtime_delta()
>> if (vtime->*time >= TICK_NSEC)
>> account_*_time()
>> account_system_index_time()
>> task_group_account_field()
>> __this_cpu_add(kernel_cpustat.cpustat[index], tmp); <---- here
>>
>> is not folding time into kernel_cpustat when the task vtime isn't at
>> least a tick's worth. And then when we switch to another task, we leak
>> time.
> Looks right. Last time the patch was posted I misunderstood the issue.
>
>> There's another problem here, vtime_task_switch_generic() should use a
>> single call to sched_clock() to compute the old vtime_delta and set the
>> new vtime->starttime, otherwise there's a time hole there as well.
> Right, but does it really matter? It's just a few nanosecs ignored
> between two tasks switching.
>
>> This is all quite the maze and it really wants cleaning up, not be made
>> worse.
>>
>> So I think you want to do two things:
>>
>> - pull kernel_cpustat updates out of task_group_account_field()
>> and put them into vtime_task_switch_generic() to be purely
>> vtime->starttime based.
> So you want to force the update on all context switches? We used that TICK_NSEC
> limit before updating in order to lower some overhead.
>
> There is also user <-> kernel involved.
>
> How about handling that from the read side? (below untested)
>
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 78a233d43757..f0f1af337e49 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -814,9 +814,9 @@ u64 task_gtime(struct task_struct *t)
> do {
> seq = read_seqcount_begin(&vtime->seqcount);
>
> - gtime = t->gtime;
> + gtime = t->gtime + vtime->gtime;
> if (vtime->state == VTIME_GUEST)
> - gtime += vtime->gtime + vtime_delta(vtime);
> + gtime += vtime_delta(vtime);
>
> } while (read_seqcount_retry(&vtime->seqcount, seq));
>
> @@ -845,8 +845,8 @@ bool task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
> ret = false;
> seq = read_seqcount_begin(&vtime->seqcount);
>
> - *utime = t->utime;
> - *stime = t->stime;
> + *utime = t->utime + vtime->utime;
> + *stime = t->stime + vtime->stime;
>
> /* Task is sleeping or idle, nothing to add */
> if (vtime->state < VTIME_SYS)
> @@ -860,9 +860,9 @@ bool task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
> * add pending nohz time to the right place.
> */
> if (vtime->state == VTIME_SYS)
> - *stime += vtime->stime + delta;
> + *stime += delta;
> else
> - *utime += vtime->utime + delta;
> + *utime += delta;
> } while (read_seqcount_retry(&vtime->seqcount, seq));
>
> return ret;
> @@ -896,11 +896,22 @@ static int vtime_state_fetch(struct vtime *vtime, int cpu)
>
> static u64 kcpustat_user_vtime(struct vtime *vtime)
> {
> - if (vtime->state == VTIME_USER)
> - return vtime->utime + vtime_delta(vtime);
> - else if (vtime->state == VTIME_GUEST)
> - return vtime->gtime + vtime_delta(vtime);
> - return 0;
> + u64 delta = vtime->utime + vtime->gtime;
> +
> + if (vtime->state == VTIME_USER || vtime->state == VTIME_GUEST)
> + delta += vtime_delta(vtime);
> +
> + return delta;
> +}
> +
> +static u64 kcpustat_guest_vtime(struct vtime *vtime)
> +{
> + u64 delta = vtime->gtime;
> +
> + if (vtime->state == VTIME_GUEST)
> + delta += vtime_delta(vtime);
> +
> + return delta;
> }
>
> static int kcpustat_field_vtime(u64 *cpustat,
> @@ -931,8 +942,9 @@ static int kcpustat_field_vtime(u64 *cpustat,
> */
> switch (usage) {
> case CPUTIME_SYSTEM:
> + *val += vtime->stime;
> if (state == VTIME_SYS)
> - *val += vtime->stime + vtime_delta(vtime);
> + *val += vtime_delta(vtime);
> break;
> case CPUTIME_USER:
> if (task_nice(tsk) <= 0)
> @@ -943,12 +955,12 @@ static int kcpustat_field_vtime(u64 *cpustat,
> *val += kcpustat_user_vtime(vtime);
> break;
> case CPUTIME_GUEST:
> - if (state == VTIME_GUEST && task_nice(tsk) <= 0)
> - *val += vtime->gtime + vtime_delta(vtime);
> + if (task_nice(tsk) <= 0)
> + *val += kcpustat_guest_vtime(vtime);
> break;
> case CPUTIME_GUEST_NICE:
> - if (state == VTIME_GUEST && task_nice(tsk) > 0)
> - *val += vtime->gtime + vtime_delta(vtime);
> + if (task_nice(tsk) > 0)
> + *val += kcpustat_guest_vtime(vtime);
> break;
> default:
> break;
> @@ -1013,6 +1025,15 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
> *dst = *src;
> cpustat = dst->cpustat;
>
> + cpustat[CPUTIME_SYSTEM] += vtime->stime;
> + if (task_nice(tsk) > 0) {
> + cpustat[CPUTIME_NICE] += vtime->utime + vtime->gtime;
> + cpustat[CPUTIME_GUEST_NICE] += vtime->gtime;
> + } else {
> + cpustat[CPUTIME_USER] += vtime->utime + vtime->gtime;
> + cpustat[CPUTIME_GUEST] += vtime->gtime;
> + }
> +
> /* Task is sleeping, dead or idle, nothing to add */
> if (state < VTIME_SYS)
> continue;
> @@ -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.
> } 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));
> .
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