Re: [PATCH -next] sched/cputime: Fix the time backward issue about /proc/stat

[zhengzucheng]

在 2022/9/30 20:16, Frederic Weisbecker 写道:
> 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?
Chinese National Day holiday, sorry for not replying to you in time.

CONFIG_HZ=100
const struct kernel_cpustat *src = &kcpustat_cpu(cpu);
struct vtime *vtime = &tsk->vtime;
cpustat[CPUTIME_USER] += vtime->utime + delta;

first：
cat /proc/stat | grep cpu1
cpu1 319 0 496 41665 0 0 0 0 0 0
Task A is running on CPU 1，so vtime is A->vtime and delta is A's delta.
kcpustat_cpu_fetch_vtime: cpu=1 src->cpustat[CPUTIME_USER]=3189000000 
vtime->utime=900000 delta=100001
cpustat[CPUTIME_USER] = 3189000000 + 900000 + 100001 is 3,190,000,001, 
319 ticks

again：
cat /proc/stat | grep cpu1
cpu1 318 0 497 41674 0 0 0 0 0 0
Task B is running on CPU 1，so vtime is B->vtime and delta is B's delta.
kcpustat_cpu_fetch_vtime: cpu=1 src->cpustat[CPUTIME_USER]=3189000000 
vtime->utime=900000 delta=90000
cpustat[CPUTIME_USER] = 3189000000 + 900000 + 90000 is 3,189,990,000, 
318 ticks

The root cause is that the value of task B's "vtime->utime + delta" may 
be smaller than task A.
> 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));
> > > .
> .