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

[Frederic Weisbecker]

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@xxxxxxxxxx>
> > 
> > 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;
 		} 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));