Re: [PATCH 15/15] sched/cputime: Handle dyntick-idle steal time correctly

Frederic Weisbecker
2026-03-24T14:53:08Z

Le Tue, Mar 03, 2026 at 04:47:45PM +0530, Shrikanth Hegde a écrit :
>
>
> On 2/6/26 7:52 PM, Frederic Weisbecker wrote:
> > The dyntick-idle steal time is currently accounted when the tick
> > restarts but the stolen idle time is not substracted from the idle time
> > that was already accounted. This is to avoid observing the idle time
> > going backward as the dyntick-idle cputime accessors can't reliably know
> > in advance the stolen idle time.
> >
> > In order to maintain a forward progressing idle cputime while
> > substracting idle steal time from it, keep track of the previously
> > accounted idle stolen time and substract it from _later_ idle cputime
> > accounting.
> >
>
> s/substract/subtract ?

Right.

>
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > ---
> > include/linux/kernel_stat.h | 1 +
> > kernel/sched/cputime.c | 21 +++++++++++++++------
> > 2 files changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
> > index 512104b0ff49..24a54a6151ba 100644
> > --- a/include/linux/kernel_stat.h
> > +++ b/include/linux/kernel_stat.h
> > @@ -39,6 +39,7 @@ struct kernel_cpustat {
> > bool idle_elapse;
> > seqcount_t idle_sleeptime_seq;
> > u64 idle_entrytime;
> > + u64 idle_stealtime;
> > #endif
> > u64 cpustat[NR_STATS];
> > };
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 92fa2f037b6e..7e79288eb327 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -424,19 +424,25 @@ static inline void irqtime_account_process_tick(struct task_struct *p, int user_
> > static void kcpustat_idle_stop(struct kernel_cpustat *kc, u64 now)
> > {
> > u64 *cpustat = kc->cpustat;
> > - u64 delta;
> > + u64 delta, steal, steal_delta;
> > if (!kc->idle_elapse)
> > return;
> > delta = now - kc->idle_entrytime;
> > + steal = steal_account_process_time(delta);
> > write_seqcount_begin(&kc->idle_sleeptime_seq);
> > + steal_delta = min_t(u64, kc->idle_stealtime, delta);
> > + delta -= steal_delta;
>
> I didn;t get this logic. Why do we need idle_stealtime?
>
> Lets say 10ms was steal time and 50ms was delta. but idle_stealtime is
> sum of past accumulated steal time. we only need to subtract steal time there no?
>
> Shouldn't this be delta -= steal ?

That would be a risk to observe backward idle accounting:

Time CPU 0 CPU 1
---- ----- -----
0 sec kcpustat_idle_start()
<#VMEXIT>
...
1 sec </#VMEXIT>
arch_cpu_idle() // returns 2
2 sec kcpustat_idle_stop() kcpustat_field(CPUTIME_IDLE, 0)
cpustat[CPUTIME_IDLE] = 2 - 1
// returns 1
kcpustat_field(CPUTIME_IDLE, 0)

We could instead read remotely the paravirt clock, but then
steal_account_process_time() would need to always hold the ->idle_sleeptime_seq,
though it should happen to work without given the ordering.

Anyway to avoid any surprise I accumulate the steal time of an idle cycle to be
substracted on the next idle cycle.

Thanks.

--
Frederic Weisbecker
SUSE Labs