Re: [Xen-devel] [PATCH 1/1] sched/cputime: do not decrease steal time after live migration on xen

[Stanislaw Gruszka <sgruszka@xxxxxxxxxx>]

On Tue, Oct 10, 2017 at 12:59:26PM +0200, Ingo Molnar wrote:
> 
> (Cc:-ed more gents involved in kernel/sched/cputime.c work. Full patch quoted 
> below.)
> 
> * Dongli Zhang <dongli.zhang@xxxxxxxxxx> wrote:
> 
> > After guest live migration on xen, steal time in /proc/stat
> > (cpustat[CPUTIME_STEAL]) might decrease because steal returned by
> > paravirt_steal_clock() might be less than this_rq()->prev_steal_time.
> > 
> > For instance, steal time of each vcpu is 335 before live migration.
> > 
> > cpu  198 0 368 200064 1962 0 0 1340 0 0
> > cpu0 38 0 81 50063 492 0 0 335 0 0
> > cpu1 65 0 97 49763 634 0 0 335 0 0
> > cpu2 38 0 81 50098 462 0 0 335 0 0
> > cpu3 56 0 107 50138 374 0 0 335 0 0
> > 
> > After live migration, steal time is reduced to 312.
> > 
> > cpu  200 0 370 200330 1971 0 0 1248 0 0
> > cpu0 38 0 82 50123 500 0 0 312 0 0
> > cpu1 65 0 97 49832 634 0 0 312 0 0
> > cpu2 39 0 82 50167 462 0 0 312 0 0
> > cpu3 56 0 107 50207 374 0 0 312 0 0
> > 
> > The code in this patch is borrowed from do_stolen_accounting() which has
> > already been removed from linux source code since commit ecb23dc6 ("xen:
> > add steal_clock support on x86").
> > 
> > Similar and more severe issue would impact prior linux 4.8-4.10 as
> > discussed by Michael Las at
> > https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest.
> > Unlike the issue discussed by Michael Las which would overflow steal time
> > and lead to 100% st usage in top command for linux 4.8-4.10, the issue for
> > linux 4.11+ would only decrease but not overflow steal time after live
> > migration.
> > 
> > References: 
> > https://0xstubs.org/debugging-a-flaky-cpu-steal-time-counter-on-a-paravirtualized-xen-guest
> > Signed-off-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
> > ---
> >  kernel/sched/cputime.c | 13 ++++++++++---
> >  1 file changed, 10 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> > index 14d2dbf..57d09cab 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -238,10 +238,17 @@ static __always_inline u64 
> > steal_account_process_time(u64 maxtime)
> >  {
> >  #ifdef CONFIG_PARAVIRT
> >     if (static_key_false(&paravirt_steal_enabled)) {
> > -           u64 steal;
> > +           u64 steal, steal_time;
> > +           s64 steal_delta;
> > +
> > +           steal_time = paravirt_steal_clock(smp_processor_id());
> > +           steal = steal_delta = steal_time - this_rq()->prev_steal_time;
> > +
> > +           if (unlikely(steal_delta < 0)) {
> > +                   this_rq()->prev_steal_time = steal_time;

I don't think setting prev_steal_time to smaller value is right
thing to do. 

Beside, I don't think we need to check for overflow condition for
cputime variables (it will happen after 279 years :-). So instead
of introducing signed steal_delta variable I would just add
below check, which should be sufficient to fix the problem:

        if (unlikely(steal <= this_rq()->prev_steal_time))
                return 0;

Thanks
Stanislaw

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