Re: [stable 4.4: PATCH] sched/cputime: Fix prev steal time accouting during CPU hotplug

[Paolo Bonzini]

On 02/08/2017 18:36, Andres Oportus wrote:
> +Paolo Bonzini
> 
> On Mon, Jul 31, 2017 at 9:18 PM, Andres Oportus
> <andresoportus@xxxxxxxxxxx <mailto:andresoportus@xxxxxxxxxxx>> wrote:
> 
>     commit 3d89e5478bf550a50c99e93adf659369798263b0 upstream.

Not an easy call because this is a guest patch that depends on a host
patch.  But since it is a corner case (CPU hotplug) and it was
backported to 4.7 already,

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Paolo

>     Commit:
> 
>       e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU
>     hotplug")
> 
>     ... set rq->prev_* to 0 after a CPU hotplug comes back, in order to
>     fix the case where (after CPU hotplug) steal time is smaller than
>     rq->prev_steal_time.
> 
>     However, this should never happen. Steal time was only smaller
>     because of the
>     KVM-specific bug fixed by the previous patch.  Worse, the previous patch
>     triggers a bug on CPU hot-unplug/plug operation: because
>     rq->prev_steal_time is cleared, all of the CPU's past steal time will be
>     accounted again on hot-plug.
> 
>     Since the root cause has been fixed, we can just revert
>     commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs.
>     CPU hotplug").
> 
>     Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx
>     <mailto:wanpeng.li@xxxxxxxxxxx>>
>     Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx
>     <mailto:peterz@xxxxxxxxxxxxx>>
>     Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx
>     <mailto:pbonzini@xxxxxxxxxx>>
>     Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx <mailto:fweisbec@xxxxxxxxx>>
>     Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx
>     <mailto:torvalds@xxxxxxxxxxxxxxxxxxxx>>
>     Cc: Mike Galbraith <efault@xxxxxx <mailto:efault@xxxxxx>>
>     Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx <mailto:peterz@xxxxxxxxxxxxx>>
>     Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx <mailto:rkrcmar@xxxxxxxxxx>>
>     Cc: Rik van Riel <riel@xxxxxxxxxx <mailto:riel@xxxxxxxxxx>>
>     Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx <mailto:tglx@xxxxxxxxxxxxx>>
>     Fixes: 'commit e9532e69b8d1 ("sched/cputime: Fix steal time
>     accounting vs. CPU hotplug")'
>     Link:
>     http://lkml.kernel.org/r/1465813966-3116-3-git-send-email-wanpeng.li@xxxxxxxxxxx
>     <http://lkml.kernel.org/r/1465813966-3116-3-git-send-email-wanpeng.li@xxxxxxxxxxx>
>     Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx <mailto:mingo@xxxxxxxxxx>>
>     Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx
>     <mailto:gregkh@xxxxxxxxxxxxxxxxxxx>>
>     Signed-off-by: Andres Oportus <andresoportus@xxxxxxxxxx
>     <mailto:andresoportus@xxxxxxxxxx>>
>     ---
>      kernel/sched/core.c  |  1 -
>      kernel/sched/sched.h | 13 -------------
>      2 files changed, 14 deletions(-)
> 
>     diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>     index c436426..b103b9c 100644
>     --- a/kernel/sched/core.c
>     +++ b/kernel/sched/core.c
>     @@ -5553,7 +5553,6 @@ migration_call(struct notifier_block *nfb,
>     unsigned long action, void *hcpu)
> 
>             case CPU_UP_PREPARE:
>                     rq->calc_load_update = calc_load_update;
>     -               account_reset_rq(rq);
>                     break;
> 
>             case CPU_ONLINE:
>     diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>     index 4e5db65..55d92a1 100644
>     --- a/kernel/sched/sched.h
>     +++ b/kernel/sched/sched.h
>     @@ -1770,16 +1770,3 @@ static inline u64 irq_time_read(int cpu)
>      }
>      #endif /* CONFIG_64BIT */
>      #endif /* CONFIG_IRQ_TIME_ACCOUNTING */
>     -
>     -static inline void account_reset_rq(struct rq *rq)
>     -{
>     -#ifdef CONFIG_IRQ_TIME_ACCOUNTING
>     -       rq->prev_irq_time = 0;
>     -#endif
>     -#ifdef CONFIG_PARAVIRT
>     -       rq->prev_steal_time = 0;
>     -#endif
>     -#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
>     -       rq->prev_steal_time_rq = 0;
>     -#endif
>     -}
>     --
>     1.9.1
> 
>