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

From: Paolo Bonzini
Date: Wed Aug 02 2017 - 12:47:14 EST


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
>
>