linux-kernel - Re: [PATCH v6 2/3] sched/cputime: Fix prev steal time accouting during cpu hotplug

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Date:	Mon, 13 Jun 2016 12:44:12 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH v6 2/3] sched/cputime: Fix prev steal time accouting
 during cpu hotplug



On 13/06/2016 12:32, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.com>
> 
> 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 is smaller than
> rq->prev_steal_time.
> 
> However, this should never happen. steal 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.
> 
> Fixes: 'commit e9532e69b8d1 ("sched/cputime: Fix steal time accounting vs. CPU hotplug")'
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  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 7f2cae4..7d45bb3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7213,7 +7213,6 @@ static void sched_rq_cpu_starting(unsigned int cpu)
>  	struct rq *rq = cpu_rq(cpu);
>  
>  	rq->calc_load_update = calc_load_update;
> -	account_reset_rq(rq);
>  	update_max_interval();
>  }
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 72f1f30..de607e4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1809,16 +1809,3 @@ static inline void cpufreq_trigger_update(u64 time) {}
>  #else /* arch_scale_freq_capacity */
>  #define arch_scale_freq_invariant()	(false)
>  #endif
> -
> -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
> -}
> 

Acked-by: Paolo Bonzini <pbonzini@...hat.com>