Date:   Wed, 10 Nov 2021 14:30:31 -0500
From:   Phil Auld <pauld@redhat.com>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hasegawa Hitomi <hasegawa-hitomi@fujitsu.com>,
        Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH 2/2] sched/cputime: Fix getrusage(RUSAGE_THREAD) with
 nohz_full
Message-ID: <YYwd17co5iwSnDzK@lorien.usersys.redhat.com>
References: <20211026141055.57358-1-frederic@kernel.org>
 <20211026141055.57358-3-frederic@kernel.org>
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In-Reply-To: <20211026141055.57358-3-frederic@kernel.org>
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Hi,

On Tue, Oct 26, 2021 at 04:10:55PM +0200 Frederic Weisbecker wrote:
> getrusage(RUSAGE_THREAD) with nohz_full may return shorter utime/stime
> than the actual time.
> 
> task_cputime_adjusted() snapshots utime and stime and then adjust their
> sum to match the scheduler maintained cputime.sum_exec_runtime.
> Unfortunately in nohz_full, sum_exec_runtime is only updated once per
> second in the worst case, causing a discrepancy against utime and stime
> that can be updated anytime by the reader using vtime.
> 
> To fix this situation, perform an update of cputime.sum_exec_runtime
> when the cputime snapshot reports the task as actually running while
> the tick is disabled. The related overhead is then contained within the
> relevant situations.
> 
> Reported-by: Hasegawa Hitomi <hasegawa-hitomi@fujitsu.com>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Hasegawa Hitomi <hasegawa-hitomi@fujitsu.com>
> ---
>  include/linux/sched/cputime.h |  5 +++--
>  kernel/sched/cputime.c        | 12 +++++++++---
>  2 files changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/sched/cputime.h b/include/linux/sched/cputime.h
> index 6c9f19a33865..ce3c58286062 100644
> --- a/include/linux/sched/cputime.h
> +++ b/include/linux/sched/cputime.h
> @@ -18,15 +18,16 @@
>  #endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
>  
>  #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
> -extern void task_cputime(struct task_struct *t,
> +extern bool task_cputime(struct task_struct *t,
>  			 u64 *utime, u64 *stime);
>  extern u64 task_gtime(struct task_struct *t);
>  #else
> -static inline void task_cputime(struct task_struct *t,
> +static inline bool task_cputime(struct task_struct *t,
>  				u64 *utime, u64 *stime)
>  {
>  	*utime = t->utime;
>  	*stime = t->stime;
> +	return false;
>  }
>  
>  static inline u64 task_gtime(struct task_struct *t)
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 872e481d5098..9392aea1804e 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -615,7 +615,8 @@ void task_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
>  		.sum_exec_runtime = p->se.sum_exec_runtime,
>  	};
>  
> -	task_cputime(p, &cputime.utime, &cputime.stime);
> +	if (task_cputime(p, &cputime.utime, &cputime.stime))
> +		cputime.sum_exec_runtime = task_sched_runtime(p);
>  	cputime_adjust(&cputime, &p->prev_cputime, ut, st);
>  }
>  EXPORT_SYMBOL_GPL(task_cputime_adjusted);
> @@ -828,19 +829,21 @@ u64 task_gtime(struct task_struct *t)
>   * add up the pending nohz execution time since the last
>   * cputime snapshot.
>   */
> -void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
> +bool task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  {
>  	struct vtime *vtime = &t->vtime;
>  	unsigned int seq;
>  	u64 delta;
> +	int ret;
>  
>  	if (!vtime_accounting_enabled()) {
>  		*utime = t->utime;
>  		*stime = t->stime;
> -		return;
> +		return false;
>  	}
>  
>  	do {
> +		ret = false;
>  		seq = read_seqcount_begin(&vtime->seqcount);
>  
>  		*utime = t->utime;
> @@ -850,6 +853,7 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  		if (vtime->state < VTIME_SYS)
>  			continue;
>  
> +		ret = true;
>  		delta = vtime_delta(vtime);
>  
>  		/*
> @@ -861,6 +865,8 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
>  		else
>  			*utime += vtime->utime + delta;
>  	} while (read_seqcount_retry(&vtime->seqcount, seq));
> +
> +	return ret;
>  }
>  
>  static int vtime_state_fetch(struct vtime *vtime, int cpu)
> -- 
> 2.25.1
> 


Could someone please pick this (or, rather, these) up?

Acked-by: Phil Auld <pauld@redhat.com>

Thanks!


Phil

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