linux-kernel - Re: [PATCH v2] sched, timer: Use atomics for thread_group

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Date:	Mon, 2 Mar 2015 20:40:33 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Scott Norton <scott.norton@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched, timer: Use atomics for thread_group_cputimer
	to improve scalability

Well, I forgot everything about this code, but let me ask anyway ;)

On 03/02, Jason Low wrote:
>
> -static void update_gt_cputime(struct task_cputime *a, struct task_cputime *b)
> +static inline void __update_gt_cputime(atomic64_t *cputime, u64 sum_cputime)
>  {
> -	if (b->utime > a->utime)
> -		a->utime = b->utime;
> -
> -	if (b->stime > a->stime)
> -		a->stime = b->stime;
> +	u64 curr_cputime;
> +	/*
> +	 * Set cputime to sum_cputime if sum_cputime > cputime. Use cmpxchg
> +	 * to avoid race conditions with concurrent updates to cputime.
> +	 */
> +retry:
> +	curr_cputime = atomic64_read(cputime);
> +	if (sum_cputime > curr_cputime) {
> +		if (atomic64_cmpxchg(cputime, curr_cputime, sum_cputime) != curr_cputime)
> +			goto retry;
> +	}
> +}
>  
> -	if (b->sum_exec_runtime > a->sum_exec_runtime)
> -		a->sum_exec_runtime = b->sum_exec_runtime;
> +static void update_gt_cputime(struct thread_group_cputimer *cputimer, struct task_cputime *sum)
> +{
> +	__update_gt_cputime(&cputimer->utime, sum->utime);
> +	__update_gt_cputime(&cputimer->stime, sum->stime);
> +	__update_gt_cputime(&cputimer->sum_exec_runtime, sum->sum_exec_runtime);
>  }

And this is called if !cputimer_running().

So who else can update these atomic64_t's ? The caller is called under ->siglock.
IOW, do we really need to cmpxchg/retry ?

Just curious, I am sure I missed something.

> @@ -222,13 +239,10 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
>  		 * it.
>  		 */
>  		thread_group_cputime(tsk, &sum);
> -		raw_spin_lock_irqsave(&cputimer->lock, flags);
> -		cputimer->running = 1;
> -		update_gt_cputime(&cputimer->cputime, &sum);
> -	} else
> -		raw_spin_lock_irqsave(&cputimer->lock, flags);
> -	*times = cputimer->cputime;
> -	raw_spin_unlock_irqrestore(&cputimer->lock, flags);
> +		update_gt_cputime(cputimer, &sum);
> +		ACCESS_ONCE(cputimer->running) = 1;

WRITE_ONCE() looks better... but it is not clear to me why do we need it
at all.

Oleg.

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