linux-kernel - Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

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Message-ID: <20140815164953.GA20197@redhat.com>
Date:	Fri, 15 Aug 2014 18:49:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
	seqlock

On 08/15, Oleg Nesterov wrote:
>
> However, if we only want to make sys_times() more scalable(), then
> perhaps the "lockless" version of thread_group_cputime() makes more
> sense. And given that do_sys_times() uses current we can simplify it;
> is_dead is not possible and we do not need to take ->siglock twice:
>
> 	void current_group_cputime(struct task_cputime *times)
> 	{
> 		struct task_struct *tsk = current, *t;
> 		struct spinlock_t *siglock = &tsk->sighand->siglock;
> 		struct signal_struct *sig = tsk->signal;
> 		bool lockless = true;
> 		u64 exec;
>
> 	 retry:
> 		spin_lock_irq(siglock);
> 		times->utime = sig->utime;
> 		times->stime = sig->stime;
> 		times->sum_exec_runtime = exec = sig->sum_sched_runtime;
>
> 		if (lockless)
> 			spin_unlock_irq(siglock);
>
> 		rcu_read_lock();
> 		for_each_thread(tsk, t) {
> 			cputime_t utime, stime;
> 			task_cputime(t, &utime, &stime);
> 			times->utime += utime;
> 			times->stime += stime;
> 			times->sum_exec_runtime += task_sched_runtime(t);
> 		}
> 		rcu_read_unlock();
>
> 		if (lockless) {
> 			lockless = false;
> 			spin_unlock_wait(siglock);
> 			smp_rmb();
> 			if (exec != sig->sum_sched_runtime)
> 				goto retry;
> 		} else {
> 			spin_unlock_irq(siglock);
> 		}
> 	}

Just in case... Yes, sure, "seqlock_t stats_lock" is more scalable. Just
I do not know it's worth the trouble.

Oleg.

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