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

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Message-ID: <20140814183413.GA6959@redhat.com>
Date:	Thu, 14 Aug 2014 20:34:13 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.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 08/14, Oleg Nesterov wrote:
>
> But just for record, the "lockless" version doesn't look that bad to me,
>
> 	void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times)
> 	{
> 		struct signal_struct *sig = tsk->signal;
> 		bool lockless, is_dead;
> 		struct task_struct *t;
> 		unsigned long flags;
> 		u64 exec;
>
> 		lockless = true;
> 		is_dead = !lock_task_sighand(p, &flags);
> 	 retry:
> 		times->utime = sig->utime;
> 		times->stime = sig->stime;
> 		times->sum_exec_runtime = exec = sig->sum_sched_runtime;
> 		if (is_dead)
> 			return;
>
> 		if (lockless)
> 			unlock_task_sighand(p, &flags);
>
> 		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;
> 			is_dead = !lock_task_sighand(p, &flags);
> 			if (is_dead || exec != sig->sum_sched_runtime)
> 				goto retry;
> 		}
> 		unlock_task_sighand(p, &flags);
> 	}
>
> The obvious problem is that we should shift lock_task_sighand() from the
> callers to thread_group_cputime() first, or add thread_group_cputime_lockless()
> and change the current users one by one.

OTOH, it is simple to convert do_sys_times() and posix_cpu_clock_get_task()
to use the lockless version, and avoid the new stats_lock and other changes
it needs.

> And of course, stats_lock is more generic.

Yes, this is true in any case.

So I simply do not know.

Oleg.

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