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Date: Thu, 14 Aug 2014 19:48:49 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Rik van Riel <riel@redhat.com>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>,
        Frank Mayhar <fmayhar@google.com>,
        Frederic Weisbecker <fweisbec@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sanjay Rao <srao@redhat.com>, Larry Woodman <lwoodman@redhat.com>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
	seqlock
Message-ID: <20140814174849.GA5091@redhat.com>
References: <20140812142539.01851e52@annuminas.surriel.com> <20140812191218.GA15210@redhat.com> <53EA94DD.5040900@redhat.com> <20140813172230.GA6296@redhat.com> <20140813133526.1eb5526f@cuia.bos.redhat.com> <20140813180807.GA8098@redhat.com> <53EBADB1.2020403@redhat.com> <20140813184511.GA9663@redhat.com> <20140813170324.544aaf2d@cuia.bos.redhat.com> <20140814132239.GA24465@redhat.com>
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On 08/14, Oleg Nesterov wrote:
>
> OK, lets forget about alternative approach for now. We can reconsider
> it later. At least I have to admit that seqlock is more straighforward.

Yes.

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.

And of course, stats_lock is more generic.

Oleg.