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Date: Fri, 15 Aug 2014 18:49:53 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <umgwanakikbuti@gmail.com>, Rik van Riel <riel@redhat.com>,
        linux-kernel@vger.kernel.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: <20140815164953.GA20197@redhat.com>
References: <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> <20140814174849.GA5091@redhat.com> <1408079971.5536.37.camel@marge.simpson.net> <20140815062819.GY19379@twins.programming.kicks-ass.net> <20140815163651.GA19331@redhat.com>
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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.