linux-kernel - Re: [PATCH RFC] time: drop do_sys

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Date:	Wed, 13 Aug 2014 19:22:30 +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: drop do_sys_times spinlock

On 08/12, Rik van Riel wrote:
>
> Any other ideas?

To simplify, lets suppose that we only need sum_exec_runtime.

Perhaps we can do something like this

	u64 thread_group_sched_runtime(void)
	{
		struct task_struct *tsk = current;
		spinlock_t *siglock = &tsk->sighand->siglock; /* stable */
		struct task_struct *t;
		u64 x1, x2;

	retry:
		x1 = tsk->signal->sum_sched_runtime;
		rmb();
		spin_unlock_wait(siglock);
		rmb();

		x2 = 0;
		rcu_read_lock();
		for_each_thread(tsk, t)
			x2 += task_sched_runtime(t);
		rcu_read_unlock();

		rmb();
		spin_unlock_wait(siglock);
		rmb();

		if (x1 != tsk->signal->sum_sched_runtime)
			goto retry;

		return x1 + x2;
	}

?

We do not care if for_each_thread() misses the new thread, we can pretend
thread_group_sched_runtime() was called before clone.

We do not care if a thread with sum_sched_runtime == 0 exits, obviously.

Otherwise "x1 != tsk->signal->sum_sched_runtime" should tell us that we
raced with __exit_signal().

Oleg.

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