linux-kernel - Re: [RFC PATCH 6/6] stop_machine: optimize stop_work

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Message-ID: <20150629085646.GC3644@twins.programming.kicks-ass.net>
Date:	Mon, 29 Jun 2015 10:56:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	der.herr@...r.at, dave@...olabs.net, riel@...hat.com,
	viro@...IV.linux.org.uk, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 6/6] stop_machine: optimize stop_work_alloc()

On Fri, Jun 26, 2015 at 04:15:26AM +0200, Oleg Nesterov wrote:
> wait_event()/wake_up_all() in stop_work_alloc/stop_work_free logic
> is very suboptimal because of non-exclusive wakeups. So we add the
> wait_queue_func_t alloc_wake() helper which wakes the waiter up only
> a) if it actually waits for a stop_work in the "freed" cpumask, and
> b) only after we already set ->stop_owner = waiter.
> 
> So if 2 stop_machine()'s race with each other, the loser will likely
> call schedule() only once and we will have a single wakeup.

So I think I can beat lockdep into submission (ugly but still) do we
want to use an actual per-cpu mutex instead?
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