linux-kernel - Re: [PATCH RT] sched/workqueue: Only wake up idle workers if not blocked on sleeping spin lock

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Date:	Fri, 3 May 2013 17:53:40 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RT] sched/workqueue: Only wake up idle workers if not
 blocked on sleeping spin lock

* Steven Rostedt | 2013-03-18 15:12:49 [-0400]:

>In -rt, most spin_locks() turn into mutexes. One of these spin_lock
>conversions is performed on the workqueue gcwq->lock. When the idle
>worker is worken, the first thing it will do is grab that same lock and
>it too will block, possibly jumping into the same code, but because
>nr_running would already be decremented it prevents an infinite loop.
>
>But this is still a waste of CPU cycles, and it doesn't follow the method
>of mainline, as new workers should only be woken when a worker thread is
>truly going to sleep, and not just blocked on a spin_lock().
>
>Check the saved_state too before waking up new workers.
>
>Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Taking for v3.8-rt.

Sebastian
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