linux-kernel - Re: [PATCH] thp: reduce khugepaged freezing latency

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Message-ID: <20111109170657.GE1260@google.com>
Date:	Wed, 9 Nov 2011 09:06:57 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Andrea Arcangeli <aarcange@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...e.com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jiri Slaby <jirislaby@...il.com>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] thp: reduce khugepaged freezing latency

Hello,

On Wed, Nov 09, 2011 at 08:59:25AM -0800, Tejun Heo wrote:
> Freezer depends on the usual "set_current_state(INTERRUPTIBLE); check
> freezing; schedule(); check freezing" construct and sends
> INTERRUPTIBLE wake up after setting freezing state.  The
> synchronization hasn't been completely clear but recently been cleaned
> up, so as long as freezing condition is tested after INTERRUPTIBLE is
> set before going to sleep, the event won't go missing.
> 
> Maybe we need a helper here, which would be named horribly -
> schedule_timeout_interruptible_freezable().  (cc'ing Oleg) Oleg, maybe
> we need schedule_timeout(@sleep_type) too?

Ah, crap, still waking up.  Sorry about that.  So, yes, there's a race
condition above.  You need to set TASK_INTERRUPTIBLE before testing
freezing and use schedule_timeout() instead of
schedule_timeout_interruptible().  Was getting confused with
prepare_to_wait().  That said, why not use prepare_to_wait() instead?

Thanks.

-- 
tejun
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