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

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Message-ID: <20111109180900.GF1260@google.com>
Date:	Wed, 9 Nov 2011 10:09:00 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"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, Anrea.

On Wed, Nov 09, 2011 at 06:29:42PM +0100, Andrea Arcangeli wrote:
> My point is if what happens is:
> 
>    freezer CPU		   khugepaged
>    ------
>    assert freezing
>    wake_up(interruptible)
> 			   __set_current_state(interruptible)
> 			   schedule()
> 
> are we still hanging then?

Yeap, you're right.  I was thinking INTERRUPTILBE was being set before
try_to_freeze().

> And I think it's silly to use wait_event_freezable_timeout if I
> don't have any waitqueue to wait on.

I'm confused.  You're doing add_wait_queue() before
schedule_timeout_interruptible().  prepare_to_wait() is essentially
add_wait_queue() + set_current_state().  What am I missing?  ie. why
not do the following?

	prepare_to_wait(INTERRUPTIBLE);
	try_to_freeze();
	schedule_timeout();
	try_to_freeze();
	finish_wait();

or even simpler,

	wait_event_freezable_timeout(wq, false, timeout);

In terms of overhead, there is no appreciable difference from

	add_wait_queue();
	schedule_timeout_interruptible();
	remove_wait_queue()

Or is the logic there scheduled to change?

> +signed long __sched schedule_timeout_freezable(signed long timeout)
> +{
> +	do
> +		set_current_state(TASK_INTERRUPTIBLE);
> +	while (try_to_freeze());
> +	return schedule_timeout(timeout);
> +}
> +EXPORT_SYMBOL(schedule_timeout_freezable);

Hmmm... I don't know.  I really hope all freezable tasks stick to
higher level interface.  It's way too easy to get things wrong and eat
either freezing or actual wakeup condition.

Thank you.

-- 
tejun
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