linux-kernel - Re: [PATCH 5/12] ksm: keep quiet while list empty

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Date:	Wed, 5 Aug 2009 12:54:49 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	ieidus@...hat.com, aarcange@...hat.com, riel@...hat.com,
	chrisw@...hat.com, nickpiggin@...oo.com.au,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 5/12] ksm: keep quiet while list empty

On Tue, 4 Aug 2009, Andrew Morton wrote:
> On Mon, 3 Aug 2009 13:14:03 +0100 (BST)
> Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:
> 
> > +		if (ksmd_should_run()) {
> >  			schedule_timeout_interruptible(
> >  				msecs_to_jiffies(ksm_thread_sleep_millisecs));
> >  		} else {
> >  			wait_event_interruptible(ksm_thread_wait,
> > -					(ksm_run & KSM_RUN_MERGE) ||
> > -					kthread_should_stop());
> > +				ksmd_should_run() || kthread_should_stop());
> >  		}
> 
> Yields

(Phew, for a moment I thought you were asking us to use yield() here.)

> 
> 
> 		if (ksmd_should_run()) {
> 			schedule_timeout_interruptible(
> 				msecs_to_jiffies(ksm_thread_sleep_millisecs));
> 		} else {
> 			wait_event_interruptible(ksm_thread_wait,
> 				ksmd_should_run() || kthread_should_stop());
> 		}
> 
> can it be something like
> 
> 		wait_event_interruptible_timeout(ksm_thread_wait,
> 			ksmd_should_run() || kthread_should_stop(),
> 			msecs_to_jiffies(ksm_thread_sleep_millisecs));
> 
> ?

I'd be glad to simplify what we have there, but I think your proposal
ends up doing exactly what we're trying to avoid, doesn't it?  Won't
it briefly wake up ksmd every ksm_thread_sleep_millisecs, even when
there's nothing for it to do?

> 
> That would also reduce the latency in responding to kthread_should_stop().

That's not a high priority consideration.  So far as I can tell, the only
use for that test is at startup, if the sysfs_create_group mysteriously
fails.  It's mostly a leftover from when you could have CONFIG_KSM=m:

I did wonder whether to go back and add some SLAB_PANICs etc now,
but in the end I was either too lazy or too deferential to Izik's
fine error handling (you choose which to believe: both, actually).

Hugh
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