linux-kernel - Re: [PATCH] mm, vmstat: Use cond_resched only when !CONFIG

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Message-ID: <BANLkTiksAjyCBAPdCB58tAWhXcdqXM4EcA@mail.gmail.com>
Date:	Tue, 31 May 2011 09:58:40 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Christoph Lameter <cl@...ux.com>,
	Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] mm, vmstat: Use cond_resched only when !CONFIG_PREEMPT

On Tue, May 31, 2011 at 9:18 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Tue, 31 May 2011 09:13:47 +0600
> Rakib Mullick <rakib.mullick@...il.com> wrote:
>
>> On Tue, May 31, 2011 at 5:38 AM, KAMEZAWA Hiroyuki
>> <kamezawa.hiroyu@...fujitsu.com> wrote:
>> > On Mon, 30 May 2011 22:59:04 +0600
>> > Rakib Mullick <rakib.mullick@...il.com> wrote:
>> >
>> >> commit 468fd62ed9 (vmstats: add cond_resched() to refresh_cpu_vm_stats()) added cond_resched() in refresh_cpu_vm_stats. Purpose of that patch was to allow other threads to run in non-preemptive case. This patch, makes sure that cond_resched() gets called when !CONFIG_PREEMPT is set. In a preemptiable kernel we don't need to call cond_resched().
>> >>
>> >> Signed-off-by: Rakib Mullick <rakib.mullick@...il.com>
>> >
>> > Hmm, what benefit do we get by adding this extra #ifdef in the code directly ?
>> > Other cond_resched() callers are not guilty in !CONFIG_PREEMPT ?
>> >
>> Well, in preemptible kernel this context will get preempted if
>> requires, so we don't need cond_resched(). If you checkout the git log
>> of the mentioned commit, you'll find the explanation. It says:
>>         "Adding a cond_resched() to allow other threads to run in the
>> non-preemptive
>>     case."
>>
>
> IOW, my question is "why only this cond_resched() should be fixed ?"

cond_resched() forces this thread to be scheduled. I'm just trying
pointing out the use of cond_resched(), until unless I'm not missing
anything.

> What's bad with all cond_resched() in the kernel as no-op in CONFIG_PREEMPT ?
>
cond_resched() basically checks whether it needs to be scheduled or
not. But, we know in advance that we don't need cond_resched in
CONFIG_PREEMPT.

Thanks,
Rakib

> Thanks,
> -Kame
>
>
>
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