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

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Date:	Tue, 31 May 2011 09:13:47 +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 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."

So, let cond_resched() be in non-preemptive case.

Thanks,
Rakib

> Thanks,
> -Kame
>
>> ---
>>
>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>> index 20c18b7..72cf857 100644
>> --- a/mm/vmstat.c
>> +++ b/mm/vmstat.c
>> @@ -461,7 +461,11 @@ void refresh_cpu_vm_stats(int cpu)
>>                               p->expire = 3;
>>  #endif
>>                       }
>> +
>> +#ifndef CONFIG_PREEMPT
>>               cond_resched();
>> +#endif
>> +
>>  #ifdef CONFIG_NUMA
>>               /*
>>                * Deal with draining the remote pageset of this
>>
>>
>
>
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