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Message-ID: <4FB1B37A.5050501@gmail.com>
Date: Mon, 14 May 2012 21:38:02 -0400
From: KOSAKI Motohiro <kosaki.motohiro@...il.com>
To: Rusty Russell <rusty@...tcorp.com.au>
CC: KOSAKI Motohiro <kosaki.motohiro@...il.com>,
Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
LKML <linux-kernel@...r.kernel.org>, anton@...ba.org,
Arnd Bergmann <arnd@...db.de>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Mike Travis <travis@....com>,
Thomas Gleixner <tglx@...utronix.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PULL] cpumask: finally make them variable size w/ CPUMASK_OFFSTACK.
>> This code still slow than original. when calling reclaim path, new allocation is almost always
>> fail. then, your code almost always invoke all cpu batch invalidation. i.e. many ipi.
>
> I don't know this code. Does that happen often?Do we really need to
> optimize the out-of-memory path?
we don't need optimize out-of-memory path. but it's not out-of-memory path. our reclaim code
has two steps 1) purge small file cache (try_to_free_pages) 2) get new page (get_page_from_freelist).
but if you have smp box, it's racy. To success 1) doesn't guarantee to success 2). then, drain_all_pages()
is called frequently than you expected.
> But I should have used on_each_cpu_cond() helper which does this for us
> (except it falls back to individial IPIs) which would make this code
> neater.
Ah, yes. that definitely makes sense.
>>>> 2) When CONFIG_CPUMASK_OFFSTACK=n and NR_CPUS is relatively large, cpumask on stack may
>>>> cause stack overflow. because of, alloc_pages() can be called from
>>>> very deep call stack.
>>>
>>> You can't have large NR_CPUS without CONFIG_CPUMASK_OFFSTACK=y,
>>> otherwise you'll get many other stack overflows, too.
>>
>> Original code put cpumask bss instead stack then. :-)
>
> Yes, and this is what it looks like if we convert it directly, but I
> still don't want to encourage people to do this :(
>
> Cheers,
> Rusty.
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1179,7 +1179,7 @@ void drain_all_pages(void)
> * Allocate in the BSS so we wont require allocation in
> * direct reclaim path for CONFIG_CPUMASK_OFFSTACK=y
> */
> - static cpumask_t cpus_with_pcps;
> + static DECLARE_BITMAP(cpus_with_pcps, NR_CPUS);
>
> /*
> * We don't care about racing with CPU hotplug event
> @@ -1197,11 +1197,12 @@ void drain_all_pages(void)
> }
> }
> if (has_pcps)
> - cpumask_set_cpu(cpu,&cpus_with_pcps);
> + cpumask_set_cpu(cpu, to_cpumask(cpus_with_pcps));
> else
> - cpumask_clear_cpu(cpu,&cpus_with_pcps);
> + cpumask_clear_cpu(cpu, to_cpumask(cpus_with_pcps));
> }
> - on_each_cpu_mask(&cpus_with_pcps, drain_local_pages, NULL, 1);
> + on_each_cpu_mask(to_cpumask(cpus_with_pcps),
> + drain_local_pages, NULL, 1);
> }
Looks good to me. thanks.
Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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