linux-kernel - Re: [resend][PATCH v2] mlock() doesn't wait to finish lru_add_drain

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Date:	Mon, 12 Oct 2009 20:35:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <onestero@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [resend][PATCH v2] mlock() doesn't wait to finish
 lru_add_drain_all()

On Tue, 13 Oct 2009 12:18:17 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> The problem is in __lru_cache_add().
> 
> ============================================================
> void __lru_cache_add(struct page *page, enum lru_list lru)
> {
>         struct pagevec *pvec = &get_cpu_var(lru_add_pvecs)[lru];
> 
>         page_cache_get(page);
>         if (!pagevec_add(pvec, page))
>                 ____pagevec_lru_add(pvec, lru);
>         put_cpu_var(lru_add_pvecs);
> }
> ============================================================
> 
> current typical scenario is
> 1. preempt disable
> 2. assign lru_add_pvec
> 3. page_cache_get()
> 4. pvec->pages[pvec->nr++] = page;
> 5. preempt enable
> 
> but the preempt disabling assume drain_cpu_pagevecs() run on process context.
> we need to convert it with irq_disabling.

Nope, preempt_disable()/enable() can be performed in hard IRQ context. 
I see nothing in __lru_cache_add() which would cause problems when run
from hard IRQ.

Apart from latency, of course.  Doing a full smp_call_function() in
lru_add_drain_all() might get expensive if it's ever called with any
great frequency.

A smart implementation might take a peek at other cpu's queues and omit
the cross-CPU call if the queue is empty, for example..

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