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

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Date:	Tue, 13 Oct 2009 10:17:48 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	kosaki.motohiro@...fujitsu.com,
	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()

Hi

> On Fri,  9 Oct 2009 11:21:55 +0900 (JST)
> KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> 
> > Recently, Mike Galbraith reported mlock() makes hang-up very long time in
> > his system. Peter Zijlstra explainted the reason.
> > 
> >   Suppose you have 2 cpus, cpu1 is busy doing a SCHED_FIFO-99 while(1),
> >   cpu0 does mlock()->lru_add_drain_all(), which does
> >   schedule_on_each_cpu(), which then waits for all cpus to complete the
> >   work. Except that cpu1, which is busy with the RT task, will never run
> >   keventd until the RT load goes away.
> > 
> >   This is not so much an actual deadlock as a serious starvation case.
> > 
> > His system has two partions using cpusets and RT-task partion cpu doesn't
> > have any PCP cache. thus, this result was pretty unexpected.
> > 
> > The fact is, mlock() doesn't need to wait to finish lru_add_drain_all().
> > if mlock() can't turn on PG_mlock, vmscan turn it on later.
> > 
> > Thus, this patch replace it with lru_add_drain_all_async().
> 
> So why don't we just remove the lru_add_drain_all() call from sys_mlock()?

There are small reason. the administrators and the testers (include me)
look at Mlock field in /proc/meminfo.
They natually expect Mlock field match with actual number of mlocked pages
if the system don't have any stress. Otherwise, we can't make mlock test case ;)


> How did you work out why the lru_add_drain_all() is present in
> sys_mlock() anyway?  Neither the code nor the original changelog tell
> us.  Who do I thwap for that?  Nick and his reviewers.  Sigh.

[Umm, My dictionaly don't tell me the meaning of "thwap".  An meaning of
an imitative word strongly depend on culture. Thus, I probably
misunderstand this paragraph.]

I've understand the existing reason by looooooong time review.


> There are many callers of lru_add_drain_all() all over the place.  Each
> of those is vulnerable to the same starvation issue, is it not?

There are.

> If so, it would be better to just fix up lru_add_drain_all().  Afaict
> all of its functions can be performed in hard IRQ context, so we can
> use smp_call_function()?

There is a option. but it have one downside, it require lru_add_pvecs
related function call irq_disable().

__lru_cache_add() is often called from page fault path. then we need
performance mesurement.




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