linux-kernel - Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct reclaim path completely

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Date:	Thu, 24 Mar 2011 16:03:03 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-mm <linux-mm@...ck.org>,
	Andrey Vagin <avagin@...nvz.org>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 1/5] vmscan: remove all_unreclaimable check from direct reclaim path completely

> On Thu, Mar 24, 2011 at 3:16 PM, KOSAKI Motohiro
> <kosaki.motohiro@...fujitsu.com> wrote:
> > Hi
> >
> >> Thanks for your effort, Kosaki.
> >> But I still doubt this patch is good.
> >>
> >> This patch makes early oom killing in hibernation as it skip
> >> all_unreclaimable check.
> >> Normally,  hibernation needs many memory so page_reclaim pressure
> >> would be big in small memory system. So I don't like early give up.
> >
> > Wait. When occur big pressure? hibernation reclaim pressure
> > (sc->nr_to_recliam) depend on physical memory size. therefore
> > a pressure seems to don't depend on the size.
> 
> It depends on physical memory size and /sys/power/image_size.
> If you want to tune image size bigger, reclaim pressure would be big.

Ok, _If_ I want.
However, I haven't seen desktop people customize it.


> >> Do you think my patch has a problem? Personally, I think it's very
> >> simple and clear. :)
> >
> > To be honest, I dislike following parts. It's madness on madness.
> >
> >        static bool zone_reclaimable(struct zone *zone)
> >        {
> >                if (zone->all_unreclaimable)
> >                        return false;
> >
> >                return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
> >        }
> >
> >
> > The function require a reviewer know
> >
> >  o pages_scanned and all_unreclaimable are racy
> 
> Yes. That part should be written down of comment.
> 
> >  o at hibernation, zone->all_unreclaimable can be false negative,
> >   but can't be false positive.
> 
> The comment of all_unreclaimable already does explain it well, I think.

Where is?


> > And, a function comment of all_unreclaimable() says
> >
> >         /*
> >          * As hibernation is going on, kswapd is freezed so that it can't mark
> >          * the zone into all_unreclaimable. It can't handle OOM during hibernation.
> >          * So let's check zone's unreclaimable in direct reclaim as well as kswapd.
> >          */
> >
> > But, now it is no longer copy of kswapd algorithm.
> 
> The comment don't say it should be a copy of kswapd.

I meant the comments says

          * So let's check zone's unreclaimable in direct reclaim as well as kswapd.

but now it isn't aswell as kswapd.

I think it's critical important. If people can't understand why the
algorithm was choosed, anyone will break the code again sooner or later.


> > If you strongly prefer this idea even if you hear above explanation,
> > please consider to add much and much comments. I can't say
> > current your patch is enough readable/reviewable.
> 
> My patch isn't a formal patch for merge but just a concept to show.
> If you agree the idea, of course, I will add more concrete comment
> when I send formal patch.
> 
> Before, I would like to get a your agreement. :)
> If you solve my concern(early give up in hibernation) in your patch, I
> don't insist on my patch, either.

Ok. Let's try.

Please concern why priority=0 is not enough. zone_reclaimable_pages(zone) * 6 
is a conservative value of worry about multi thread race. While one task
is reclaiming, others can allocate/free memory concurrently. therefore,
even after priority=0, we have a chance getting reclaimable pages on lru.
But, in hibernation case, almost all tasks was freezed before hibernation
call shrink_all_memory(). therefore, there is no race. priority=0 reclaim
can cover all lru pages.

Is this enough explanation for you?


> 
> Thanks for the comment, Kosaki.




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