linux-kernel - Re: [RFC] mm: bail out in shrin_inactive

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Message-ID: <20160801234639.GA6770@bbox>
Date:	Tue, 2 Aug 2016 08:46:39 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC] mm: bail out in shrin_inactive_list

On Fri, Jul 29, 2016 at 10:11:30AM -0400, Johannes Weiner wrote:
> On Mon, Jul 25, 2016 at 04:51:59PM +0900, Minchan Kim wrote:
> > With node-lru, if there are enough reclaimable pages in highmem
> > but nothing in lowmem, VM can try to shrink inactive list although
> > the requested zone is lowmem.
> > 
> > The problem is direct reclaimer scans inactive list is fulled with
> > highmem pages to find a victim page at a reqested zone or lower zones
> > but the result is that VM should skip all of pages. It just burns out
> > CPU. Even, many direct reclaimers are stalled by too_many_isolated
> > if lots of parallel reclaimer are going on although there are no
> > reclaimable memory in inactive list.
> > 
> > I tried the experiment 4 times in 32bit 2G 8 CPU KVM machine
> > to get elapsed time.
> > 
> > 	hackbench 500 process 2
> > 
> > = Old =
> > 
> > 1st: 289s 2nd: 310s 3rd: 112s 4th: 272s
> > 
> > = Now =
> > 
> > 1st: 31s  2nd: 132s 3rd: 162s 4th: 50s.
> > 
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> > I believe proper fix is to modify get_scan_count. IOW, I think
> > we should introduce lruvec_reclaimable_lru_size with proper
> > classzone_idx but I don't know how we can fix it with memcg
> > which doesn't have zone stat now. should introduce zone stat
> > back to memcg? Or, it's okay to ignore memcg?
> 
> You can fully ignore memcg and kmemcg. They only care about the
> balance sheet - page in, page out - never mind the type of page.
> 
> If you are allocating a slab object and there is no physical memory,
> you'll wake kswapd or enter direct reclaim with the restricted zone
> index. If you then try to charge the freshly allocated page or object
> but hit the limit, kmem or otherwise, you'll enter memcg reclaim that
> is not restricted and only cares about getting usage + pages < limit.

Thanks. I got understood.

> 
> I agree that it might be better to put this logic in get_scan_count()
> and set both nr[lru] as well as *lru_pages according to the pages that
> are eligible for the given reclaim index.
> 
> if (global_reclaim(sc))
>   add zone stats from 0 to sc->reclaim_idx
> else
>   use lruvec_lru_size()

Yeb, I already sent it.
http://lkml.kernel.org/r/1469604588-6051-2-git-send-email-minchan@kernel.org

Thanks for the review, Johannes!

> 
> It's a bit unfortunate that abstractions like the lruvec fall apart
> when we have to reconstruct zones ad-hoc now, but I don't see any
> obvious way around it...