linux-kernel - Re: [PATCH] Revert "mm: remove __GFP_NO

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Date:	Fri, 16 Nov 2012 14:14:47 -0500
From:	Josh Boyer <jwboyer@...il.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Zdenek Kabelac <zkabelac@...hat.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Jiri Slaby <jslaby@...e.cz>, Valdis.Kletnieks@...edu,
	Jiri Slaby <jirislaby@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>
Subject: Re: [PATCH] Revert "mm: remove __GFP_NO_KSWAPD"

On Mon, Nov 12, 2012 at 6:37 AM, Mel Gorman <mgorman@...e.de> wrote:
> With "mm: vmscan: scale number of pages reclaimed by reclaim/compaction
> based on failures" reverted, Zdenek Kabelac reported the following
>
>         Hmm,  so it's just took longer to hit the problem and observe
>         kswapd0 spinning on my CPU again - it's not as endless like before -
>         but still it easily eats minutes - it helps to  turn off  Firefox
>         or TB  (memory hungry apps) so kswapd0 stops soon - and restart
>         those apps again.  (And I still have like >1GB of cached memory)
>
>         kswapd0         R  running task        0    30      2 0x00000000
>          ffff8801331efae8 0000000000000082 0000000000000018 0000000000000246
>          ffff880135b9a340 ffff8801331effd8 ffff8801331effd8 ffff8801331effd8
>          ffff880055dfa340 ffff880135b9a340 00000000331efad8 ffff8801331ee000
>         Call Trace:
>          [<ffffffff81555bf2>] preempt_schedule+0x42/0x60
>          [<ffffffff81557a95>] _raw_spin_unlock+0x55/0x60
>          [<ffffffff81192971>] put_super+0x31/0x40
>          [<ffffffff81192a42>] drop_super+0x22/0x30
>          [<ffffffff81193b89>] prune_super+0x149/0x1b0
>          [<ffffffff81141e2a>] shrink_slab+0xba/0x510
>
> The sysrq+m indicates the system has no swap so it'll never reclaim
> anonymous pages as part of reclaim/compaction. That is one part of the
> problem but not the root cause as file-backed pages could also be reclaimed.
>
> The likely underlying problem is that kswapd is woken up or kept awake
> for each THP allocation request in the page allocator slow path.
>
> If compaction fails for the requesting process then compaction will be
> deferred for a time and direct reclaim is avoided. However, if there
> are a storm of THP requests that are simply rejected, it will still
> be the the case that kswapd is awake for a prolonged period of time
> as pgdat->kswapd_max_order is updated each time. This is noticed by
> the main kswapd() loop and it will not call kswapd_try_to_sleep().
> Instead it will loopp, shrinking a small number of pages and calling
> shrink_slab() on each iteration.
>
> The temptation is to supply a patch that checks if kswapd was woken for
> THP and if so ignore pgdat->kswapd_max_order but it'll be a hack and not
> backed up by proper testing. As 3.7 is very close to release and this is
> not a bug we should release with, a safer path is to revert "mm: remove
> __GFP_NO_KSWAPD" for now and revisit it with the view to ironing out the
> balance_pgdat() logic in general.
>
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Does anyone know if this is queued to go into 3.7 somewhere?  I looked
a bit and can't find it in a tree.  We have a few reports of Fedora
rawhide users hitting this.

josh
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