All of lore.kernel.org
 help / color / mirror / Atom feed
From: Mel Gorman <mgorman@suse.de>
To: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>, Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Minchan Kim <minchan.kim@gmail.com>,
	Dave Jones <davej@redhat.com>, Jan Kara <jack@suse.cz>,
	Andy Isaacson <adi@hexapodia.org>,
	Johannes Weiner <jweiner@redhat.com>, Nai Xia <nai.xia@gmail.com>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 11/11] mm: Isolate pages for immediate reclaim on their own LRU
Date: Fri, 30 Dec 2011 11:27:23 +0000	[thread overview]
Message-ID: <20111230112723.GD15729@suse.de> (raw)
In-Reply-To: <4EFCC008.30803@redhat.com>

On Thu, Dec 29, 2011 at 02:31:20PM -0500, Rik van Riel wrote:
> On 12/29/2011 11:59 AM, Mel Gorman wrote:
> 
> >I considered a few ways of fixing this. The obvious one is to add a
> >new page flag but that is difficult to justify as the high-cpu-usage
> >problem should only occur when there is a lot of writeback to slow
> >storage which I believe is a rare case. It is not a suitable use for
> >an extended page flag.
> 
> Actually, don't we already have three LRU related
> bits in the page flags?
> 

Yes - PG_active, PG_unevictable and PG_swapbacked

> We could stop using those as bit flags, and use
> them as a number instead. That way we could encode
> up to 7 or 8 (depending on how we use all-zeroes)
> LRU lists with the number of bits we have now.
> 

I wondered about this and I felt there were two problems.

One was reading and updating them atomically.  To do this safely,
the page would either need to be locked, have the page isolated from
the LRU without any other references or be protected by the zone->lru
lock. For the most part we are accessing these bits under the page lock
and in cases such as rotate_reclaimable_page()[1] or truncation that do
not necessarily hold the page lock, we would depend on the zone->lru to
prevent parallel changes (particularly updating PageActive).  I did not
spot a case where we were not protected by some combination of the page
lock and zone->lru so it should be fine but there might be a corner
case I missed. Can you think of one? If a case is missed, it means
that it is possible to get an invalid LRU index leading to corruption.

The other problem is that certain operations become more expensive. We
can no longer check one bit for PageActive for example. We'd have
to read the LRU index and see if it corresponds to an activated
page or not. This is not insurmountable but there would be a small
hit for any path that currently checks PageSwapBacked, PageActive
or PageUnevictable.

[1] I noticed another bug in the LRU immediate patch. It's possible
to call pagevec_putback_from_immediate on a page isolated for reclaim
because the check for PageLRU is wrong.

-- 
Mel Gorman
SUSE Labs

WARNING: multiple messages have this Message-ID (diff)
From: Mel Gorman <mgorman@suse.de>
To: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>, Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Minchan Kim <minchan.kim@gmail.com>,
	Dave Jones <davej@redhat.com>, Jan Kara <jack@suse.cz>,
	Andy Isaacson <adi@hexapodia.org>,
	Johannes Weiner <jweiner@redhat.com>, Nai Xia <nai.xia@gmail.com>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 11/11] mm: Isolate pages for immediate reclaim on their own LRU
Date: Fri, 30 Dec 2011 11:27:23 +0000	[thread overview]
Message-ID: <20111230112723.GD15729@suse.de> (raw)
In-Reply-To: <4EFCC008.30803@redhat.com>

On Thu, Dec 29, 2011 at 02:31:20PM -0500, Rik van Riel wrote:
> On 12/29/2011 11:59 AM, Mel Gorman wrote:
> 
> >I considered a few ways of fixing this. The obvious one is to add a
> >new page flag but that is difficult to justify as the high-cpu-usage
> >problem should only occur when there is a lot of writeback to slow
> >storage which I believe is a rare case. It is not a suitable use for
> >an extended page flag.
> 
> Actually, don't we already have three LRU related
> bits in the page flags?
> 

Yes - PG_active, PG_unevictable and PG_swapbacked

> We could stop using those as bit flags, and use
> them as a number instead. That way we could encode
> up to 7 or 8 (depending on how we use all-zeroes)
> LRU lists with the number of bits we have now.
> 

I wondered about this and I felt there were two problems.

One was reading and updating them atomically.  To do this safely,
the page would either need to be locked, have the page isolated from
the LRU without any other references or be protected by the zone->lru
lock. For the most part we are accessing these bits under the page lock
and in cases such as rotate_reclaimable_page()[1] or truncation that do
not necessarily hold the page lock, we would depend on the zone->lru to
prevent parallel changes (particularly updating PageActive).  I did not
spot a case where we were not protected by some combination of the page
lock and zone->lru so it should be fine but there might be a corner
case I missed. Can you think of one? If a case is missed, it means
that it is possible to get an invalid LRU index leading to corruption.

The other problem is that certain operations become more expensive. We
can no longer check one bit for PageActive for example. We'd have
to read the LRU index and see if it corresponds to an activated
page or not. This is not insurmountable but there would be a small
hit for any path that currently checks PageSwapBacked, PageActive
or PageUnevictable.

[1] I noticed another bug in the LRU immediate patch. It's possible
to call pagevec_putback_from_immediate on a page isolated for reclaim
because the check for PageLRU is wrong.

-- 
Mel Gorman
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

  reply	other threads:[~2011-12-30 11:27 UTC|newest]

Thread overview: 100+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-12-14 15:41 [PATCH 0/11] Reduce compaction-related stalls and improve asynchronous migration of dirty pages v6 Mel Gorman
2011-12-14 15:41 ` Mel Gorman
2011-12-14 15:41 ` [PATCH 01/11] mm: compaction: Allow compaction to isolate dirty pages Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-14 15:41 ` [PATCH 02/11] mm: compaction: Use synchronous compaction for /proc/sys/vm/compact_memory Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-14 15:41 ` [PATCH 03/11] mm: vmscan: Check if we isolated a compound page during lumpy scan Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-15 23:21   ` Rik van Riel
2011-12-15 23:21     ` Rik van Riel
2011-12-14 15:41 ` [PATCH 04/11] mm: vmscan: Do not OOM if aborting reclaim to start compaction Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-15 23:36   ` Rik van Riel
2011-12-15 23:36     ` Rik van Riel
2011-12-14 15:41 ` [PATCH 05/11] mm: compaction: Determine if dirty pages can be migrated without blocking within ->migratepage Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  3:32   ` Rik van Riel
2011-12-16  3:32     ` Rik van Riel
2011-12-16 23:20   ` Andrew Morton
2011-12-16 23:20     ` Andrew Morton
2011-12-17  3:03     ` Nai Xia
2011-12-17  3:03       ` Nai Xia
2011-12-17  3:26       ` Andrew Morton
2011-12-17  3:26         ` Andrew Morton
2011-12-19 11:05     ` Mel Gorman
2011-12-19 11:05       ` Mel Gorman
2011-12-19 13:12       ` nai.xia
2011-12-19 13:12         ` nai.xia
2011-12-14 15:41 ` [PATCH 06/11] mm: compaction: make isolate_lru_page() filter-aware again Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  3:34   ` Rik van Riel
2011-12-16  3:34     ` Rik van Riel
2011-12-18  1:53   ` Minchan Kim
2011-12-18  1:53     ` Minchan Kim
2011-12-14 15:41 ` [PATCH 07/11] mm: page allocator: Do not call direct reclaim for THP allocations while compaction is deferred Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  4:10   ` Rik van Riel
2011-12-16  4:10     ` Rik van Riel
2011-12-14 15:41 ` [PATCH 08/11] mm: compaction: Introduce sync-light migration for use by compaction Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  4:31   ` Rik van Riel
2011-12-16  4:31     ` Rik van Riel
2011-12-18  2:05   ` Minchan Kim
2011-12-18  2:05     ` Minchan Kim
2011-12-19 11:45     ` Mel Gorman
2011-12-19 11:45       ` Mel Gorman
2011-12-20  7:18       ` Minchan Kim
2011-12-20  7:18         ` Minchan Kim
2012-01-13 21:25   ` Andrew Morton
2012-01-13 21:25     ` Andrew Morton
2012-01-16 11:33     ` Mel Gorman
2012-01-16 11:33       ` Mel Gorman
2011-12-14 15:41 ` [PATCH 09/11] mm: vmscan: When reclaiming for compaction, ensure there are sufficient free pages available Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  4:35   ` Rik van Riel
2011-12-16  4:35     ` Rik van Riel
2011-12-14 15:41 ` [PATCH 10/11] mm: vmscan: Check if reclaim should really abort even if compaction_ready() is true for one zone Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  4:38   ` Rik van Riel
2011-12-16  4:38     ` Rik van Riel
2011-12-16 11:29     ` Mel Gorman
2011-12-16 11:29       ` Mel Gorman
2011-12-14 15:41 ` [PATCH 11/11] mm: Isolate pages for immediate reclaim on their own LRU Mel Gorman
2011-12-14 15:41   ` Mel Gorman
2011-12-16  4:47   ` Rik van Riel
2011-12-16  4:47     ` Rik van Riel
2011-12-16 12:26     ` Mel Gorman
2011-12-16 12:26       ` Mel Gorman
2011-12-16 15:17   ` Johannes Weiner
2011-12-16 15:17     ` Johannes Weiner
2011-12-16 16:07     ` Mel Gorman
2011-12-16 16:07       ` Mel Gorman
2011-12-19 16:14       ` Johannes Weiner
2011-12-19 16:14         ` Johannes Weiner
2011-12-17 16:08   ` Minchan Kim
2011-12-17 16:08     ` Minchan Kim
2011-12-19 13:26     ` Mel Gorman
2011-12-19 13:26       ` Mel Gorman
2011-12-20  7:10       ` Minchan Kim
2011-12-20  7:10         ` Minchan Kim
2011-12-20  9:55         ` Mel Gorman
2011-12-20  9:55           ` Mel Gorman
2011-12-23 19:08           ` Hugh Dickins
2011-12-23 19:08             ` Hugh Dickins
2011-12-29 16:59             ` Mel Gorman
2011-12-29 16:59               ` Mel Gorman
2011-12-29 19:31               ` Rik van Riel
2011-12-29 19:31                 ` Rik van Riel
2011-12-30 11:27                 ` Mel Gorman [this message]
2011-12-30 11:27                   ` Mel Gorman
2011-12-16 22:56 ` [PATCH 0/11] Reduce compaction-related stalls and improve asynchronous migration of dirty pages v6 Andrew Morton
2011-12-16 22:56   ` Andrew Morton
2011-12-19 14:40   ` Mel Gorman
2011-12-19 14:40     ` Mel Gorman
2011-12-16 23:37 ` Andrew Morton
2011-12-16 23:37   ` Andrew Morton
2011-12-19 14:20   ` Mel Gorman
2011-12-19 14:20     ` Mel Gorman
  -- strict thread matches above, loose matches on Subject: below --
2011-12-01 17:36 [PATCH 0/11] Reduce compaction-related stalls and improve asynchronous migration of dirty pages v5 Mel Gorman
2011-12-01 17:36 ` [PATCH 11/11] mm: Isolate pages for immediate reclaim on their own LRU Mel Gorman
2011-12-01 17:36   ` Mel Gorman

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20111230112723.GD15729@suse.de \
    --to=mgorman@suse.de \
    --cc=aarcange@redhat.com \
    --cc=adi@hexapodia.org \
    --cc=akpm@linux-foundation.org \
    --cc=davej@redhat.com \
    --cc=hughd@google.com \
    --cc=jack@suse.cz \
    --cc=jweiner@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=minchan.kim@gmail.com \
    --cc=minchan@kernel.org \
    --cc=nai.xia@gmail.com \
    --cc=riel@redhat.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.