linux-kernel - [PATCH v3 00/11] mm: fixup changers of per cpu pageset's ->high and ->batch

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Date:	Wed, 10 Apr 2013 11:23:28 -0700
From:	Cody P Schafer <cody@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Gilad Ben-Yossef <gilad@...yossef.com>,
	Simon Jeons <simon.jeons@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Mel Gorman <mgorman@...e.de>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>
Subject: [PATCH v3 00/11] mm: fixup changers of per cpu pageset's ->high and ->batch

"Problems" with the current code:
 1. there is a lack of synchronization in setting ->high and ->batch in
    percpu_pagelist_fraction_sysctl_handler()
 2. stop_machine() in zone_pcp_update() is unnecissary.
 3. zone_pcp_update() does not consider the case where percpu_pagelist_fraction is non-zero

To fix:
 1. add memory barriers, a safe ->batch value, an update side mutex when
    updating ->high and ->batch, and use ACCESS_ONCE() for ->batch users that
    expect a stable value.
 2. avoid draining pages in zone_pcp_update(), rely upon the memory barriers added to fix #1
 3. factor out quite a few functions, and then call the appropriate one.

Note that it results in a change to the behavior of zone_pcp_update(), which is
used by memory_hotplug. I'm rather certain that I've diserned (and preserved)
the essential behavior (changing ->high and ->batch), and only eliminated
unneeded actions (draining the per cpu pages), but this may not be the case.

Further note that the draining of pages that previously took place in
zone_pcp_update() occured after repeated draining when attempting to offline a
page, and after the offline has "succeeded". It appears that the draining was
added to zone_pcp_update() to avoid refactoring setup_pageset() into 2
funtions.

--

Since v2: https://lkml.org/lkml/2013/4/9/718

 - note ACCESS_ONCE() in fix #1 above.
 - consolidate ->batch & ->high update protocol into a single funtion (Gilad).
 - add missing ACCESS_ONCE() on ->batch

Since v1: https://lkml.org/lkml/2013/4/5/444

 - instead of using on_each_cpu(), use memory barriers (Gilad) and an update side mutex.
 - add "Problem" #3 above, and fix.
 - rename function to match naming style of similar function
 - move unrelated comment

--

Cody P Schafer (11):
  mm/page_alloc: factor out setting of pcp->high and pcp->batch.
  mm/page_alloc: prevent concurrent updaters of pcp ->batch and ->high
  mm/page_alloc: insert memory barriers to allow async update of pcp
    batch and high
  mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE
  mm/page_alloc: convert zone_pcp_update() to rely on memory barriers
    instead of stop_machine()
  mm/page_alloc: when handling percpu_pagelist_fraction, don't unneedly
    recalulate high
  mm/page_alloc: factor setup_pageset() into pageset_init() and
    pageset_set_batch()
  mm/page_alloc: relocate comment to be directly above code it refers
    to.
  mm/page_alloc: factor zone_pageset_init() out of setup_zone_pageset()
  mm/page_alloc: in zone_pcp_update(), uze zone_pageset_init()
  mm/page_alloc: rename setup_pagelist_highmark() to match naming of
    pageset_set_batch()

 mm/page_alloc.c | 151 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 90 insertions(+), 61 deletions(-)

-- 
1.8.2.1

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