From: Cody P Schafer <cody@linux.vnet.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>,
        Simon Jeons <simon.jeons@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        Mel Gorman <mgorman@suse.de>, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Cody P Schafer <cody@linux.vnet.ibm.com>
Subject: [PATCH v2 00/10] mm: fixup changers of per cpu pageset's ->high and ->batch
Date: Tue,  9 Apr 2013 16:28:09 -0700
Message-Id: <1365550099-6795-1-git-send-email-cody@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2651
Lines: 58

"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, and an update side mutex when updating ->high and ->batch
 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.

--
Changes since v1:

 - 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 (10):
  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: 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 | 124 +++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 73 insertions(+), 51 deletions(-)

-- 
1.8.2

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