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SubjectRe: [PATCH 01/11] mm: page_alloc: set_migratetype_isolate: drain PCP prior to isolating
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On Sun, 01 Jan 2012 17:06:53 +0100, Gilad Ben-Yossef <gilad@benyossef.com> wrote:

> 2012/1/1 Michal Nazarewicz <mina86@mina86.com>:
>> Looks interesting, I'm not entirely sure why it does not end up a race
>> condition, but in case of __zone_drain_all_pages() we already hold

> If a page is in the PCP list when we check, you'll send the IPI and all is well.
>
> If it isn't when we check and gets added later you could just the same have
> situation where we send the IPI, try to do try an empty PCP list and then
> the page gets added. So we are not adding a race condition that is not there
> already :-)

Right, makes sense.

>> zone->lock, so my fears are somehow gone.. I'll give it a try, and prepare
>> a patch for __zone_drain_all_pages().

> I plan to send V5 of the IPI noise patch after some testing. It has a new
> version of the drain_all_pages, with no allocation in the reclaim path
> and no locking. You might want to wait till that one is out to base on it.

This shouldn't be a problem for my case as set_migratetype_isolate() is hardly
ever called in reclaim path. :)

The change so far seems rather obvious:

mm/page_alloc.c | 14 +++++++++++++-
1 files changed, 13 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 424d36a..eaa686b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1181,7 +1181,19 @@ static void __zone_drain_local_pages(void *arg)
*/
static void __zone_drain_all_pages(struct zone *zone)
{
- on_each_cpu(__zone_drain_local_pages, zone, 1);
+ struct per_cpu_pageset *pcp;
+ cpumask_var_t cpus;
+ int cpu;
+
+ if (likely(zalloc_cpumask_var(&cpus, GFP_ATOMIC | __GFP_NOWARN))) {
+ for_each_online_cpu(cpu)
+ if (per_cpu_ptr(zone->pageset, cpu)->pcp.count)
+ cpumask_set_cpu(cpu, cpus);
+ on_each_cpu_mask(cpus, __zone_drain_local_pages, zone, 1);
+ free_cpumask_var(cpus);
+ } else {
+ on_each_cpu(__zone_drain_local_pages, zone, 1);
+ }
}

#ifdef CONFIG_HIBERNATION
--
Best regards, _ _
.o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michał “mina86” Nazarewicz (o o)
ooo +----<email/xmpp: mpn@google.com>--------------ooO--(_)--Ooo--
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