Re: [PATCH 01/11] mm: page_alloc: set_migratetype_isolate: drain PCP prior to isolating

From: "Michal Nazarewicz" <mina86@mina86.com>
To: "Gilad Ben-Yossef" <gilad@benyossef.com>
Cc: "Marek Szyprowski" <m.szyprowski@samsung.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org, linux-mm@kvack.org,
	linaro-mm-sig@lists.linaro.org,
	"Kyungmin Park" <kyungmin.park@samsung.com>,
	"Russell King" <linux@arm.linux.org.uk>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
	"Daniel Walker" <dwalker@codeaurora.org>,
	"Mel Gorman" <mel@csn.ul.ie>, "Arnd Bergmann" <arnd@arndb.de>,
	"Jesse Barker" <jesse.barker@linaro.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Shariq Hasnain" <shariq.hasnain@linaro.org>,
	"Chunsang Jeong" <chunsang.jeong@linaro.org>,
	"Dave Hansen" <dave@linux.vnet.ibm.com>,
	"Benjamin Gaignard" <benjamin.gaignard@linaro.org>
Subject: Re: [PATCH 01/11] mm: page_alloc: set_migratetype_isolate: drain PCP prior to isolating
Date: Sun, 01 Jan 2012 19:52:16 +0100	[thread overview]
Message-ID: <op.v7e5demq3l0zgt@mpn-glaptop> (raw)
In-Reply-To: <CAOtvUMfKDiLwxaH5FCS6wC=CgPiDz3ZAPbVv4b=Oxdx4ZpMCYw@mail.gmail.com>

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

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