Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1758045Ab2KWIue (ORCPT <rfc822;w@1wt.eu>);
	Fri, 23 Nov 2012 03:50:34 -0500
Received: from mail-ie0-f174.google.com ([209.85.223.174]:33886 "EHLO
	mail-ie0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754007Ab2KWIud (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 23 Nov 2012 03:50:33 -0500
Message-ID: <50AF38D2.6090106@gmail.com>
Date: Fri, 23 Nov 2012 16:50:26 +0800
From: Jaegeuk Hanse <jaegeuk.hanse@gmail.com>
User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/17.0 Thunderbird/17.0
MIME-Version: 1.0
To: Johannes Weiner <hannes@cmpxchg.org>
CC: Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: kswapd endless loop for compaction
References: <20121120190440.GA24381@cmpxchg.org>
In-Reply-To: <20121120190440.GA24381@cmpxchg.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2112
Lines: 57

On 11/21/2012 03:04 AM, Johannes Weiner wrote:
> Hi guys,
>
> while testing a 3.7-rc5ish kernel, I noticed that kswapd can drop into
> a busy spin state without doing reclaim.  printk-style debugging told
> me that this happens when the distance between a zone's high watermark
> and its low watermark is less than two huge pages (DMA zone).
>
> 1. The first loop in balance_pgdat() over the zones finds all zones to
> be above their high watermark and only does goto out (all_zones_ok).
>
> 2. pgdat_balanced() at the out: label also just checks the high
> watermark, so the node is considered balanced and the order is not
> reduced.
>
> 3. In the `if (order)' block after it, compaction_suitable() checks if
> the zone's low watermark + twice the huge page size is okay, which
> it's not necessarily in a small zone, and so COMPACT_SKIPPED makes it
> it go back to loop_again:.
>
> This will go on until somebody else allocates and breaches the high
> watermark and then hopefully goes on to reclaim the zone above low
> watermark + 2 * THP.
>
> I'm not really sure what the correct solution is.  Should we modify
> the zone_watermark_ok() checks in balance_pgdat() to take into account
> the higher watermark requirements for reclaim on behalf of compaction?
> Change the check in compaction_suitable() / not use it directly?

Hi Johannes,

If depend on compaction get enough contigous pages, why

if (CONPACT_BUILD && order &&
     compaction_suitable(zone, order) !=
         COMPACTION_SKIPPED)
     testorder = 0;

can't guarantee low watermark + twice the huge page size is okay?

Regards,
Jaegeuk

> Thanks,
> Johannes
>
> --
> 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/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/