linux-kernel - Re: Interactivity regression since v3.11 in mm/vmscan.c

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Message-ID: <20140605133747.GB2942@dhcp22.suse.cz>
Date:	Thu, 5 Jun 2014 15:37:47 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Felipe Contreras <felipe.contreras@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: Interactivity regression since v3.11 in mm/vmscan.c

On Thu 05-06-14 06:33:40, Felipe Contreras wrote:
> Hi,

Hi,
 
> For a while I've noticed that my machine bogs down in certain
> situations, usually while doing heavy I/O operations, it is not just the
> I/O operations, but everything, including the graphical interface, even
> the mouse pointer.
> 
> As far as I can recall this did not happen in the past.
> 
> I noticed this specially on certain operations, for example updating a
> a game on Steam (to an exteranl USB 3.0 device), or copying TV episodes
> to a USB memory stick (probably flash-based).

We had a similar report for opensuse. The common part was that there was
an IO to a slow USB device going on.
 
> Today I decided to finally hunt down the problem, so I created a
> synthetic test that basically consists on copying a bunch of files from
> one drive to another (from an SSD to an external USB 3.0). This is
> pretty similar to what I noticed; the graphical interface slows down.
> 
> Then I bisected the issue and it turns out that indeed it wasn't
> happening in the past, it started happening in v3.11, and it was
> triggered by this commit:
> 
>   e2be15f (mm: vmscan: stall page reclaim and writeback pages based on
>   dirty/writepage pages encountered)
> 
> Then I went back to the latest stable version (v3.14.5), and commented
> out the line I think is causing the slow down:
> 
>   if (nr_unqueued_dirty == nr_taken || nr_immediate)
> 	  congestion_wait(BLK_RW_ASYNC, HZ/10);

Yes, I came to the same check. I didn't have any confirmation yet so
thanks for your confirmation. I've suggested to reduce this
congestion_wait only to kswapd:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 32c661d66a45..ef6a1c0e788c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1566,7 +1566,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
 		 * implies that pages are cycling through the LRU faster than
 		 * they are written so also forcibly stall.
 		 */
-		if (nr_unqueued_dirty == nr_taken || nr_immediate)
+		if ((nr_unqueued_dirty == nr_taken || nr_immediate) && current_is_kswapd())
 			congestion_wait(BLK_RW_ASYNC, HZ/10);
 	}

But I am still not sure whether this is the right way to fix it. Direct
reclaimers can be throttled later on wait_iff_congested. I guess the
original intention was to throttle kswapd to not scan LRU full of dirty
pages like crazy. So I think it makes some sense to reduce the
congestion_wait only to kswapd.

 
> After that I don't notice the slow down any more.
> 
> Anybody has any ideas how to fix the issue properly?
> 
> -- 
> Felipe Contreras
> 
> --
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-- 
Michal Hocko
SUSE Labs
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