Date:	Thu, 19 Aug 2010 12:31:39 +0200
From:	Nikola Ciprich <extmaillist@...uxbox.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jiri Slaby <jslaby@...e.cz>,
	"stable@...nel.org" <stable@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pedro Ribeiro <pedrib@...il.com>,
	Mel Gorman <mel@...ux.vnet.ibm.com>, nikola.ciprich@...uxbox.cz
Subject: Re: "vmscan: raise the bar to PAGEOUT_IO_SYNC stalls" to stable?

Hello everybody,
is there any chance this one could also make it to long term
supported 2.6.32 series?
I guess this will take more work, but I guess there are quite
a few users that would appreciate it much :)
thanks!
nik

On Thu, Aug 19, 2010 at 02:05:16PM +0800, Wu Fengguang wrote:
> Hi Jiri,
> 
> On Thu, Aug 12, 2010 at 03:11:03PM +0800, Jiri Slaby wrote:
> > Hi Wu,
> > 
> > maybe you've already sent a backported version of e31f3698cd34 for
> > 2.6.34 stable. If you haven't yet, I'm attaching my version in case you
> > don't want to duplicate work. There is a change where lumpy_reclaim is
> > passed as a parameter, since struct scan_control doesn't contain that
> > yet in 2.6.34.
> 
> This patch for -stable looks good, thank you!
> 
> Greg, this patch has received pretty positive feedbacks from some users.
> (others feel no changes: there are more sources of responsiveness stalls)
> KOSAKI and me think it's important and safe enough for -stable kernels.
> The patch looks large, however it's mainly cleanups. The real change
> is merely about raising (DEF_PRIORITY-2) to (DEF_PRIORITY/3) in the
> test condition.
> 
> Thanks,
> Fengguang
> 
> > From e31f3698cd3499e676f6b0ea12e3528f569c4fa3 Mon Sep 17 00:00:00 2001
> > From: Wu Fengguang <fengguang.wu@...el.com>
> > Date: Mon, 9 Aug 2010 17:20:01 -0700
> > Subject: vmscan: raise the bar to PAGEOUT_IO_SYNC stalls
> > 
> > Fix "system goes unresponsive under memory pressure and lots of
> > dirty/writeback pages" bug.
> > 
> > 	http://lkml.org/lkml/2010/4/4/86
> > 
> > In the above thread, Andreas Mohr described that
> > 
> > 	Invoking any command locked up for minutes (note that I'm
> > 	talking about attempted additional I/O to the _other_,
> > 	_unaffected_ main system HDD - such as loading some shell
> > 	binaries -, NOT the external SSD18M!!).
> > 
> > This happens when the two conditions are both meet:
> > - under memory pressure
> > - writing heavily to a slow device
> > 
> > OOM also happens in Andreas' system.  The OOM trace shows that 3 processes
> > are stuck in wait_on_page_writeback() in the direct reclaim path.  One in
> > do_fork() and the other two in unix_stream_sendmsg().  They are blocked on
> > this condition:
> > 
> > 	(sc->order && priority < DEF_PRIORITY - 2)
> > 
> > which was introduced in commit 78dc583d (vmscan: low order lumpy reclaim
> > also should use PAGEOUT_IO_SYNC) one year ago.  That condition may be too
> > permissive.  In Andreas' case, 512MB/1024 = 512KB.  If the direct reclaim
> > for the order-1 fork() allocation runs into a range of 512KB
> > hard-to-reclaim LRU pages, it will be stalled.
> > 
> > It's a severe problem in three ways.
> > 
> > Firstly, it can easily happen in daily desktop usage.  vmscan priority can
> > easily go below (DEF_PRIORITY - 2) on _local_ memory pressure.  Even if
> > the system has 50% globally reclaimable pages, it still has good
> > opportunity to have 0.1% sized hard-to-reclaim ranges.  For example, a
> > simple dd can easily create a big range (up to 20%) of dirty pages in the
> > LRU lists.  And order-1 to order-3 allocations are more than common with
> > SLUB.  Try "grep -v '1 :' /proc/slabinfo" to get the list of high order
> > slab caches.  For example, the order-1 radix_tree_node slab cache may
> > stall applications at swap-in time; the order-3 inode cache on most
> > filesystems may stall applications when trying to read some file; the
> > order-2 proc_inode_cache may stall applications when trying to open a
> > /proc file.
> > 
> > Secondly, once triggered, it will stall unrelated processes (not doing IO
> > at all) in the system.  This "one slow USB device stalls the whole system"
> > avalanching effect is very bad.
> > 
> > Thirdly, once stalled, the stall time could be intolerable long for the
> > users.  When there are 20MB queued writeback pages and USB 1.1 is writing
> > them in 1MB/s, wait_on_page_writeback() will stuck for up to 20 seconds.
> > Not to mention it may be called multiple times.
> > 
> > So raise the bar to only enable PAGEOUT_IO_SYNC when priority goes below
> > DEF_PRIORITY/3, or 6.25% LRU size.  As the default dirty throttle ratio is
> > 20%, it will hardly be triggered by pure dirty pages.  We'd better treat
> > PAGEOUT_IO_SYNC as some last resort workaround -- its stall time is so
> > uncomfortably long (easily goes beyond 1s).
> > 
> > The bar is only raised for (order < PAGE_ALLOC_COSTLY_ORDER) allocations,
> > which are easy to satisfy in 1TB memory boxes.  So, although 6.25% of
> > memory could be an awful lot of pages to scan on a system with 1TB of
> > memory, it won't really have to busy scan that much.
> > 
> > Andreas tested an older version of this patch and reported that it mostly
> > fixed his problem.  Mel Gorman helped improve it and KOSAKI Motohiro will
> > fix it further in the next patch.
> > 
> > Reported-by: Andreas Mohr <andi@...as.de>
> > Reviewed-by: Minchan Kim <minchan.kim@...il.com>
> > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > Cc: Rik van Riel <riel@...hat.com>
> > Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> > ---
> >  mm/vmscan.c |   53 +++++++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 45 insertions(+), 8 deletions(-)
> > 
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1118,6 +1118,48 @@ static int too_many_isolated(struct zone
> >  }
> >  
> >  /*
> > + * Returns true if the caller should wait to clean dirty/writeback pages.
> > + *
> > + * If we are direct reclaiming for contiguous pages and we do not reclaim
> > + * everything in the list, try again and wait for writeback IO to complete.
> > + * This will stall high-order allocations noticeably. Only do that when really
> > + * need to free the pages under high memory pressure.
> > + */
> > +static inline bool should_reclaim_stall(unsigned long nr_taken,
> > +					unsigned long nr_freed,
> > +					int priority,
> > +					int lumpy_reclaim,
> > +					struct scan_control *sc)
> > +{
> > +	int lumpy_stall_priority;
> > +
> > +	/* kswapd should not stall on sync IO */
> > +	if (current_is_kswapd())
> > +		return false;
> > +
> > +	/* Only stall on lumpy reclaim */
> > +	if (!lumpy_reclaim)
> > +		return false;
> > +
> > +	/* If we have relaimed everything on the isolated list, no stall */
> > +	if (nr_freed == nr_taken)
> > +		return false;
> > +
> > +	/*
> > +	 * For high-order allocations, there are two stall thresholds.
> > +	 * High-cost allocations stall immediately where as lower
> > +	 * order allocations such as stacks require the scanning
> > +	 * priority to be much higher before stalling.
> > +	 */
> > +	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
> > +		lumpy_stall_priority = DEF_PRIORITY;
> > +	else
> > +		lumpy_stall_priority = DEF_PRIORITY / 3;
> > +
> > +	return priority <= lumpy_stall_priority;
> > +}
> > +
> > +/*
> >   * shrink_inactive_list() is a helper for shrink_zone().  It returns the number
> >   * of reclaimed pages
> >   */
> > @@ -1209,14 +1251,9 @@ static unsigned long shrink_inactive_lis
> >  		nr_scanned += nr_scan;
> >  		nr_freed = shrink_page_list(&page_list, sc, PAGEOUT_IO_ASYNC);
> >  
> > -		/*
> > -		 * If we are direct reclaiming for contiguous pages and we do
> > -		 * not reclaim everything in the list, try again and wait
> > -		 * for IO to complete. This will stall high-order allocations
> > -		 * but that should be acceptable to the caller
> > -		 */
> > -		if (nr_freed < nr_taken && !current_is_kswapd() &&
> > -		    lumpy_reclaim) {
> > +		/* Check if we should syncronously wait for writeback */
> > +		if (should_reclaim_stall(nr_taken, nr_freed, priority,
> > +					lumpy_reclaim, sc)) {
> >  			congestion_wait(BLK_RW_ASYNC, HZ/10);
> >  
> >  			/*
> 
> --
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