Re: [PATCH 3/8] mm/vmscan: Throttle reclaim when no progress is being made

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Nov 23, 2021 at 05:19:12PM -0800, Darrick J. Wong wrote:
> On Fri, Oct 22, 2021 at 03:46:46PM +0100, Mel Gorman wrote:
> > Memcg reclaim throttles on congestion if no reclaim progress is made.
> > This makes little sense, it might be due to writeback or a host of
> > other factors.
> > 
> > For !memcg reclaim, it's messy. Direct reclaim primarily is throttled
> > in the page allocator if it is failing to make progress. Kswapd
> > throttles if too many pages are under writeback and marked for
> > immediate reclaim.
> > 
> > This patch explicitly throttles if reclaim is failing to make progress.
> 
> Hi Mel,
> 
> Ever since Christoph broke swapfiles, I've been carrying around a little
> fstest in my dev tree[1] that tries to exercise paging things in and out
> of a swapfile.  Sadly I've been trapped in about three dozen customer
> escalations for over a month, which means I haven't been able to do much
> upstream in weeks.  Like submit this test upstream. :(
> 
> Now that I've finally gotten around to trying out a 5.16-rc2 build, I
> notice that the runtime of this test has gone from ~5s to 2 hours.
> Among other things that it does, the test sets up a cgroup with a memory
> controller limiting the memory usage to 25MB, then runs a program that
> tries to dirty 50MB of memory.  There's 2GB of memory in the VM, so
> we're not running reclaim globally, but the cgroup gets throttled very
> severely.
> 
> AFAICT the system is mostly idle, but it's difficult to tell because ps
> and top also get stuck waiting for this cgroup for whatever reason.  My
> uninformed spculation is that usemem_and_swapoff takes a page fault
> while dirtying the 50MB memory buffer, prepares to pull a page in from
> swap, tries to evict another page to stay under the memcg limit, but
> that decides that it's making no progress and calls
> reclaim_throttle(..., VMSCAN_THROTTLE_NOPROGRESS).
> 
> The sleep is uninterruptible, so I can't even kill -9 fstests to shut it
> down.  Eventually we either finish the test or (for the mlock part) the
> OOM killer actually kills the process, but this takes a very long time.
> 
> Any thoughts?  For now I can just hack around this by skipping
> reclaim_throttle if cgroup_reclaim() == true, but that's probably not
> the correct fix. :)

Update: after adding timing information to usemem_and_swapoff, it looks
like dirtying the 50MB buffer takes ~22s (up from 0.06s on 5.15).  The
mlock call stalls for ~280s until the OOM killer kills it (up from
nearly instantaneous on 5.15), and the swapon/swapoff variant takes
20 minutes to hours depending on the run.

--D

> --D
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfstests-dev.git/commit/?h=test-swapfile-io&id=0d0ad843cea366d0ab0a7d8d984e5cd1deba5b43
> 
> > 
> > [vbabka@suse.cz: Remove redundant code]
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> > Acked-by: Vlastimil Babka <vbabka@suse.cz>
> > ---
> >  include/linux/mmzone.h        |  1 +
> >  include/trace/events/vmscan.h |  4 +++-
> >  mm/memcontrol.c               | 10 +---------
> >  mm/vmscan.c                   | 28 ++++++++++++++++++++++++++++
> >  4 files changed, 33 insertions(+), 10 deletions(-)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 9ccd8d95291b..00e305cfb3ec 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -276,6 +276,7 @@ enum lru_list {
> >  enum vmscan_throttle_state {
> >  	VMSCAN_THROTTLE_WRITEBACK,
> >  	VMSCAN_THROTTLE_ISOLATED,
> > +	VMSCAN_THROTTLE_NOPROGRESS,
> >  	NR_VMSCAN_THROTTLE,
> >  };
> >  
> > diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
> > index d4905bd9e9c4..f25a6149d3ba 100644
> > --- a/include/trace/events/vmscan.h
> > +++ b/include/trace/events/vmscan.h
> > @@ -29,11 +29,13 @@
> >  
> >  #define _VMSCAN_THROTTLE_WRITEBACK	(1 << VMSCAN_THROTTLE_WRITEBACK)
> >  #define _VMSCAN_THROTTLE_ISOLATED	(1 << VMSCAN_THROTTLE_ISOLATED)
> > +#define _VMSCAN_THROTTLE_NOPROGRESS	(1 << VMSCAN_THROTTLE_NOPROGRESS)
> >  
> >  #define show_throttle_flags(flags)						\
> >  	(flags) ? __print_flags(flags, "|",					\
> >  		{_VMSCAN_THROTTLE_WRITEBACK,	"VMSCAN_THROTTLE_WRITEBACK"},	\
> > -		{_VMSCAN_THROTTLE_ISOLATED,	"VMSCAN_THROTTLE_ISOLATED"}	\
> > +		{_VMSCAN_THROTTLE_ISOLATED,	"VMSCAN_THROTTLE_ISOLATED"},	\
> > +		{_VMSCAN_THROTTLE_NOPROGRESS,	"VMSCAN_THROTTLE_NOPROGRESS"}	\
> >  		) : "VMSCAN_THROTTLE_NONE"
> >  
> >  
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 6da5020a8656..8b33152c9b85 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3465,19 +3465,11 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
> >  
> >  	/* try to free all pages in this cgroup */
> >  	while (nr_retries && page_counter_read(&memcg->memory)) {
> > -		int progress;
> > -
> >  		if (signal_pending(current))
> >  			return -EINTR;
> >  
> > -		progress = try_to_free_mem_cgroup_pages(memcg, 1,
> > -							GFP_KERNEL, true);
> > -		if (!progress) {
> > +		if (!try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, true))
> >  			nr_retries--;
> > -			/* maybe some writeback is necessary */
> > -			congestion_wait(BLK_RW_ASYNC, HZ/10);
> > -		}
> > -
> >  	}
> >  
> >  	return 0;
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 1e54e636b927..0450f6867d61 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -3323,6 +3323,33 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
> >  	return zone_watermark_ok_safe(zone, 0, watermark, sc->reclaim_idx);
> >  }
> >  
> > +static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
> > +{
> > +	/* If reclaim is making progress, wake any throttled tasks. */
> > +	if (sc->nr_reclaimed) {
> > +		wait_queue_head_t *wqh;
> > +
> > +		wqh = &pgdat->reclaim_wait[VMSCAN_THROTTLE_NOPROGRESS];
> > +		if (waitqueue_active(wqh))
> > +			wake_up(wqh);
> > +
> > +		return;
> > +	}
> > +
> > +	/*
> > +	 * Do not throttle kswapd on NOPROGRESS as it will throttle on
> > +	 * VMSCAN_THROTTLE_WRITEBACK if there are too many pages under
> > +	 * writeback and marked for immediate reclaim at the tail of
> > +	 * the LRU.
> > +	 */
> > +	if (current_is_kswapd())
> > +		return;
> > +
> > +	/* Throttle if making no progress at high prioities. */
> > +	if (sc->priority < DEF_PRIORITY - 2)
> > +		reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS, HZ/10);
> > +}
> > +
> >  /*
> >   * This is the direct reclaim path, for page-allocating processes.  We only
> >   * try to reclaim pages from zones which will satisfy the caller's allocation
> > @@ -3407,6 +3434,7 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> >  			continue;
> >  		last_pgdat = zone->zone_pgdat;
> >  		shrink_node(zone->zone_pgdat, sc);
> > +		consider_reclaim_throttle(zone->zone_pgdat, sc);
> >  	}
> >  
> >  	/*
> > -- 
> > 2.31.1
> >