Re: [PATCH 1/9] mm: fix 100% CPU kswapd busyloop on unreclaimable nodes

[Michal Hocko]

On Fri 03-03-17 10:26:09, Minchan Kim wrote:
> Hi Johannes,
> 
> On Tue, Feb 28, 2017 at 04:39:59PM -0500, Johannes Weiner wrote:
> > Jia He reports a problem with kswapd spinning at 100% CPU when
> > requesting more hugepages than memory available in the system:
> > 
> > $ echo 4000 >/proc/sys/vm/nr_hugepages
> > 
> > top - 13:42:59 up  3:37,  1 user,  load average: 1.09, 1.03, 1.01
> > Tasks:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
> > %Cpu(s):  0.0 us, 12.5 sy,  0.0 ni, 85.5 id,  2.0 wa,  0.0 hi,  0.0 si,  0.0 st
> > KiB Mem:  31371520 total, 30915136 used,   456384 free,      320 buffers
> > KiB Swap:  6284224 total,   115712 used,  6168512 free.    48192 cached Mem
> > 
> >   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
> >    76 root      20   0       0      0      0 R 100.0 0.000 217:17.29 kswapd3
> > 
> > At that time, there are no reclaimable pages left in the node, but as
> > kswapd fails to restore the high watermarks it refuses to go to sleep.
> > 
> > Kswapd needs to back away from nodes that fail to balance. Up until
> > 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes")
> > kswapd had such a mechanism. It considered zones whose theoretically
> > reclaimable pages it had reclaimed six times over as unreclaimable and
> > backed away from them. This guard was erroneously removed as the patch
> > changed the definition of a balanced node.
> > 
> > However, simply restoring this code wouldn't help in the case reported
> > here: there *are* no reclaimable pages that could be scanned until the
> > threshold is met. Kswapd would stay awake anyway.
> > 
> > Introduce a new and much simpler way of backing off. If kswapd runs
> > through MAX_RECLAIM_RETRIES (16) cycles without reclaiming a single
> > page, make it back off from the node. This is the same number of shots
> > direct reclaim takes before declaring OOM. Kswapd will go to sleep on
> > that node until a direct reclaimer manages to reclaim some pages, thus
> > proving the node reclaimable again.
> > 
> > v2: move MAX_RECLAIM_RETRIES to mm/internal.h (Michal)
> > 
> > Reported-by: Jia He <hejianet@xxxxxxxxx>
> > Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Tested-by: Jia He <hejianet@xxxxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> >  include/linux/mmzone.h |  2 ++
> >  mm/internal.h          |  6 ++++++
> >  mm/page_alloc.c        |  9 ++-------
> >  mm/vmscan.c            | 27 ++++++++++++++++++++-------
> >  mm/vmstat.c            |  2 +-
> >  5 files changed, 31 insertions(+), 15 deletions(-)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 8e02b3750fe0..d2c50ab6ae40 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -630,6 +630,8 @@ typedef struct pglist_data {
> >  	int kswapd_order;
> >  	enum zone_type kswapd_classzone_idx;
> >  
> > +	int kswapd_failures;		/* Number of 'reclaimed == 0' runs */
> > +
> >  #ifdef CONFIG_COMPACTION
> >  	int kcompactd_max_order;
> >  	enum zone_type kcompactd_classzone_idx;
> > diff --git a/mm/internal.h b/mm/internal.h
> > index ccfc2a2969f4..aae93e3fd984 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -81,6 +81,12 @@ static inline void set_page_refcounted(struct page *page)
> >  extern unsigned long highest_memmap_pfn;
> >  
> >  /*
> > + * Maximum number of reclaim retries without progress before the OOM
> > + * killer is consider the only way forward.
> > + */
> > +#define MAX_RECLAIM_RETRIES 16
> > +
> > +/*
> >   * in mm/vmscan.c:
> >   */
> >  extern int isolate_lru_page(struct page *page);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 614cd0397ce3..f50e36e7b024 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3516,12 +3516,6 @@ bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
> >  }
> >  
> >  /*
> > - * Maximum number of reclaim retries without any progress before OOM killer
> > - * is consider as the only way to move forward.
> > - */
> > -#define MAX_RECLAIM_RETRIES 16
> > -
> > -/*
> >   * Checks whether it makes sense to retry the reclaim to make a forward progress
> >   * for the given allocation request.
> >   * The reclaim feedback represented by did_some_progress (any progress during
> > @@ -4527,7 +4521,8 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask)
> >  			K(node_page_state(pgdat, NR_WRITEBACK_TEMP)),
> >  			K(node_page_state(pgdat, NR_UNSTABLE_NFS)),
> >  			node_page_state(pgdat, NR_PAGES_SCANNED),
> > -			!pgdat_reclaimable(pgdat) ? "yes" : "no");
> > +			pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES ?
> > +				"yes" : "no");
> >  	}
> >  
> >  	for_each_populated_zone(zone) {
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 26c3b405ef34..407b27831ff7 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2626,6 +2626,15 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> >  	} while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
> >  					 sc->nr_scanned - nr_scanned, sc));
> >  
> > +	/*
> > +	 * Kswapd gives up on balancing particular nodes after too
> > +	 * many failures to reclaim anything from them and goes to
> > +	 * sleep. On reclaim progress, reset the failure counter. A
> > +	 * successful direct reclaim run will revive a dormant kswapd.
> > +	 */
> > +	if (reclaimable)
> > +		pgdat->kswapd_failures = 0;
> > +
> >  	return reclaimable;
> >  }
> >  
> > @@ -2700,10 +2709,6 @@ static void shrink_zones(struct zonelist *zonelist, struct scan_control *sc)
> >  						 GFP_KERNEL | __GFP_HARDWALL))
> >  				continue;
> >  
> > -			if (sc->priority != DEF_PRIORITY &&
> > -			    !pgdat_reclaimable(zone->zone_pgdat))
> > -				continue;	/* Let kswapd poll it */
> > -
> >  			/*
> >  			 * If we already have plenty of memory free for
> >  			 * compaction in this zone, don't free any more.
> > @@ -3134,6 +3139,10 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
> >  	if (waitqueue_active(&pgdat->pfmemalloc_wait))
> >  		wake_up_all(&pgdat->pfmemalloc_wait);
> >  
> > +	/* Hopeless node, leave it to direct reclaim */
> 
> I hope to clear what we want by deferring the job to direct reclaim.
> Direct reclaim is much limited reclaim worker by serveral things(e.g.,
> avoid writeback for stack overflow, NOIO|NOFS context)

This is true but if kswapd cannot reclaim anything at all then we do not
have much choice left

> so what do we
> want for direct reclaimer to do even if kswapd can make forward
> progress? OOM?

yes resp. back off for costly high order requests and leave the node
unbalanced.

> > +	if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
> > +		return true;
> > +
> >  	for (i = 0; i <= classzone_idx; i++) {
> >  		struct zone *zone = pgdat->node_zones + i;
> >  
> > @@ -3316,6 +3325,9 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
> >  			sc.priority--;
> >  	} while (sc.priority >= 1);
> >  
> > +	if (!sc.nr_reclaimed)
> > +		pgdat->kswapd_failures++;
> 
> sc.nr_reclaimed is reset to zero in above big loop's beginning so most of time,
> it pgdat->kswapd_failures is increased.

But then we increase the counter in kswapd_shrink_node or do I miss your
point? Are you suggesting to use the aggregate nr_reclaimed over all
priorities because the last round might have made no progress?

-- 
Michal Hocko
SUSE Labs