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SubjectRe: [PATCH 1/9] mm: fix 100% CPU kswapd busyloop on unreclaimable nodes
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@gmail.com>
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > Tested-by: Jia He <hejianet@gmail.com>
> > Acked-by: Michal Hocko <mhocko@suse.com>
> > ---
> > 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

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