On Mon 13-03-17 15:19:20, Shakeel Butt wrote:
> Recently kswapd has been modified to give up after MAX_RECLAIM_RETRIES
s@Recently@Since "mm: fix 100% CPU kswapd busyloop on unreclaimable nodes"@
> number of unsucessful iterations. Before going to sleep, kswapd thread
> will unconditionally wakeup all threads sleeping on pfmemalloc_wait.
> However the awoken threads will recheck the watermarks and wake the
> kswapd thread and sleep again on pfmemalloc_wait. There is a chance
> of continuous back and forth between kswapd and direct reclaiming
> threads if the kswapd keep failing and thus defeat the purpose of
> adding backoff mechanism to kswapd.
I would be probably more explicit about this being a livelock which
prevents the machine to reclaim anything or go OOM because _all_ direct
reclaimers might end up in in throttle_direct_reclaim so there is nobody
to make a forward progress.
> So, add kswapd_failures check
> on the throttle_direct_reclaim condition.
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
OK, seems like the simplest way forward. But we definitely have to do
something about throttle_direct_reclaim long term.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> v2:
> Instead of separate helper function for checking kswapd_failures,
> added the check into pfmemalloc_watermark_ok() and renamed that
> function.
>
> mm/vmscan.c | 15 +++++++++------
> 1 file changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bae698484e8e..afa5b20ab6d8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2783,7 +2783,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> return 0;
> }
>
> -static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> +static bool allow_direct_reclaim(pg_data_t *pgdat)
> {
> struct zone *zone;
> unsigned long pfmemalloc_reserve = 0;
> @@ -2791,6 +2791,9 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> int i;
> bool wmark_ok;
>
> + if (pgdat->kswapd_failures >= MAX_RECLAIM_RETRIES)
> + return true;
> +
> for (i = 0; i <= ZONE_NORMAL; i++) {
> zone = &pgdat->node_zones[i];
> if (!managed_zone(zone))
> @@ -2873,7 +2876,7 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
>
> /* Throttle based on the first usable node */
> pgdat = zone->zone_pgdat;
> - if (pfmemalloc_watermark_ok(pgdat))
> + if (allow_direct_reclaim(pgdat))
> goto out;
> break;
> }
> @@ -2895,14 +2898,14 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
> */
> if (!(gfp_mask & __GFP_FS)) {
> wait_event_interruptible_timeout(pgdat->pfmemalloc_wait,
> - pfmemalloc_watermark_ok(pgdat), HZ);
> + allow_direct_reclaim(pgdat), HZ);
>
> goto check_pending;
> }
>
> /* Throttle until kswapd wakes the process */
> wait_event_killable(zone->zone_pgdat->pfmemalloc_wait,
> - pfmemalloc_watermark_ok(pgdat));
> + allow_direct_reclaim(pgdat));
>
> check_pending:
> if (fatal_signal_pending(current))
> @@ -3102,7 +3105,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, int classzone_idx)
> {
> /*
> * The throttled processes are normally woken up in balance_pgdat() as
> - * soon as pfmemalloc_watermark_ok() is true. But there is a potential
> + * soon as allow_direct_reclaim() is true. But there is a potential
> * race between when kswapd checks the watermarks and a process gets
> * throttled. There is also a potential race if processes get
> * throttled, kswapd wakes, a large process exits thereby balancing the
> @@ -3271,7 +3274,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
> * able to safely make forward progress. Wake them
> */
> if (waitqueue_active(&pgdat->pfmemalloc_wait) &&
> - pfmemalloc_watermark_ok(pgdat))
> + allow_direct_reclaim(pgdat))
> wake_up_all(&pgdat->pfmemalloc_wait);
>
> /* Check if kswapd should be suspending */
> --
> 2.12.0.246.ga2ecc84866-goog
>
--
Michal Hocko
SUSE Labs
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