On Fri 10-03-17 11:46:20, Shakeel Butt wrote:
> Recently kswapd has been modified to give up after MAX_RECLAIM_RETRIES
> 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. So, add kswapd_failures check
> on the throttle_direct_reclaim condition.
I have to say I really do not like this. kswapd_failures shouldn't
really be checked outside of the kswapd context. The
pfmemalloc_watermark_ok/throttle_direct_reclaim is quite complex even
without putting another variable into it. I wish we rather replace this
throttling by something else. Johannes had an idea to throttle by the
number of reclaimers.
Anyway, I am wondering whether we can hit this issue in
practice? Have you seen it happening or is this a result of the code
review? I would assume that that !zone_reclaimable_pages check in
pfmemalloc_watermark_ok should help to some degree.
> Signed-off-by: Shakeel Butt <shake...@google.com>
> ---
> mm/vmscan.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bae698484e8e..b2d24cc7a161 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2819,6 +2819,12 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> return wmark_ok;
> }
>
> +static bool should_throttle_direct_reclaim(pg_data_t *pgdat)
> +{
> + return (pgdat->kswapd_failures < MAX_RECLAIM_RETRIES &&
> + !pfmemalloc_watermark_ok(pgdat));
> +}
> +
> /*
> * Throttle direct reclaimers if backing storage is backed by the network
> * and the PFMEMALLOC reserve for the preferred node is getting dangerously
> @@ -2873,7 +2879,7 @@ 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 (!should_throttle_direct_reclaim(pgdat))
> goto out;
> break;
> }
> @@ -2895,14 +2901,14 @@ 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);
> + !should_throttle_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));
> + !should_throttle_direct_reclaim(pgdat));
>
> check_pending:
> if (fatal_signal_pending(current))
> --
> 2.12.0.246.ga2ecc84866-goog
>
--
Michal Hocko
SUSE Labs
