On Tue, 22 Apr 2014 09:38:52 +0100 Mel Gorman <mgorman@xxxxxxx> wrote:
> throttle_direct_reclaim() is meant to trigger during swap-over-network
> during which the min watermark is treated as a pfmemalloc reserve. It
> throttes on the first node in the zonelist but this is flawed.
>
> On a NUMA machine running a 32-bit kernel (I know) allocation requests
> freom CPUs on node 1 would detect no pfmemalloc reserves and the process
> gets throttled. This patch adjusts throttling of direct reclaim to throttle
> based on the first node in the zonelist that has a usable ZONE_NORMAL or
> lower zone.
I'm unable to determine from the above whether we should backport this
fix. Please don't forget to describe the end-user visible effects of
a bug when that isn't obvious.
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2507,10 +2507,17 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
>
> for (i = 0; i <= ZONE_NORMAL; i++) {
> zone = &pgdat->node_zones[i];
> + if (!populated_zone(zone))
> + continue;
What's this? Performance tweak? Or does min_wmark_pages() return
non-zero for an unpopulated zone, which seems odd.
> pfmemalloc_reserve += min_wmark_pages(zone);
> free_pages += zone_page_state(zone, NR_FREE_PAGES);
> }
>
> + /* If there are no reserves (unexpected config) then do not throttle */
> + if (!pfmemalloc_reserve)
> + return true;
> +
> wmark_ok = free_pages > pfmemalloc_reserve / 2;
>
> /* kswapd must be awake if processes are being throttled */
> @@ -2535,9 +2542,9 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat)
> static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
> nodemask_t *nodemask)
> {
> + struct zoneref *z;
> struct zone *zone;
> - int high_zoneidx = gfp_zone(gfp_mask);
> - pg_data_t *pgdat;
> + pg_data_t *pgdat = NULL;
>
> /*
> * Kernel threads should not be throttled as they may be indirectly
> @@ -2556,10 +2563,24 @@ static bool throttle_direct_reclaim(gfp_t gfp_mask, struct zonelist *zonelist,
> if (fatal_signal_pending(current))
> goto out;
>
> - /* Check if the pfmemalloc reserves are ok */
> - first_zones_zonelist(zonelist, high_zoneidx, NULL, &zone);
> - pgdat = zone->zone_pgdat;
> - if (pfmemalloc_watermark_ok(pgdat))
> + /*
> + * Check if the pfmemalloc reserves are ok by finding the first node
> + * with a usable ZONE_NORMAL or lower zone
> + */
That comment tells us what the code does but not why it does it.
- Why do we ignore zones >= ZONE_NORMAL?
- Why do we throttle when there may be as-yet-unexamined nodes which
have reclaimable pages?
> + for_each_zone_zonelist_nodemask(zone, z, zonelist,
> + gfp_mask, nodemask) {
Those two lines have spaces-instead-of-tabs.
> + if (zone_idx(zone) > ZONE_NORMAL)
> + continue;
> +
> + /* Throttle based on the first usable node */
> + pgdat = zone->zone_pgdat;
> + if (pfmemalloc_watermark_ok(pgdat))
> + goto out;
> + break;
> + }
> +
> + /* If no zone was usable by the allocation flags then do not throttle */
> + if (!pgdat)
> goto out;
--
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