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SubjectRe: [PATCH 20/31] mm, vmscan: only wakeup kswapd once per node for the requested classzone
On Fri, Jul 01, 2016 at 09:01:28PM +0100, Mel Gorman wrote:
> kswapd is woken when zones are below the low watermark but the wakeup
> decision is not taking the classzone into account. Now that reclaim is
> node-based, it is only required to wake kswapd once per node and only if
> all zones are unbalanced for the requested classzone.
>
> Note that one node might be checked multiple times if the zonelist is
> ordered by node because there is no cheap way of tracking what nodes have
> already been visited. For zone-ordering, each node should be checked only
> once.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/page_alloc.c | 8 ++++++--
> mm/vmscan.c | 13 +++++++++++--
> 2 files changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 441f482bf9a2..2fe2fbb4f2ad 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3410,10 +3410,14 @@ static void wake_all_kswapds(unsigned int order, const struct alloc_context *ac)
> {
> struct zoneref *z;
> struct zone *zone;
> + pg_data_t *last_pgdat = NULL;
>
> for_each_zone_zonelist_nodemask(zone, z, ac->zonelist,
> - ac->high_zoneidx, ac->nodemask)
> - wakeup_kswapd(zone, order, ac_classzone_idx(ac));
> + ac->high_zoneidx, ac->nodemask) {
> + if (last_pgdat != zone->zone_pgdat)
> + wakeup_kswapd(zone, order, ac_classzone_idx(ac));
> + last_pgdat = zone->zone_pgdat;
> + }
> }

In wakeup_kswapd(), there is a check if it is a populated zone or not.
If first zone in node is not a populated zone, wakeup_kswapd() would be
skipped. Though, I'm not sure if zonelist can include a un-populated
zone. Perhaps, moving populated zone check in wakeup_kswapd() to here
would be a safe code.

Thanks.

>
> static inline unsigned int
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c1c8b77d8cb4..e02091be0e12 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3420,6 +3420,7 @@ static int kswapd(void *p)
> void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
> {
> pg_data_t *pgdat;
> + int z;
>
> if (!populated_zone(zone))
> return;
> @@ -3433,8 +3434,16 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
> pgdat->kswapd_order = max(pgdat->kswapd_order, order);
> if (!waitqueue_active(&pgdat->kswapd_wait))
> return;
> - if (zone_balanced(zone, order, 0))
> - return;
> +
> + /* Only wake kswapd if all zones are unbalanced */
> + for (z = 0; z <= classzone_idx; z++) {
> + zone = pgdat->node_zones + z;
> + if (!populated_zone(zone))
> + continue;
> +
> + if (zone_balanced(zone, order, classzone_idx))
> + return;
> + }
>
> trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
> wake_up_interruptible(&pgdat->kswapd_wait);
> --
> 2.6.4
>
> --
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