On Tue, Nov 24, 2015 at 04:59:40PM -0500, Johannes Weiner wrote:
...
> @@ -2396,6 +2396,7 @@ static bool shrink_zone(struct zone *zone, struct
> scan_control *sc,
> memcg = mem_cgroup_iter(root, NULL, &reclaim);
> do {
> unsigned long lru_pages;
> + unsigned long reclaimed;
> unsigned long scanned;
> struct lruvec *lruvec;
> int swappiness;
> @@ -2408,6 +2409,7 @@ static bool shrink_zone(struct zone *zone, struct
> scan_control *sc,
>
> lruvec = mem_cgroup_zone_lruvec(zone, memcg);
> swappiness = mem_cgroup_swappiness(memcg);
> + reclaimed = sc->nr_reclaimed;
> scanned = sc->nr_scanned;
>
> shrink_lruvec(lruvec, swappiness, sc, &lru_pages);
> @@ -2418,6 +2420,11 @@ static bool shrink_zone(struct zone *zone, struct
> scan_control *sc,
> memcg, sc->nr_scanned - scanned,
> lru_pages);
>
> + /* Record the group's reclaim efficiency */
> + vmpressure(sc->gfp_mask, memcg, false,
> + sc->nr_scanned - scanned,
> + sc->nr_reclaimed - reclaimed);
> +
Suppose we have the following cgroup configuration.
A __ B
\_ C
A is empty (which is natural for the unified hierarchy AFAIU). B has
some workload running in it, and C generates socket pressure. Due to the
socket pressure coming from C we start reclaim in A, which results in
thrashing of B, but we might not put sockets under pressure in A or C,
because vmpressure does not account pages scanned/reclaimed in B when
generating a vmpressure event for A or C. This might result in
aggressive reclaim and thrashing in B w/o generating a signal for C to
stop growing socket buffers.
Do you think such a situation is possible? If so, would it make sense to
switch to post-order walk in shrink_zone and pass sub-tree
scanned/reclaimed stats to vmpressure for each scanned memcg?
Thanks,
Vladimir
> /*
> * Direct reclaim and kswapd have to scan all memory
> * cgroups to fulfill the overall scan target for the
> @@ -2449,7 +2456,8 @@ static bool shrink_zone(struct zone *zone, struct
> scan_control *sc,
> reclaim_state->reclaimed_slab = 0;
> }
>
> - vmpressure(sc->gfp_mask, sc->target_mem_cgroup,
> + /* Record the subtree's reclaim efficiency */
> + vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
> sc->nr_scanned - nr_scanned,
> sc->nr_reclaimed - nr_reclaimed);
>
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