Re: [PATCH] mm-vmscan-dont-mess-with-pgdat-flags-in-memcg-reclaim-v2-fix

Shakeel Butt
2018-04-06T14:37:10Z

On Fri, Apr 6, 2018 at 6:52 AM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> On 04/06/2018 05:13 AM, Shakeel Butt wrote:
>> Question: Should this 'flags' be per-node? Is it ok for a congested
>> memcg to call wait_iff_congested for all nodes?
>
> Indeed, congestion state should be pre-node. If memcg on node A is
> congested, there is no point is stalling memcg reclaim from node B.
>
> Make congestion state per-cgroup-per-node and record it in
> 'struct mem_cgroup_per_node'.
>
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> ---
> include/linux/memcontrol.h | 5 +++--
> mm/vmscan.c | 39 +++++++++++++++++++++++++--------------
> 2 files changed, 28 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 8b394bbf1c86..af9eed2e3e04 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -120,6 +120,9 @@ struct mem_cgroup_per_node {
> unsigned long usage_in_excess;/* Set to the value by which */
> /* the soft limit is exceeded*/
> bool on_tree;
> + bool congested; /* memcg has many dirty pages */
> + /* backed by a congested BDI */
> +
> struct mem_cgroup *memcg; /* Back pointer, we cannot */
> /* use container_of */
> };
> @@ -189,8 +192,6 @@ struct mem_cgroup {
> /* vmpressure notifications */
> struct vmpressure vmpressure;
>
> - unsigned long flags;
> -
> /*
> * Should the accounting and control be hierarchical, per subtree?
> */
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 99688299eba8..78214c899710 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -200,16 +200,27 @@ static bool sane_reclaim(struct scan_control *sc)
> return false;
> }
>
> -static void set_memcg_bit(enum pgdat_flags flag,
> - struct mem_cgroup *memcg)
> +static void set_memcg_congestion(pg_data_t *pgdat,
> + struct mem_cgroup *memcg,
> + bool congested)
> {
> - set_bit(flag, &memcg->flags);
> + struct mem_cgroup_per_node *mz;
> +
> + if (!memcg)
> + return;
> +
> + mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> + WRITE_ONCE(mz->congested, congested);
> }
>
> -static int test_memcg_bit(enum pgdat_flags flag,
> +static bool memcg_congested(pg_data_t *pgdat,
> struct mem_cgroup *memcg)
> {
> - return test_bit(flag, &memcg->flags);
> + struct mem_cgroup_per_node *mz;
> +
> + mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> + return READ_ONCE(mz->congested);
> +
> }
> #else
> static bool global_reclaim(struct scan_control *sc)
> @@ -222,15 +233,16 @@ static bool sane_reclaim(struct scan_control *sc)
> return true;
> }
>
> -static inline void set_memcg_bit(enum pgdat_flags flag,
> - struct mem_cgroup *memcg)
> +static inline void set_memcg_congestion(struct pglist_data *pgdat,
> + struct mem_cgroup *memcg, bool congested)
> {
> }
>
> -static inline int test_memcg_bit(enum pgdat_flags flag,
> - struct mem_cgroup *memcg)
> +static inline bool memcg_congested(struct pglist_data *pgdat,
> + struct mem_cgroup *memcg)
> {
> - return 0;
> + return false;
> +
> }
> #endif
>
> @@ -2482,7 +2494,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
> static bool pgdat_memcg_congested(pg_data_t *pgdat, struct mem_cgroup *memcg)
> {
> return test_bit(PGDAT_CONGESTED, &pgdat->flags) ||
> - (memcg && test_memcg_bit(PGDAT_CONGESTED, memcg));
> + (memcg && memcg_congested(pgdat, memcg));

I am wondering if we should check all ancestors for congestion as
well. Maybe a parallel memcg reclaimer might have set some ancestor of
this memcg to congested.

> }
>
> static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> @@ -2617,7 +2629,7 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
> */
> if (!global_reclaim(sc) && sane_reclaim(sc) &&
> sc->nr.dirty && sc->nr.dirty == sc->nr.congested)
> - set_memcg_bit(PGDAT_CONGESTED, root);
> + set_memcg_congestion(pgdat, root, true);
>
> /*
> * Stall direct reclaim for IO completions if underlying BDIs
> @@ -2844,6 +2856,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
> continue;
> last_pgdat = zone->zone_pgdat;
> snapshot_refaults(sc->target_mem_cgroup, zone->zone_pgdat);
> + set_memcg_congestion(last_pgdat, sc->target_mem_cgroup, false);
> }
>
> delayacct_freepages_end();
> @@ -3067,7 +3080,6 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
> * the priority and make it zero.
> */
> shrink_node_memcg(pgdat, memcg, &sc, &lru_pages);
> - clear_bit(PGDAT_CONGESTED, &memcg->flags);
>
> trace_mm_vmscan_memcg_softlimit_reclaim_end(sc.nr_reclaimed);
>
> @@ -3113,7 +3125,6 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> noreclaim_flag = memalloc_noreclaim_save();
> nr_reclaimed = do_try_to_free_pages(zonelist, &sc);
> memalloc_noreclaim_restore(noreclaim_flag);
> - clear_bit(PGDAT_CONGESTED, &memcg->flags);
>
> trace_mm_vmscan_memcg_reclaim_end(nr_reclaimed);
>
> --
> 2.16.1
>