Re: [patch 1/4] mm: memcontrol: reduce reclaim invocations for higher order requests

Michal Hocko
2014-08-07T13:08:27Z

On Mon 04-08-14 17:14:54, Johannes Weiner wrote:
> Instead of passing the request size to direct reclaim, memcg just
> manually loops around reclaiming SWAP_CLUSTER_MAX pages until the
> charge can succeed. That potentially wastes scan progress when huge
> page allocations require multiple invocations, which always have to
> restart from the default scan priority.
>
> Pass the request size as a reclaim target to direct reclaim and leave
> it to that code to reach the goal.

THP charge then will ask for 512 pages to be (direct) reclaimed. That
is _a lot_ and I would expect long stalls to achieve this target. I
would also expect quick priority drop down and potential over-reclaim
for small and moderately sized memcgs (e.g. memcg with 1G worth of pages
would need to drop down below DEF_PRIORITY-2 to have a chance to scan
that many pages). All that done for a charge which can fallback to a
single page charge.

The current code is quite hostile to THP when we are close to the limit
but solving this by introducing long stalls instead doesn't sound like a
proper approach to me.

> Charging will still have to loop in case concurrent allocations steal
> reclaim effort, but at least it doesn't have to loop to meet even the
> basic request size. This also prepares the memcg reclaim API for use
> with the planned high limit, to reclaim excess with a single call.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> include/linux/swap.h | 3 ++-
> mm/memcontrol.c | 15 +++++++++------
> mm/vmscan.c | 3 ++-
> 3 files changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 1b72060f093a..473a3ae4cdd6 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -327,7 +327,8 @@ extern void lru_cache_add_active_or_unevictable(struct page *page,
> extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> gfp_t gfp_mask, nodemask_t *mask);
> extern int __isolate_lru_page(struct page *page, isolate_mode_t mode);
> -extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem,
> +extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> + unsigned long nr_pages,
> gfp_t gfp_mask, bool noswap);
> extern unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *mem,
> gfp_t gfp_mask, bool noswap,
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ec4dcf1b9562..ddffeeda2d52 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1793,6 +1793,7 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> }
>
> static unsigned long mem_cgroup_reclaim(struct mem_cgroup *memcg,
> + unsigned long nr_pages,
> gfp_t gfp_mask,
> unsigned long flags)
> {
> @@ -1808,7 +1809,8 @@ static unsigned long mem_cgroup_reclaim(struct mem_cgroup *memcg,
> for (loop = 0; loop < MEM_CGROUP_MAX_RECLAIM_LOOPS; loop++) {
> if (loop)
> drain_all_stock_async(memcg);
> - total += try_to_free_mem_cgroup_pages(memcg, gfp_mask, noswap);
> + total += try_to_free_mem_cgroup_pages(memcg, nr_pages,
> + gfp_mask, noswap);
> /*
> * Allow limit shrinkers, which are triggered directly
> * by userspace, to catch signals and stop reclaim
> @@ -1816,7 +1818,7 @@ static unsigned long mem_cgroup_reclaim(struct mem_cgroup *memcg,
> */
> if (total && (flags & MEM_CGROUP_RECLAIM_SHRINK))
> break;
> - if (mem_cgroup_margin(memcg))
> + if (mem_cgroup_margin(memcg) >= nr_pages)
> break;
> /*
> * If nothing was reclaimed after two attempts, there
> @@ -2572,7 +2574,8 @@ retry:
> if (!(gfp_mask & __GFP_WAIT))
> goto nomem;
>
> - nr_reclaimed = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
> + nr_reclaimed = mem_cgroup_reclaim(mem_over_limit, nr_pages,
> + gfp_mask, flags);
>
> if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
> goto retry;
> @@ -3718,7 +3721,7 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> if (!ret)
> break;
>
> - mem_cgroup_reclaim(memcg, GFP_KERNEL,
> + mem_cgroup_reclaim(memcg, 1, GFP_KERNEL,
> MEM_CGROUP_RECLAIM_SHRINK);
> curusage = res_counter_read_u64(&memcg->res, RES_USAGE);
> /* Usage is reduced ? */
> @@ -3777,7 +3780,7 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
> if (!ret)
> break;
>
> - mem_cgroup_reclaim(memcg, GFP_KERNEL,
> + mem_cgroup_reclaim(memcg, 1, GFP_KERNEL,
> MEM_CGROUP_RECLAIM_NOSWAP |
> MEM_CGROUP_RECLAIM_SHRINK);
> curusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
> @@ -4028,7 +4031,7 @@ static int mem_cgroup_force_empty(struct mem_cgroup *memcg)
> if (signal_pending(current))
> return -EINTR;
>
> - progress = try_to_free_mem_cgroup_pages(memcg, GFP_KERNEL,
> + progress = try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL,
> false);
> if (!progress) {
> nr_retries--;
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d698f4f7b0f2..7db33f100db4 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2747,6 +2747,7 @@ unsigned long mem_cgroup_shrink_node_zone(struct mem_cgroup *memcg,
> }
>
> unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> + unsigned long nr_pages,
> gfp_t gfp_mask,
> bool noswap)
> {
> @@ -2754,7 +2755,7 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
> unsigned long nr_reclaimed;
> int nid;
> struct scan_control sc = {
> - .nr_to_reclaim = SWAP_CLUSTER_MAX,
> + .nr_to_reclaim = max(nr_pages, SWAP_CLUSTER_MAX),
> .gfp_mask = (gfp_mask & GFP_RECLAIM_MASK) |
> (GFP_HIGHUSER_MOVABLE & ~GFP_RECLAIM_MASK),
> .target_mem_cgroup = memcg,
> --
> 2.0.3
>

--
Michal Hocko
SUSE Labs