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SubjectRe: [BUGFIX][PATCH] memcg: fix behavior of per cpu charge cache draining.
I have a few minor comments.

On Wed, 8 Jun 2011 14:05:18 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> This patch is made against mainline git tree.
> ==
> From d1372da4d3c6f8051b5b1cf7b5e8b45a8094b388 Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Date: Wed, 8 Jun 2011 13:51:11 +0900
> Subject: [BUGFIX][PATCH] memcg: fix behavior of per cpu charge cache draining.
>
> For performance, memory cgroup caches some "charge" from res_counter
> into per cpu cache. This works well but because it's cache,
> it needs to be flushed in some cases. Typical cases are
> 1. when someone hit limit.
> 2. when rmdir() is called and need to charges to be 0.
>
> But "1" has problem.
>
> Recently, with large SMP machines, we see many kworker/%d:%d when
> memcg hit limit. It is because of flushing memcg's percpu cache.
> Bad things in implementation are
>
> a) it's called before calling try_to_free_mem_cgroup_pages()
> so, it's called immidiately when a task hit limit.
> (I thought it was better to avoid to run into memory reclaim.
> But it was wrong decision.)
>
> b) Even if a cpu contains a cache for memcg not related to
> a memcg which hits limit, drain code is called.
>
> This patch fixes a) and b) by
>
> A) delay calling of flushing until one run of try_to_free...
> Then, the number of calling is much decreased.
> B) check percpu cache contains a useful data or not.
> plus
> C) check asynchronous percpu draining doesn't run on the cpu.
>
> Reported-by: Ying Han <yinghan@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> mm/memcontrol.c | 44 ++++++++++++++++++++++++++++----------------
> 1 files changed, 28 insertions(+), 16 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bd9052a..c22c0eb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -359,7 +359,7 @@ enum charge_type {
> static void mem_cgroup_get(struct mem_cgroup *mem);
> static void mem_cgroup_put(struct mem_cgroup *mem);
> static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
> -static void drain_all_stock_async(void);
> +static void drain_all_stock_async(struct mem_cgroup *mem);
>
> static struct mem_cgroup_per_zone *
> mem_cgroup_zoneinfo(struct mem_cgroup *mem, int nid, int zid)
> @@ -1670,8 +1670,6 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> victim = mem_cgroup_select_victim(root_mem);
> if (victim == root_mem) {
> loop++;
> - if (loop >= 1)
> - drain_all_stock_async();
> if (loop >= 2) {
> /*
> * If we have not been able to reclaim
> @@ -1723,6 +1721,7 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> return total;
> } else if (mem_cgroup_margin(root_mem))
> return total;
> + drain_all_stock_async(root_mem);
> }
> return total;
> }
> @@ -1934,9 +1933,11 @@ struct memcg_stock_pcp {
> struct mem_cgroup *cached; /* this never be root cgroup */
> unsigned int nr_pages;
> struct work_struct work;
> + unsigned long flags;
> +#define ASYNC_FLUSHING (0)
> };
> static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> -static atomic_t memcg_drain_count;
> +static atomic_t memcg_drain_count; /* Indicates there is synchronous flusher */
>
> /*
> * Try to consume stocked charge on this cpu. If success, one page is consumed
> @@ -1984,6 +1985,7 @@ static void drain_local_stock(struct work_struct *dummy)
> {
> struct memcg_stock_pcp *stock = &__get_cpu_var(memcg_stock);
> drain_stock(stock);
> + clear_bit(ASYNC_FLUSHING, &stock->flags);
> }
>
> /*
> @@ -2006,28 +2008,38 @@ static void refill_stock(struct mem_cgroup *mem, unsigned int nr_pages)
> * Tries to drain stocked charges in other cpus. This function is asynchronous
> * and just put a work per cpu for draining localy on each cpu. Caller can
> * expects some charges will be back to res_counter later but cannot wait for
> - * it.
> + * it. This runs only when per-cpu stock contains information of memcg which
> + * is under specified root_mem and no other flush runs.
> */
> -static void drain_all_stock_async(void)
> +static void drain_all_stock_async(struct mem_cgroup *root_mem)
> {
> int cpu;
> - /* This function is for scheduling "drain" in asynchronous way.
> - * The result of "drain" is not directly handled by callers. Then,
> - * if someone is calling drain, we don't have to call drain more.
> - * Anyway, WORK_STRUCT_PENDING check in queue_work_on() will catch if
> - * there is a race. We just do loose check here.
> +
> + /*
> + * If synchronous flushing (which flushes all cpus's cache) runs,
> + * do nothing.
> */
> - if (atomic_read(&memcg_drain_count))
> + if (unlikely(atomic_read(&memcg_drain_count)))
> return;
> - /* Notify other cpus that system-wide "drain" is running */
> - atomic_inc(&memcg_drain_count);
> get_online_cpus();
> for_each_online_cpu(cpu) {
> struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> - schedule_work_on(cpu, &stock->work);
> + struct mem_cgroup *mem;
> + bool do_flush;
> +
> + rcu_read_lock();

Should this rcu_read_lock() be placed here ? IIUC, it's necessary only for css_is_ancestor().

> + mem = stock->cached;
> + if (!mem) {
> + rcu_read_unlock();
> + continue;
> + }
> + do_flush = ((mem == root_mem) ||
> + css_is_ancestor(&mem->css, &root_mem->css));

Adding "root_mem->use_hierarchy" is better to avoid flusing the cache as long as possible.


Thanks,
Daisuke Nishimura.

> + rcu_read_unlock();
> + if (do_flush && !test_and_set_bit(ASYNC_FLUSHING, &stock->flags))
> + schedule_work_on(cpu, &stock->work);
> }
> put_online_cpus();
> - atomic_dec(&memcg_drain_count);
> /* We don't wait for flush_work */
> }
>
> --
> 1.7.4.1
>
>


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