Re: [PATCH 3/4] memcg: get rid of percpu_charge_mutex lock

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 21 Jul 2011 09:58:24 +0200
Michal Hocko <mhocko@suse.cz> wrote:

> percpu_charge_mutex protects from multiple simultaneous per-cpu charge
> caches draining because we might end up having too many work items.
> At least this was the case until 26fe6168 (memcg: fix percpu cached
> charge draining frequency) when we introduced a more targeted draining
> for async mode.
> Now that also sync draining is targeted we can safely remove mutex
> because we will not send more work than the current number of CPUs.
> FLUSHING_CACHED_CHARGE protects from sending the same work multiple
> times and stock->nr_pages == 0 protects from pointless sending a work
> if there is obviously nothing to be done. This is of course racy but we
> can live with it as the race window is really small (we would have to
> see FLUSHING_CACHED_CHARGE cleared while nr_pages would be still
> non-zero).
> The only remaining place where we can race is synchronous mode when we
> rely on FLUSHING_CACHED_CHARGE test which might have been set by other
> drainer on the same group but we should wait in that case as well.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>

A concern.

> ---
>  mm/memcontrol.c |   12 ++----------
>  1 files changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8180cd9..9d49a12 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2065,7 +2065,6 @@ struct memcg_stock_pcp {
>  #define FLUSHING_CACHED_CHARGE	(0)
>  };
>  static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> -static DEFINE_MUTEX(percpu_charge_mutex);
>  
>  /*
>   * Try to consume stocked charge on this cpu. If success, one page is consumed
> @@ -2166,7 +2165,8 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
>  
>  	for_each_online_cpu(cpu) {
>  		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> -		if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> +		if (root_mem == stock->cached &&
> +				test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
>  			flush_work(&stock->work);

Doesn't this new check handle hierarchy ?
css_is_ancestor() will be required if you do this check.

BTW, this change should be in other patch, I think.

Thanks,
-Kame

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