Re: [BUGFIX][PATCH] memcg: fix behavior of per cpu charge cachedraining.

[KAMEZAWA Hiroyuki]

On Wed, 8 Jun 2011 15:29:01 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> On Wed, 8 Jun 2011 14:49:34 +0900
> Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:
> 
> > I have a few minor comments.
> > 
> > On Wed, 8 Jun 2011 14:05:18 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > 
> > > This patch is made against mainline git tree.
> > > ==
> > > From d1372da4d3c6f8051b5b1cf7b5e8b45a8094b388 Mon Sep 17 00:00:00 2001
> > > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > > ---
> > >  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().
> > 
> 
> I thought rcu_read_lock() is required before getting a pointer to be acceseed.
> 
> But hmm...at second thought..
>  1. stock->cached is flushed before memcg is destroyed..
>  2. force_empty()(before destroy memcg) and this function can
>     do mutual execution by some lock.
> 
> Then, it's safe to access stock->cached. Ok. I'll move this (with some comment)
> 
> > > +		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.
> > 
> 
> ok. here is v2.
> 

I found typo ;(.
I'll post v3 in a new thread. Very sorry.

Thanks,
-Kame

>

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