Re: [PATCH 2/2] memcg: coalescing charges per cpu

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

On Thu, 8 Oct 2009 15:26:20 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Fri, 2 Oct 2009 14:03:43 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > 
> > This is a patch for coalescing access to res_counter at charging by
> > percpu caching. At charge, memcg charges 64pages and remember it in
> > percpu cache. Because it's cache, drain/flush is done if necessary.
> > 
> > This version uses public percpu area.
> >  2 benefits of using public percpu area.
> >  1. Sum of stocked charge in the system is limited to # of cpus
> >     not to the number of memcg. This shows better synchonization.
> >  2. drain code for flush/cpuhotplug is very easy (and quick)
> > 
> > The most important point of this patch is that we never touch res_counter
> > in fast path. The res_counter is system-wide shared counter which is modified
> > very frequently. We shouldn't touch it as far as we can for avoiding
> > false sharing.
> > 
> > ...
> > 
> > +/* size of first charge trial. "32" comes from vmscan.c's magic value */
> > +#define CHARGE_SIZE	(32 * PAGE_SIZE)
> > +struct memcg_stock_pcp {
> > +	struct mem_cgroup *cached;
> > +	int charge;
> > +	struct work_struct work;
> > +};
> > +static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> > +static DEFINE_MUTEX(memcg_drain_mutex);
> > +
> > +static bool consume_stock(struct mem_cgroup *mem)
> > +{
> > +	struct memcg_stock_pcp *stock;
> > +	bool ret = true;
> > +
> > +	stock = &get_cpu_var(memcg_stock);
> > +	if (mem == stock->cached && stock->charge)
> > +		stock->charge -= PAGE_SIZE;
> > +	else
> > +		ret = false;
> > +	put_cpu_var(memcg_stock);
> > +	return ret;
> > +}
> 
> It's unobvious what the return value from this function means.  A nice
> comment would help.

will do.

> 
> > +static void drain_stock(struct memcg_stock_pcp *stock)
> > +{
> > +	struct mem_cgroup *old = stock->cached;
> > +
> > +	if (stock->charge) {
> > +		res_counter_uncharge(&old->res, stock->charge);
> > +		if (do_swap_account)
> > +			res_counter_uncharge(&old->memsw, stock->charge);
> > +	}
> > +	stock->cached = NULL;
> > +	stock->charge = 0;
> > +}
> > +
> > +static void drain_local_stock(struct work_struct *dummy)
> > +{
> > +	struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
> > +	drain_stock(stock);
> > +	put_cpu_var(memcg_stock);
> > +}
> 
> drain_local_stock() is only ever run by a thread which is pinned to a
> particular CPU, so we can use plain old __get_cpu_var() here and remove
> the put_cpu_var(), methinks.  If this is done, a comment shuold be
> added explaining why we can use that optimisation.
> 

Ok. I'll try and test that.


> > +static void refill_stock(struct mem_cgroup *mem, int val)
> > +{
> > +	struct memcg_stock_pcp *stock = &get_cpu_var(memcg_stock);
> > +
> > +	if (stock->cached != mem) {
> > +		drain_stock(stock);
> > +		stock->cached = mem;
> > +	}
> > +	stock->charge += val;
> > +	put_cpu_var(memcg_stock);
> > +}
> > +
> > +static void drain_all_stock_async(void)
> > +{
> > +	int cpu;
> > +	/* Contention means someone tries to flush. */
> > +	if (!mutex_trylock(&memcg_drain_mutex))
> > +		return;
> 
> Any time I see a trylock I ask "hm, what happens when it fails - what
> are the consequences and how to we later rerun the aborted operation".
> 
I'll add more comments.

> That's unobvious here and merits a comment.
> 
> > +	for_each_online_cpu(cpu) {
> 
> Did we need get_online_cpus() here?
> 

Ahhhh, yes. It's necessary.

> > +		struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> > +		if (work_pending(&stock->work))
> > +			continue;
> > +		INIT_WORK(&stock->work, drain_local_stock);
> > +		schedule_work_on(cpu, &stock->work);
> > +	}
> > +	mutex_unlock(&memcg_drain_mutex);
> > +	/* We don't wait for flush_work */
> > +}
> > +
> > +static void drain_all_stock_sync(void)
> > +{
> > +	/* called when force_empty is called */
> > +	mutex_lock(&memcg_drain_mutex);
> > +	schedule_on_each_cpu(drain_local_stock);
> > +	mutex_unlock(&memcg_drain_mutex);
> > +}
> 
> In fact it would be nice were each of these functions to have a brief
> note describing their role.
> 
Okay. I'll add.

> > +static int __cpuinit memcg_stock_cpu_callback(struct notifier_block *nb,
> > +					unsigned long action,
> > +					void *hcpu)
> > +{
> > +#ifdef CONFIG_HOTPLUG_CPU
> > +	int cpu = (unsigned long)hcpu;
> > +	struct memcg_stock_pcp *stock;
> > +
> > +	if (action != CPU_DEAD)
> > +		return NOTIFY_OK;
> > +	stock = &per_cpu(memcg_stock, cpu);
> > +	drain_stock(stock);
> > +#endif
> > +	return NOTIFY_OK;
> > +}
> 
> Is the ifdef needed?  Using hotcpu_notifier() should cause this entire
> function to disappear from vmlinux when CONFIG_HOTPLUG_CPU=n.
> 
Ah, I didn't notice that. I'll try.

Thank you.

Regards,
-Kame

> >  /*
> >   * Unlike exported interface, "oom" parameter is added. if oom==true,
> >   * oom-killer can be invoked.
> > @@ -1269,6 +1370,7 @@ static int __mem_cgroup_try_charge(struc
> >  	struct mem_cgroup *mem, *mem_over_limit;
> >  	int nr_retries = MEM_CGROUP_RECLAIM_RETRIES;
> >  	struct res_counter *fail_res;
> > +	int csize = CHARGE_SIZE;
> >  
> >  	if (unlikely(test_thread_flag(TIF_MEMDIE))) {
> >  		/* Don't account this! */
> > @@ -1293,23 +1395,25 @@ static int __mem_cgroup_try_charge(struc
> >  		return 0;
> >  
> >  	VM_BUG_ON(css_is_removed(&mem->css));
> > +	if (mem_cgroup_is_root(mem))
> > +		goto done;
> >  
> >  	while (1) {
> >  		int ret = 0;
> >  		unsigned long flags = 0;
> >  
> > -		if (mem_cgroup_is_root(mem))
> > -			goto done;
> > -		ret = res_counter_charge(&mem->res, PAGE_SIZE, &fail_res);
> > +		if (consume_stock(mem))
> > +			goto charged;
> > +
> > +		ret = res_counter_charge(&mem->res, csize, &fail_res);
> >  		if (likely(!ret)) {
> >  			if (!do_swap_account)
> >  				break;
> > -			ret = res_counter_charge(&mem->memsw, PAGE_SIZE,
> > -							&fail_res);
> > +			ret = res_counter_charge(&mem->memsw, csize, &fail_res);
> >  			if (likely(!ret))
> >  				break;
> >  			/* mem+swap counter fails */
> > -			res_counter_uncharge(&mem->res, PAGE_SIZE);
> > +			res_counter_uncharge(&mem->res, csize);
> >  			flags |= MEM_CGROUP_RECLAIM_NOSWAP;
> >  			mem_over_limit = mem_cgroup_from_res_counter(fail_res,
> >  									memsw);
> > @@ -1318,6 +1422,11 @@ static int __mem_cgroup_try_charge(struc
> >  			mem_over_limit = mem_cgroup_from_res_counter(fail_res,
> >  									res);
> >  
> > +		/* reduce request size and retry */
> > +		if (csize > PAGE_SIZE) {
> > +			csize = PAGE_SIZE;
> > +			continue;
> > +		}
> >  		if (!(gfp_mask & __GFP_WAIT))
> >  			goto nomem;
> >  
> > @@ -1347,6 +1456,9 @@ static int __mem_cgroup_try_charge(struc
> >  			goto nomem;
> >  		}
> >  	}
> > +	if (csize > PAGE_SIZE)
> > +		refill_stock(mem, csize - PAGE_SIZE);
> > +charged:
> >  	/*
> >  	 * Insert ancestor (and ancestor's ancestors), to softlimit RB-tree.
> >  	 * if they exceeds softlimit.
> > @@ -2463,6 +2575,7 @@ move_account:
> >  			goto out;
> >  		/* This is for making all *used* pages to be on LRU. */
> >  		lru_add_drain_all();
> > +		drain_all_stock_sync();
> >  		ret = 0;
> >  		for_each_node_state(node, N_HIGH_MEMORY) {
> >  			for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
> > @@ -3181,6 +3294,7 @@ mem_cgroup_create(struct cgroup_subsys *
> >  		root_mem_cgroup = mem;
> >  		if (mem_cgroup_soft_limit_tree_init())
> >  			goto free_out;
> > +		hotcpu_notifier(memcg_stock_cpu_callback, 0);
> >  
> >  	} else {
> >  		parent = mem_cgroup_from_cont(cont->parent);
> 

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