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Message-Id: <20091008152620.e81d6209.akpm@linux-foundation.org>
Date: Thu, 8 Oct 2009 15:26:20 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 2/2] memcg: coalescing charges per cpu
On Fri, 2 Oct 2009 14:03:43 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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.
> +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.
> +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".
That's unobvious here and merits a comment.
> + for_each_online_cpu(cpu) {
Did we need get_online_cpus() here?
> + 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.
> +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.
> /*
> * 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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