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Date: Wed, 16 Apr 2014 11:22:45 +0200
From: Michal Hocko <mhocko@...e.cz>
To: Jianyu Zhan <nasa4836@...il.com>
Cc: hannes@...xchg.org, bsingharora@...il.com,
kamezawa.hiroyu@...fujitsu.com, akpm@...ux-foundation.org,
cgroups@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/memcontrol.c: make mem_cgroup_read_stat() read all
interested stat item in one go
On Fri 11-04-14 01:11:08, Jianyu Zhan wrote:
> Currently, mem_cgroup_read_stat() is used for user interface. The
> user accounts memory usage by memory cgroup and he _always_ requires
> exact value because he accounts memory. So we don't use quick-and-fuzzy
> -read-and-do-periodic-synchronization way. Thus, we iterate all cpus
> for one read.
>
> And we mem_cgroup_usage() and mem_cgroup_recursive_stat() both finally
> call into mem_cgroup_read_stat().
>
> However, these *stat snapshot* operations are implemented in a quite
> coarse way: it takes M*N iteration for each stat item(M=nr_memcgs,
> N=nr_possible_cpus). There are two deficiencies:
>
> 1. for every stat item, we have to iterate over all percpu value, which
> is not so cache friendly.
> 2. for every stat item, we call mem_cgroup_read_stat() once, which
> increase the probablity of contending on pcp_counter_lock.
>
> So, this patch improve this a bit.
How much and under what kind of load?
> Concretely, for all interested stat
> items, mark them in a bitmap, and then make mem_cgroup_read_stat() read
> them all in one go.
>
> This is more efficient, and to some degree make it more like *stat snapshot*.
>
> Signed-off-by: Jianyu Zhan <nasa4836@...il.com>
> ---
> mm/memcontrol.c | 91 +++++++++++++++++++++++++++++++++++++++------------------
> 1 file changed, 62 insertions(+), 29 deletions(-)
I cannot say I like the new code much more than the previous one and
I've never seen the old one being a bottleneck. So I am not entirely
fond of optimization without a good reason. (Hint, if you are optimizing
something always show us numbers which support the optimization)
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 29501f0..009357e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -347,7 +347,7 @@ struct mem_cgroup {
> struct mem_cgroup_stat_cpu __percpu *stat;
> /*
> * used when a cpu is offlined or other synchronizations
> - * See mem_cgroup_read_stat().
> + * See mem_cgroup_read_stat_vec().
> */
> struct mem_cgroup_stat_cpu nocpu_base;
> spinlock_t pcp_counter_lock;
> @@ -855,7 +855,13 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz)
> return mz;
> }
>
> -/*
> +/**
> + * @memcg: the mem_cgroup to account for.
> + * @stat_bitmask: a bitmap record which stat items to read,
> + * each mem_cgroup_stat_index has its corresponding bit.
> + * @stat_vec: a stat vector to hold the stat value for returing, caller
> + * shall take care of initializing it.
> + *
> * Implementation Note: reading percpu statistics for memcg.
> *
> * Both of vmstat[] and percpu_counter has threshold and do periodic
> @@ -874,22 +880,25 @@ mem_cgroup_largest_soft_limit_node(struct mem_cgroup_tree_per_zone *mctz)
> * common workload, threashold and synchonization as vmstat[] should be
> * implemented.
> */
> -static long mem_cgroup_read_stat(struct mem_cgroup *memcg,
> - enum mem_cgroup_stat_index idx)
> +static void mem_cgroup_read_stat_vec(struct mem_cgroup *memcg,
> + unsigned long *stat_bitmask,
> + long long *stat_vec)
> {
> - long val = 0;
> int cpu;
> + int i;
>
> get_online_cpus();
> for_each_online_cpu(cpu)
> - val += per_cpu(memcg->stat->count[idx], cpu);
> + for_each_set_bit(i, stat_bitmask, MEM_CGROUP_STAT_NSTATS)
> + stat_vec[i] += per_cpu(memcg->stat->count[i], cpu);
> +
> #ifdef CONFIG_HOTPLUG_CPU
> spin_lock(&memcg->pcp_counter_lock);
> - val += memcg->nocpu_base.count[idx];
> + for_each_set_bit(i, stat_bitmask, MEM_CGROUP_STAT_NSTATS)
> + stat_vec[i] += memcg->nocpu_base.count[i];
> spin_unlock(&memcg->pcp_counter_lock);
> #endif
> put_online_cpus();
> - return val;
> }
>
> static void mem_cgroup_swap_statistics(struct mem_cgroup *memcg,
> @@ -1674,6 +1683,7 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
> static DEFINE_MUTEX(oom_info_lock);
> struct mem_cgroup *iter;
> unsigned int i;
> + DECLARE_BITMAP(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
>
> if (!p)
> return;
> @@ -1702,16 +1712,22 @@ void mem_cgroup_print_oom_info(struct mem_cgroup *memcg, struct task_struct *p)
> res_counter_read_u64(&memcg->kmem, RES_LIMIT) >> 10,
> res_counter_read_u64(&memcg->kmem, RES_FAILCNT));
>
> + bitmap_fill(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
> + if (!do_swap_account)
> + clear_bit(MEM_CGROUP_STAT_SWAP, stat_bitmask);
> for_each_mem_cgroup_tree(iter, memcg) {
> + long long stat_vec[MEM_CGROUP_STAT_NSTATS] = {0};
> +
> pr_info("Memory cgroup stats for ");
> pr_cont_cgroup_path(iter->css.cgroup);
> pr_cont(":");
>
> + mem_cgroup_read_stat_vec(iter, stat_bitmask, stat_vec);
> for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
> if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
> continue;
> - pr_cont(" %s:%ldKB", mem_cgroup_stat_names[i],
> - K(mem_cgroup_read_stat(iter, i)));
> + pr_cont(" %s:%lldKB", mem_cgroup_stat_names[i],
> + K(stat_vec[i]));
> }
>
> for (i = 0; i < NR_LRU_LISTS; i++)
> @@ -4940,25 +4956,28 @@ out:
> return retval;
> }
>
> -
> -static unsigned long mem_cgroup_recursive_stat(struct mem_cgroup *memcg,
> - enum mem_cgroup_stat_index idx)
> +/* Callers should take care of initialize stat_vec array */
> +static void mem_cgroup_recursive_stat(struct mem_cgroup *memcg,
> + unsigned long *stat_bitmask,
> + long long *stat_vec)
> {
> struct mem_cgroup *iter;
> - long val = 0;
> + int idx;
>
> /* Per-cpu values can be negative, use a signed accumulator */
> for_each_mem_cgroup_tree(iter, memcg)
> - val += mem_cgroup_read_stat(iter, idx);
> + mem_cgroup_read_stat_vec(iter, stat_bitmask, stat_vec);
>
> - if (val < 0) /* race ? */
> - val = 0;
> - return val;
> + for_each_set_bit(idx, stat_bitmask, MEM_CGROUP_STAT_NSTATS)
> + if (stat_vec[idx] < 0) /* race ? */
> + stat_vec[idx] = 0;
> }
>
> static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> {
> u64 val;
> + DECLARE_BITMAP(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
> + long long stat_vec[MEM_CGROUP_STAT_NSTATS] = {0};
>
> if (!mem_cgroup_is_root(memcg)) {
> if (!swap)
> @@ -4967,15 +4986,21 @@ static inline u64 mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
> return res_counter_read_u64(&memcg->memsw, RES_USAGE);
> }
>
> +
> /*
> * Transparent hugepages are still accounted for in MEM_CGROUP_STAT_RSS
> * as well as in MEM_CGROUP_STAT_RSS_HUGE.
> */
> - val = mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_CACHE);
> - val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_RSS);
> -
> + bitmap_zero(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
> + set_bit(MEM_CGROUP_STAT_CACHE, stat_bitmask);
> + set_bit(MEM_CGROUP_STAT_RSS, stat_bitmask);
> if (swap)
> - val += mem_cgroup_recursive_stat(memcg, MEM_CGROUP_STAT_SWAP);
> + set_bit(MEM_CGROUP_STAT_SWAP, stat_bitmask);
> +
> + mem_cgroup_recursive_stat(memcg, stat_bitmask, stat_vec);
> +
> + val = stat_vec[MEM_CGROUP_STAT_CACHE] + stat_vec[MEM_CGROUP_STAT_RSS] +
> + (swap ? stat_vec[MEM_CGROUP_STAT_SWAP] : 0);
>
> return val << PAGE_SHIFT;
> }
> @@ -5349,12 +5374,19 @@ static int memcg_stat_show(struct seq_file *m, void *v)
> struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> struct mem_cgroup *mi;
> unsigned int i;
> + DECLARE_BITMAP(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
> + long long stat_vec[MEM_CGROUP_STAT_NSTATS] = {0};
> +
> + bitmap_fill(stat_bitmask, MEM_CGROUP_STAT_NSTATS);
> + if (!do_swap_account)
> + clear_bit(MEM_CGROUP_STAT_SWAP, stat_bitmask);
> + mem_cgroup_read_stat_vec(memcg, stat_bitmask, stat_vec);
>
> for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
> if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
> continue;
> - seq_printf(m, "%s %ld\n", mem_cgroup_stat_names[i],
> - mem_cgroup_read_stat(memcg, i) * PAGE_SIZE);
> + seq_printf(m, "%s %lld\n", mem_cgroup_stat_names[i],
> + stat_vec[i] * PAGE_SIZE);
> }
>
> for (i = 0; i < MEM_CGROUP_EVENTS_NSTATS; i++)
> @@ -5375,14 +5407,15 @@ static int memcg_stat_show(struct seq_file *m, void *v)
> memsw_limit);
> }
>
> + for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++)
> + stat_vec[i] = 0;
> + mem_cgroup_recursive_stat(memcg, stat_bitmask, stat_vec);
> for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
> - long long val = 0;
> -
> if (i == MEM_CGROUP_STAT_SWAP && !do_swap_account)
> continue;
> - for_each_mem_cgroup_tree(mi, memcg)
> - val += mem_cgroup_read_stat(mi, i) * PAGE_SIZE;
> - seq_printf(m, "total_%s %lld\n", mem_cgroup_stat_names[i], val);
> +
> + seq_printf(m, "total_%s %lld\n", mem_cgroup_stat_names[i],
> + stat_vec[i] * PAGE_SIZE);
> }
>
> for (i = 0; i < MEM_CGROUP_EVENTS_NSTATS; i++) {
> --
> 1.9.0.GIT
>
> --
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--
Michal Hocko
SUSE Labs
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