Re: [PATCH 01/19] memcg: make mem_cgroup_read_{stat|event}() iterate possible cpus instead of online

Michal Hocko
2015-06-17T14:41:43Z

On Fri 22-05-15 18:23:18, Tejun Heo wrote:
> cpu_possible_mask represents the CPUs which are actually possible
> during that boot instance. For systems which don't support CPU
> hotplug, this will match cpu_online_mask exactly in most cases. Even
> for systems which support CPU hotplug, the number of possible CPU
> slots is highly unlikely to diverge greatly from the number of online
> CPUs. The only cases where the difference between possible and online
> caused problems were when the boot code failed to initialize the
> possible mask and left it fully set at NR_CPUS - 1.
>
> As such, most per-cpu constructs allocate for all possible CPUs and
> often iterate over the possibles, which also has the benefit of
> avoiding the blocking CPU hotplug synchronization.
>
> memcg open codes per-cpu stat counting for mem_cgroup_read_stat() and
> mem_cgroup_read_events(), which iterates over online CPUs and handles
> CPU hotplug operations explicitly. This complexity doesn't actually
> buy anything. Switch to iterating over the possibles and drop the
> explicit CPU hotplug handling.
>
> Eventually, we want to convert memcg to use percpu_counter instead of
> its own custom implementation which also benefits from quick access
> w/o summing for cases where larger error margin is acceptable.
>
> This will allow mem_cgroup_read_stat() to be called from non-sleepable
> contexts which will be used by cgroup writeback.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>

I am sorry for being late in this thread.

I have seen systems where the number of possible CPUs was really high
wrt. online ones but I wouldn't worry about them. The change is an
overal improvement for usual configurations though.

Acked-by: Michal Hocko <mhocko@suse.cz>
> ---
> mm/memcontrol.c | 51 ++-------------------------------------------------
> 1 file changed, 2 insertions(+), 49 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6732c2c..d7d270a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -324,11 +324,6 @@ struct mem_cgroup {
> * percpu counter.
> */
> struct mem_cgroup_stat_cpu __percpu *stat;
> - /*
> - * used when a cpu is offlined or other synchronizations
> - * See mem_cgroup_read_stat().
> - */
> - struct mem_cgroup_stat_cpu nocpu_base;
> spinlock_t pcp_counter_lock;
>
> #if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_INET)
> @@ -815,15 +810,8 @@ static long mem_cgroup_read_stat(struct mem_cgroup *memcg,
> long val = 0;
> int cpu;
>
> - get_online_cpus();
> - for_each_online_cpu(cpu)
> + for_each_possible_cpu(cpu)
> val += per_cpu(memcg->stat->count[idx], cpu);
> -#ifdef CONFIG_HOTPLUG_CPU
> - spin_lock(&memcg->pcp_counter_lock);
> - val += memcg->nocpu_base.count[idx];
> - spin_unlock(&memcg->pcp_counter_lock);
> -#endif
> - put_online_cpus();
> return val;
> }
>
> @@ -833,15 +821,8 @@ static unsigned long mem_cgroup_read_events(struct mem_cgroup *memcg,
> unsigned long val = 0;
> int cpu;
>
> - get_online_cpus();
> - for_each_online_cpu(cpu)
> + for_each_possible_cpu(cpu)
> val += per_cpu(memcg->stat->events[idx], cpu);
> -#ifdef CONFIG_HOTPLUG_CPU
> - spin_lock(&memcg->pcp_counter_lock);
> - val += memcg->nocpu_base.events[idx];
> - spin_unlock(&memcg->pcp_counter_lock);
> -#endif
> - put_online_cpus();
> return val;
> }
>
> @@ -2191,37 +2172,12 @@ static void drain_all_stock(struct mem_cgroup *root_memcg)
> mutex_unlock(&percpu_charge_mutex);
> }
>
> -/*
> - * This function drains percpu counter value from DEAD cpu and
> - * move it to local cpu. Note that this function can be preempted.
> - */
> -static void mem_cgroup_drain_pcp_counter(struct mem_cgroup *memcg, int cpu)
> -{
> - int i;
> -
> - spin_lock(&memcg->pcp_counter_lock);
> - for (i = 0; i < MEM_CGROUP_STAT_NSTATS; i++) {
> - long x = per_cpu(memcg->stat->count[i], cpu);
> -
> - per_cpu(memcg->stat->count[i], cpu) = 0;
> - memcg->nocpu_base.count[i] += x;
> - }
> - for (i = 0; i < MEM_CGROUP_EVENTS_NSTATS; i++) {
> - unsigned long x = per_cpu(memcg->stat->events[i], cpu);
> -
> - per_cpu(memcg->stat->events[i], cpu) = 0;
> - memcg->nocpu_base.events[i] += x;
> - }
> - spin_unlock(&memcg->pcp_counter_lock);
> -}
> -
> static int memcg_cpu_hotplug_callback(struct notifier_block *nb,
> unsigned long action,
> void *hcpu)
> {
> int cpu = (unsigned long)hcpu;
> struct memcg_stock_pcp *stock;
> - struct mem_cgroup *iter;
>
> if (action == CPU_ONLINE)
> return NOTIFY_OK;
> @@ -2229,9 +2185,6 @@ static int memcg_cpu_hotplug_callback(struct notifier_block *nb,
> if (action != CPU_DEAD && action != CPU_DEAD_FROZEN)
> return NOTIFY_OK;
>
> - for_each_mem_cgroup(iter)
> - mem_cgroup_drain_pcp_counter(iter, cpu);
> -
> stock = &per_cpu(memcg_stock, cpu);
> drain_stock(stock);
> return NOTIFY_OK;
> --
> 2.4.0
>

--
Michal Hocko
SUSE Labs