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@xxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxx> 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@xxxxxxx> > --- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html