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

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, May 22, 2015 at 06:23:18PM -0400, 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>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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