Re: [PATCH] memcg: make mem_cgroup_read_stat() unsigned

[Greg Thelen <gthelen@google.com>]

Andrew Morton wrote:

> On Tue, 22 Sep 2015 15:16:32 -0700 Greg Thelen <gthelen@google.com> wrote:
>
>> mem_cgroup_read_stat() returns a page count by summing per cpu page
>> counters.  The summing is racy wrt. updates, so a transient negative sum
>> is possible.  Callers don't want negative values:
>> - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback.
>> - oom reports and memory.stat shouldn't show confusing negative usage.
>> - tree_usage() already avoids negatives.
>>
>> Avoid returning negative page counts from mem_cgroup_read_stat() and
>> convert it to unsigned.
>
> Someone please remind me why this code doesn't use the existing
> percpu_counter library which solved this problem years ago.
>
>>   for_each_possible_cpu(cpu)
>
> and which doesn't iterate across offlined CPUs.

I found [1] and [2] discussing memory layout differences between:
a) existing memcg hand rolled per cpu arrays of counters
vs
b) array of generic percpu_counter
The current approach was claimed to have lower memory overhead and
better cache behavior.

I assume it's pretty straightforward to create generic
percpu_counter_array routines which memcg could use.  Possibly something
like this could be made general enough could be created to satisfy
vmstat, but less clear.

[1] http://www.spinics.net/lists/cgroups/msg06216.html
[2] https://lkml.org/lkml/2014/9/11/1057

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