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

[Greg Thelen <gthelen@google.com>]

Andrew Morton wrote:

> On Tue, 22 Sep 2015 17:42:13 -0700 Greg Thelen <gthelen@google.com> wrote:
>
>> 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
>
> That all sounds rather bogus to me.  __percpu_counter_add() doesn't
> modify struct percpu_counter at all except for when the cpu-local
> counter overflows the configured batch size.  And for the memcg
> application I suspect we can set the batch size to INT_MAX...

Nod.  The memory usage will be a bit larger, but the code reuse is
attractive.  I dusted off Vladimir's
https://lkml.org/lkml/2014/9/11/710.  Next step is to benchmark it
before posting.

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