lkml.org 
[lkml]   [2015]   [Sep]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] memcg: make mem_cgroup_read_stat() unsigned
Date

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.


\
 
 \ /
  Last update: 2015-09-23 09:41    [W:0.041 / U:0.528 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site