Re: Access rules for current->memcg

[Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Tue 21-07-15 13:32:23, Nikolay Borisov wrote:
> 
> 
> On 07/21/2015 12:48 PM, Michal Hocko wrote:
> > On Tue 21-07-15 12:08:40, Nikolay Borisov wrote:
[...]
> >> I looked more carefully into the code and saw that the page_counters
> >> (which back memory/memsw.limit/usage_in_bytes) are charged during the
> >> try_charge whereas the per-cpu statistics (which back the info in
> >> memory.stats) are updated after committing the charge. I assume in the
> >> case where charges are not canceled the data in memory.stats and
> >> memory.memsw.max_usage_in_bytes - memory.usage_in_bytes should be identical?
> > 
> > I am not sure what you mean here. max_usage_in_bytes is a historical
> > value which was the maximum charge used at some point in time.
> > usage_in_bytes is always the _current_ value of the charge counter.
> > max_usage_in_bytes-usage_in_bytes doesn't tell you much really.
> > 
> 
> I have misunderstood me I have never, ever referred to max_usage. What I
> meant was that the information that memory.stat file provides is
> acquired by reading the memcg->stat->count[counter] and those values are
> updated when mem_cgroup_commit_charge() is invoked.

True

> And
> mem_cgroup_commit_charge is invoked AFTER mem_cgroup_try_charge, which
> updates the charge counters.

Still true and you should realize that the commit is called very shortly
after the charge. The race window is not really interesting for anything
practical.

> So my point was that whether I read the
> swap value (for example) from the memory.stats file or whether I
> manually do the maths with subtraction  as previously shown the 2 values
> should match.

Your subtraction simply doesn't work and doesn't tell you how much
memory is swapped out from the memcg as explained in other email.

> Essentially whether information should be queried form the charge
> counter or from the mem_cgroup_stat_cpu struct. Does that make sense?

memory.stat will tell you the information you are looking for. You
simply cannot calculate those numbers from the counters.
-- 
Michal Hocko
SUSE Labs