Re: [PATCH] memcg: convert threshold to bytes

[Michal Hocko <mhocko@kernel.org>]

On Wed 07-10-15 00:58:20, Andrew Morton wrote:
> On Wed, 7 Oct 2015 09:30:02 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> 
> > On Tue 06-10-15 12:22:25, Andrew Morton wrote:
> > > On Tue, 6 Oct 2015 19:01:23 +0200 Michal Hocko <mhocko@kernel.org> wrote:
> > > 
> > > > On Mon 05-10-15 14:44:22, Shaohua Li wrote:
> > > > > The page_counter_memparse() returns pages for the threshold, while
> > > > > mem_cgroup_usage() returns bytes for memory usage. Convert the threshold
> > > > > to bytes.
> > > > > 
> > > > > Looks a regression introduced by 3e32cb2e0a12b69150
> > > > 
> > > > Yes. This suggests
> > > > Cc: stable # 3.19+
> > > 
> > > But it's been this way for 2 years and nobody noticed it.  How come?
> > 
> > Maybe we do not have that many users of this API with newer kernels.
> 
> Either it's zero or all the users have worked around this bug.
> 
> > > Or at least, nobody reported it.  Maybe people *have* noticed it, and
> > > adjusted their userspace appropriately.  In which case this patch will
> > > cause breakage.
> > 
> > I dunno, I would rather have it fixed than keep bug to bug compatibility
> > because they would eventually move to a newer kernel one day when they
> > see the "breakage" anyway.
> 
> They'd only see breakage if we fixed this in the newer kernel.
> 
> We could just change the docs and leave it as-is.  That it is called
> "usage_in_bytes" makes that a bit awkward.

The whole API is bytes based. Having one which is silently page size
based is definitely wrong.
 
> A bit of googling indicates that people are using usage_in_bytes.  A
> few.  All the discussions I found clearly predate this bug.
>
> 
> So did people just stop using this?

To be honest I haven't seen any real users from my enterprise
distribution experience and I also consider the API quite unusable
because most loads simply fill up their limit with the page case so
something like a vmpressure is a much better indicator of the memory
usage.

This has been introduced before my time. Kirill has introduced it back
in 2009.

> Is there some alternative way of getting the same info?

I am not aware of any alternative nor am I aware of any strong usecases
for such a small granularity API. The consensus so far has been that any
new controller knob for the new cgrroup API has to be backed by a strong
usecase.

I am pretty sure that we want some form of memory pressure notification,
though.

> Why does memcg_write_event_control() says "DO NOT USE IN NEW FILES"
> and "DO NOT ADD NEW FILES"?

Because eventfd notification mechanism is considered a wrong API to
convey notifications. New cgroup API is supposed to use a new mechanism.
The current one will have to live with the old/legacy cgroup API though.

-- 
Michal Hocko
SUSE Labs

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