Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining

[Leonardo Brás]

On Fri, 2023-01-27 at 08:20 +0100, Michal Hocko wrote:
> On Fri 27-01-23 04:14:19, Leonardo Brás wrote:
> > On Thu, 2023-01-26 at 15:12 -0800, Roman Gushchin wrote:
> [...]
> > > I'd rather opt out of stock draining for isolated cpus: it might slightly reduce
> > > the accuracy of memory limits and slightly increase the memory footprint (all
> > > those dying memcgs...), but the impact will be limited. Actually it is limited
> > > by the number of cpus.
> > 
> > I was discussing this same idea with Marcelo yesterday morning.
> > 
> > The questions had in the topic were:
> > a - About how many pages the pcp cache will hold before draining them itself? 
> 
> MEMCG_CHARGE_BATCH (64 currently). And one more clarification. The cache
> doesn't really hold any pages. It is a mere counter of how many charges
> have been accounted for the memcg page counter. So it is not really
> consuming proportional amount of resources. It just pins the
> corresponding memcg. Have a look at consume_stock and refill_stock

I see. Thanks for pointing that out!

So in worst case scenario the memcg would have reserved 64 pages * (numcpus - 1)
that are not getting used, and may cause an 'earlier' OOM if this amount is
needed but can't be freed.

In the wave of worst case, supposing a big powerpc machine, 256 CPUs, each
holding 64k * 64 pages => 1GB memory - 4MB (one cpu using resources).
It's starting to get too big, but still ok for a machine this size.

The thing is that it can present an odd behavior: 
You have a cgroup created before, now empty, and try to run given application,
and hits OOM.
You then restart the cgroup, run the same application without an issue.

Even though it looks a good possibility, this can be perceived by user as
instability.

> 
> > b - Would it cache any kind of bigger page, or huge page in this same aspect?
> 
> The above should answer this as well as those following up I hope. If
> not let me know.

IIUC we are talking normal pages, is that it?

Best regards,
Leo