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

["Leonardo Brás" <leobras@redhat.com>]

On Fri, 2023-01-27 at 10:29 +0100, Michal Hocko wrote:
> On Fri 27-01-23 04:35:22, Leonardo Brás wrote:
> > 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)
> 
> s@numcpus@num_isolated_cpus@

I was thinking worst case scenario being (ncpus - 1) being isolated.

> 
> > that are not getting used, and may cause an 'earlier' OOM if this amount is
> > needed but can't be freed.
> 
> s@OOM@memcg OOM@
 
> > 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.
> 
> It is more about the memcg limit rather than the size of the machine.
> Again, let's focus on actual usacase. What is the usual memcg setup with
> those isolcpus

I understand it's about the limit, not actually allocated memory. When I point
the machine size, I mean what is expected to be acceptable from a user in that
machine.

> 
> > 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.
> 
> The application would either consume those cached charges or flush them
> if it is running in a different memcg. Or what do you have in mind?

1 - Create a memcg with a VM inside, multiple vcpus pinned to isolated cpus. 
2 - Run multi-cpu task inside the VM, it allocates memory for every CPU and keep
    the pcp cache
3 - Try to run a single-cpu task (pinned?) inside the VM, which uses almost all
    the available memory.
4 - memcg OOM.

Does it make sense?


> 
> > 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?
> 
> We are talking about memcg charges and those have page granularity.
> 

Thanks for the info!

Also, thanks for the feedback!
Leo