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

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, Jan 27, 2023 at 04:29:37PM -0300, Leonardo Brás wrote:
> 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?

It can happen now as well, you just need a competing drain request.

Honestly, I feel the probability of this scenario to be a real problem is fairly low.
I don't recall any complains on spurious OOMs because of races in the draining code.
Usually machines which are tight on memory are rarely have so many idle cpus.

Thanks!