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

[Michal Hocko <mhocko@suse.com>]

On Thu 26-01-23 21:32:18, Marcelo Tosatti wrote:
> On Thu, Jan 26, 2023 at 08:20:46PM +0100, Michal Hocko wrote:
> > On Thu 26-01-23 15:03:43, Marcelo Tosatti wrote:
> > > On Thu, Jan 26, 2023 at 08:41:34AM +0100, Michal Hocko wrote:
> > > > On Wed 25-01-23 15:14:48, Roman Gushchin wrote:
> > > > > On Wed, Jan 25, 2023 at 03:22:00PM -0300, Marcelo Tosatti wrote:
> > > > > > On Wed, Jan 25, 2023 at 08:06:46AM -0300, Leonardo Brás wrote:
> > > > > > > On Wed, 2023-01-25 at 09:33 +0100, Michal Hocko wrote:
> > > > > > > > On Wed 25-01-23 04:34:57, Leonardo Bras wrote:
> > > > > > > > > Disclaimer:
> > > > > > > > > a - The cover letter got bigger than expected, so I had to split it in
> > > > > > > > >     sections to better organize myself. I am not very confortable with it.
> > > > > > > > > b - Performance numbers below did not include patch 5/5 (Remove flags
> > > > > > > > >     from memcg_stock_pcp), which could further improve performance for
> > > > > > > > >     drain_all_stock(), but I could only notice the optimization at the
> > > > > > > > >     last minute.
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > 0 - Motivation:
> > > > > > > > > On current codebase, when drain_all_stock() is ran, it will schedule a
> > > > > > > > > drain_local_stock() for each cpu that has a percpu stock associated with a
> > > > > > > > > descendant of a given root_memcg.
> > > > > 
> > > > > Do you know what caused those drain_all_stock() calls? I wonder if we should look
> > > > > into why we have many of them and whether we really need them?
> > > > > 
> > > > > It's either some user's actions (e.g. reducing memory.max), either some memcg
> > > > > is entering pre-oom conditions. In the latter case a lot of drain calls can be
> > > > > scheduled without a good reason (assuming the cgroup contain multiple tasks running
> > > > > on multiple cpus).
> > > > 
> > > > I believe I've never got a specific answer to that. We
> > > > have discussed that in the previous version submission
> > > > (20221102020243.522358-1-leobras@redhat.com and specifically
> > > > Y2TQLavnLVd4qHMT@dhcp22.suse.cz). Leonardo has mentioned a mix of RT and
> > > > isolcpus. I was wondering about using memcgs in RT workloads because
> > > > that just sounds weird but let's say this is the case indeed. 
> > > 
> > > This could be the case. You can consider an "edge device" where it is
> > > necessary to run a RT workload. It might also be useful to run 
> > > non realtime applications on the same system.
> > > 
> > > > Then an RT task or whatever task that is running on an isolated
> > > > cpu can have pcp charges.
> > > 
> > > Usually the RT task (or more specifically the realtime sensitive loop
> > > of the application) runs entirely on userspace. But i suppose there
> > > could be charges on application startup.
> > 
> > What is the role of memcg then? If the memory limit is in place and the
> > workload doesn't fit in then it will get reclaimed during start up and
> > memory would need to be refaulted if not mlocked. If it is mlocked then
> > the limit cannot be enforced and the start up would likely fail as a
> > result of the memcg oom killer.
> 
> 1) Application which is not time sensitive executes on isolated CPU,
> with memcg control enabled. Per-CPU stock is created.
> 
> 2) App with memcg control enabled exits, per-CPU stock is not drained.
> 
> 3) Latency sensitive application starts, isolated per-CPU has stock to
> be drained, and:
> 
> /*
>  * Drains all per-CPU charge caches for given root_memcg resp. subtree
>  * of the hierarchy under it.
>  */
> static void drain_all_stock(struct mem_cgroup *root_memcg)

No, this is not really answering my question. See
Y9LQ615H13RmG7wL@dhcp22.suse.cz which already explains how the draining
would be triggered. This is not really happening on any operation.

I am really asking for specific workloads which are running multiple
processes on a mix of isolated and non-isolated cpus yet they share
memcg so that they can interfere. The consequences of the common memcg
are described above.
-- 
Michal Hocko
SUSE Labs