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@xxxxxxxxxx and specifically > > > > Y2TQLavnLVd4qHMT@xxxxxxxxxxxxxx). 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@xxxxxxxxxxxxxx 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
