Re: [PATCH v3 2/2] mm: memcg: introduce new event to trace shrink_memcg

[Michal Hocko <mhocko@suse.com>]

On Wed 29-11-23 19:57:52, Dmitry Rokosov wrote:
> On Wed, Nov 29, 2023 at 05:06:37PM +0100, Michal Hocko wrote:
> > On Wed 29-11-23 18:20:57, Dmitry Rokosov wrote:
> > > On Tue, Nov 28, 2023 at 10:32:50AM +0100, Michal Hocko wrote:
> > > > On Mon 27-11-23 19:16:37, Dmitry Rokosov wrote:
> > [...]
> > > > > 2) With this approach, we will not have the ability to trace a situation
> > > > > where the kernel is requesting reclaim for a specific memcg, but due to
> > > > > limits issues, we are unable to run it.
> > > > 
> > > > I do not follow. Could you be more specific please?
> > > > 
> > > 
> > > I'm referring to a situation where kswapd() or another kernel mm code
> > > requests some reclaim pages from memcg, but memcg rejects it due to
> > > limits checkers. This occurs in the shrink_node_memcgs() function.
> > 
> > Ohh, you mean reclaim protection
> > 
> > > ===
> > > 		mem_cgroup_calculate_protection(target_memcg, memcg);
> > > 
> > > 		if (mem_cgroup_below_min(target_memcg, memcg)) {
> > > 			/*
> > > 			 * Hard protection.
> > > 			 * If there is no reclaimable memory, OOM.
> > > 			 */
> > > 			continue;
> > > 		} else if (mem_cgroup_below_low(target_memcg, memcg)) {
> > > 			/*
> > > 			 * Soft protection.
> > > 			 * Respect the protection only as long as
> > > 			 * there is an unprotected supply
> > > 			 * of reclaimable memory from other cgroups.
> > > 			 */
> > > 			if (!sc->memcg_low_reclaim) {
> > > 				sc->memcg_low_skipped = 1;
> > > 				continue;
> > > 			}
> > > 			memcg_memory_event(memcg, MEMCG_LOW);
> > > 		}
> > > ===
> > > 
> > > With separate shrink begin()/end() tracepoints we can detect such
> > > problem.
> > 
> > How? You are only reporting the number of reclaimed pages and no
> > reclaimed pages could be not just because of low/min limits but
> > generally because of other reasons. You would need to report also the
> > number of scanned/isolated pages.
> >  
> 
> From my perspective, if memory control group (memcg) protection
> restrictions occur, we can identify them by the absence of the end()
> pair of begin(). Other reasons will have both tracepoints raised.

That is not really great way to detect that TBH. Trace events could be
lost and then you simply do not know what has happened.

-- 
Michal Hocko
SUSE Labs