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

[Michal Hocko]

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.
 
> > > 3) LRU and SLAB shrinkers are too common places to handle memcg-related
> > > tasks. Additionally, memcg can be disabled in the kernel configuration.
> > 
> > Right. This could be all hidden in the tracing code. You simply do not
> > print memcg id when the controller is disabled. Or just simply print 0.
> > I do not really see any major problems with that.
> > 
> > I would really prefer to focus on that direction rather than adding
> > another begin/end tracepoint which overalaps with existing begin/end
> > traces and provides much more limited information because I would bet we
> > will have somebody complaining that mere nr_reclaimed is not sufficient.
> 
> Okay, I will try to prepare a new patch version with memcg printing from
> lruvec and slab tracepoints.
> 
> Then Andrew should drop the previous patchsets, I suppose. Please advise
> on the correct workflow steps here.

Andrew usually just drops the patch from his tree and it will disappaer
from the linux-next as well.
-- 
Michal Hocko
SUSE Labs