Re: [PATCH v2 2/2] mm: memcg: introduce new event to trace shrink_memcg

[Dmitry Rokosov <ddrokosov@salutedevices.com>]

On Wed, Nov 22, 2023 at 02:24:59PM +0100, Michal Hocko wrote:
> On Wed 22-11-23 13:58:36, Dmitry Rokosov wrote:
> > Hello Michal,
> > 
> > Thank you for the quick review!
> > 
> > On Wed, Nov 22, 2023 at 11:23:24AM +0100, Michal Hocko wrote:
> > > On Wed 22-11-23 13:01:56, Dmitry Rokosov wrote:
> > > > The shrink_memcg flow plays a crucial role in memcg reclamation.
> > > > Currently, it is not possible to trace this point from non-direct
> > > > reclaim paths.
> > > 
> > > Is this really true? AFAICS we have
> > > mm_vmscan_lru_isolate
> > > mm_vmscan_lru_shrink_active
> > > mm_vmscan_lru_shrink_inactive
> > > 
> > > which are in the vry core of the memory reclaim. Sure post processing
> > > those is some work.
> > 
> > Sure, you are absolutely right. In the usual scenario, the memcg
> > shrinker utilizes two sub-shrinkers: slab and LRU. We can enable the
> > tracepoints you mentioned and analyze them. However, there is one
> > potential issue. Enabling these tracepoints will trigger the reclaim
> > events show for all pages. Although we can filter them per pid, we
> > cannot filter them per cgroup. Nevertheless, there are times when it
> > would be extremely beneficial to comprehend the effectiveness of the
> > reclaim process within the relevant cgroup. For this reason, I am adding
> > the cgroup name to the memcg tracepoints and implementing a cumulative
> > tracepoint for memcg shrink (LRU + slab)."
> 
> I can see how printing memcg in mm_vmscan_memcg_reclaim_begin makes it
> easier to postprocess per memcg reclaim. But you could do that just by
> adding that to mm_vmscan_memcg_reclaim_{begin, end}, no? Why exactly
> does this matter for kswapd and other global reclaim contexts? 

From my point of view, kswapd and other non-direct reclaim paths are
important for memcg analysis because they also influence the memcg
reclaim statistics.

The tracepoint mm_vmscan_memcg_reclaim_{begin, end} is called from the
direct memcg reclaim flow, such as:
    - a direct write to the 'reclaim' node
    - changing 'max' and 'high' thresholds
    - raising the 'force_empty' mechanism
    - the charge path
    - etc.

However, it doesn't cover global reclaim contexts, so it doesn't provide
us with the full memcg reclaim statistics.

-- 
Thank you,
Dmitry