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

[Dmitry Rokosov]

On Mon, Nov 27, 2023 at 10:33:49AM +0100, Michal Hocko wrote:
> On Thu 23-11-23 22:39:37, 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. However, direct reclaim has its own tracepoint, so there
> > is no issue there. In certain cases, when debugging memcg pressure,
> > developers may need to identify all potential requests for memcg
> > reclamation including kswapd(). The patchset introduces the tracepoints
> > mm_vmscan_memcg_shrink_{begin|end}() to address this problem.
> > 
> > Example of output in the kswapd context (non-direct reclaim):
> >     kswapd0-39      [001] .....   240.356378: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
> >     kswapd0-39      [001] .....   240.356396: mm_vmscan_memcg_shrink_end: nr_reclaimed=0 memcg=16
> >     kswapd0-39      [001] .....   240.356420: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
> >     kswapd0-39      [001] .....   240.356454: mm_vmscan_memcg_shrink_end: nr_reclaimed=1 memcg=16
> >     kswapd0-39      [001] .....   240.356479: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
> >     kswapd0-39      [001] .....   240.356506: mm_vmscan_memcg_shrink_end: nr_reclaimed=4 memcg=16
> >     kswapd0-39      [001] .....   240.356525: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
> >     kswapd0-39      [001] .....   240.356593: mm_vmscan_memcg_shrink_end: nr_reclaimed=11 memcg=16
> >     kswapd0-39      [001] .....   240.356614: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
> >     kswapd0-39      [001] .....   240.356738: mm_vmscan_memcg_shrink_end: nr_reclaimed=25 memcg=16
> >     kswapd0-39      [001] .....   240.356790: mm_vmscan_memcg_shrink_begin: order=0 gfp_flags=GFP_KERNEL memcg=16
> >     kswapd0-39      [001] .....   240.357125: mm_vmscan_memcg_shrink_end: nr_reclaimed=53 memcg=16
> 
> In the previous version I have asked why do we need this specific
> tracepoint when we already do have trace_mm_vmscan_lru_shrink_{in}active
> which already give you a very good insight. That includes the number of
> reclaimed pages but also more. I do see that we do not include memcg id
> of the reclaimed LRU, but that shouldn't be a big problem to add, no?

>From my point of view, memcg reclaim includes two points: LRU shrink and
slab shrink, as mentioned in the vmscan.c file.


static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
...
		reclaimed = sc->nr_reclaimed;
		scanned = sc->nr_scanned;

		shrink_lruvec(lruvec, sc);

		shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
			    sc->priority);
...

So, both of these operations are important for understanding whether
memcg reclaiming was successful or not, as well as its effectiveness. I
believe it would be beneficial to summarize them, which is why I have
created new tracepoints.

-- 
Thank you,
Dmitry