Re: [PATCH v2] memcg: use ratelimited stats flush in the reclaim

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Aug 14, 2024 at 04:03:13PM GMT, Nhat Pham wrote:
> On Wed, Aug 14, 2024 at 9:32 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> >
> > Ccing Nhat
> >
> > On Wed, Aug 14, 2024 at 02:57:38PM GMT, Jesper Dangaard Brouer wrote:
> > > I suspect the next whac-a-mole will be the rstat flush for the slab code
> > > that kswapd also activates via shrink_slab, that via
> > > shrinker->count_objects() invoke count_shadow_nodes().
> > >
> >
> > Actually count_shadow_nodes() is already using ratelimited version.
> > However zswap_shrinker_count() is still using the sync version. Nhat is
> > modifying this code at the moment and we can ask if we really need most
> > accurate values for MEMCG_ZSWAP_B and MEMCG_ZSWAPPED for the zswap
> > writeback heuristic.
> 
> You are referring to this, correct:
> 
> mem_cgroup_flush_stats(memcg);
> nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT;
> nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED);
> 
> It's already a bit less-than-accurate - as you pointed out in another
> discussion, it takes into account the objects and sizes of the entire
> subtree, rather than just the ones charged to the current (memcg,
> node) combo. Feel free to optimize this away!
> 
> In fact, I should probably replace this with another (atomic?) counter
> in zswap_lruvec_state struct, which tracks the post-compression size.
> That way, we'll have a better estimate of the compression factor -
> total post-compression size /  (length of LRU * page size), and
> perhaps avoid the whole stat flushing path altogether...
> 

That sounds like much better solution than relying on rstat for accurate
stats.