Re: [PATCH] Revert "mm:vmscan: fix inaccurate reclaim during proactive reclaim"

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On Tue, Jan 23, 2024 at 9:48 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> The revert isn't a straight-forward solution.
> The patch you're reverting fixed conventional reclaim and broke
> MGLRU. Your revert fixes MGLRU and breaks conventional reclaim.

This is not true -- the patch reverted regressed the active/inactive
LRU too, on execution time.

Quoting the commit message: "completion times for proactive reclaim on
much smaller non-root cgroups take ~30% longer (with or without

And I wouldn't call the original patch a fix -- it shifted the problem
from space to time, which at best is a tradeoff.

> On Tue, Jan 23, 2024 at 05:58:05AM -0800, T.J. Mercier wrote:
> > They both are able to make progress. The main difference is that a
> > single iteration of try_to_free_mem_cgroup_pages with MGLRU ends soon
> > after it reclaims nr_to_reclaim, and before it touches all memcgs. So
> > a single iteration really will reclaim only about SWAP_CLUSTER_MAX-ish
> > pages with MGLRU. WIthout MGLRU the memcg walk is not aborted
> > immediately after nr_to_reclaim is reached, so a single call to
> > try_to_free_mem_cgroup_pages can actually reclaim thousands of pages
> > even when sc->nr_to_reclaim is 32. (I.E. MGLRU overreclaims less.)
> >
> Is that a feature or a bug?
>  * 1. Memcg LRU only applies to global reclaim, and the round-robin incrementing
>  *    of their max_seq counters ensures the eventual fairness to all eligible
>  *    memcgs. For memcg reclaim, it still relies on mem_cgroup_iter().
> If it bails out exactly after nr_to_reclaim, it'll overreclaim
> less. But with steady reclaim in a complex subtree, it will always hit
> the first cgroup returned by mem_cgroup_iter() and then bail. This
> seems like a fairness issue.
> We should figure out what the right method for balancing fairness with
> overreclaim is, regardless of reclaim implementation. Because having
> two different approaches and reverting dependent things back and forth
> doesn't make sense.
> Using an LRU to rotate through memcgs over multiple reclaim cycles
> seems like a good idea. Why is this specific to MGLRU? Shouldn't this
> be a generic piece of memcg infrastructure?
> Then there is the question of why there is an LRU for global reclaim,
> but not for subtree reclaim. Reclaiming a container with multiple
> subtrees would benefit from the fairness provided by a container-level
> LRU order just as much; having fairness for root but not for subtrees
> would produce different reclaim and pressure behavior, and can cause
> regressions when moving a service from bare-metal into a container.
> Figuring out these differences and converging on a method for cgroup
> fairness would be the better way of fixing this. Because of the
> regression risk to the default reclaim implementation, I'm inclined to
> NAK this revert.

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