linux-kernel - Re: [PATCH V10] cgroup/rstat: Avoid flushing if there is an ongoing root flush

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Message-ID: <CAKEwX=PcK=kJG-yxaoTYvJGNwQ=eTGo1m=ZraqYy1SyLDs9Asw@mail.gmail.com>
Date: Thu, 12 Sep 2024 11:24:59 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Jesper Dangaard Brouer <hawk@...nel.org>, tj@...nel.org, cgroups@...r.kernel.org, 
	shakeel.butt@...ux.dev, hannes@...xchg.org, lizefan.x@...edance.com, 
	longman@...hat.com, kernel-team@...udflare.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, mfleming@...udflare.com, 
	joshua.hahnjy@...il.com
Subject: Re: [PATCH V10] cgroup/rstat: Avoid flushing if there is an ongoing
 root flush

On Thu, Sep 12, 2024 at 10:28 AM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> >
> > I'm not, but Joshua from my team is working on it :)
>
> Great, thanks for letting me know!

FWIW, I think the zswap_shrinker_count() path is fairly trivial to
take care of :)  We only need the stats itself, and you don't even
need any tree traversal tbh - technically it is most accurate to track
zswap memory usage of the memcg itself - one atomic counter per
zswap_lruvec_struct should suffice.

obj_cgroup_may_zswap() could be more troublesome - we need the entire
subtree data to make the decision, at each level :) How about this:

1. Add a per-memcg counter to track zswap memory usage.

2. At obj_cgroup_may_zswap() time, the logic is unchanged - we
traverse the tree from current memcg to root memcg, grabbing the
memcg's counter and check for usage.

3. At obj_cgroup_charge_zswap() time, we have to perform another
upward traversal again, to increment the counters. Would this be too
expensive?

We still need the whole obj_cgroup charging spiel, for memory usage
purposes, but this should allow us to remove the MEMCG_ZSWAP_B.
Similarly, another set of counters can be introduced to remove
MEMCG_ZSWAPPED...

Yosry, Joshua, how do you feel about this design? Step 3 is the part
where I'm least certain about, but it's the only way I can think of
that would avoid any flushing action. You have to pay the price of
stat updates at *some* point :)