Re: [PATCH V2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes

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

On Mon, Jun 24, 2024 at 05:46:05AM GMT, Yosry Ahmed wrote:
> On Mon, Jun 24, 2024 at 4:55 AM Jesper Dangaard Brouer <hawk@kernel.org> wrote:
> 
[...]
> I am assuming this supersedes your other patch titled "[PATCH RFC]
> cgroup/rstat: avoid thundering herd problem on root cgrp", so I will
> only respond here.
> 
> I have two comments:
> - There is no reason why this should be limited to the root cgroup. We
> can keep track of the cgroup being flushed, and use
> cgroup_is_descendant() to find out if the cgroup we want to flush is a
> descendant of it. We can use a pointer and cmpxchg primitives instead
> of the atomic here IIUC.
> 
> - More importantly, I am not a fan of skipping the flush if there is
> an ongoing one. For all we know, the ongoing flush could have just
> started and the stats have not been flushed yet. This is another
> example of non deterministic behavior that could be difficult to
> debug.

Even with the flush, there will almost always per-cpu updates which will
be missed. This can not be fixed unless we block the stats updaters as
well (which is not going to happen). So, we are already ok with this
level of non-determinism. Why skipping flushing would be worse? One may
argue 'time window is smaller' but this still does not cap the amount of
updates. So, unless there is concrete data that this skipping flushing
is detrimental to the users of stats, I don't see an issue in the
presense of periodic flusher.

> 
> I tried a similar approach before where we sleep and wait for the
> ongoing flush to complete instead, without contending on the lock,
> using completions [1]. Although that patch has a lot of complexity,

We can definitely add complexity but only if there are no simple good
enough mitigations.