Re: [PATCH v1 3/3] cgroup/rstat: introduce ratelimited rstat flushing

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Apr 18, 2024 at 2:15 PM Tejun Heo <tj@kernel.org> wrote:
>
> Hello, Yosry.
>
> On Thu, Apr 18, 2024 at 02:00:28PM -0700, Yosry Ahmed wrote:
> ...
> > I think this is an artifact of different subsystems sharing the same
> > rstat tree for no specific reason. I think almost all flushing calls
> > really need the stats from one subsystem after all.
> >
> > If we have separate trees, lock contention gets slightly better as
> > different subsystems do not compete. We can also have different
> > subsystems "customize" their trees, for e.g. by setting different
> > time-based or magnitude-based rate-limiting thresholds.
> >
> > I know this is a bigger lift, just thinking out loud :)
>
> I have no objection to separating out rstat trees so that it has
> per-controller tracking. However, the high frequency source of updates are
> cpu and memory, which tend to fire together, and the only really high
> frequency consumer seems to be memory, so I'm not too sure how much benefit
> separating the trees out would bring.

Well, we could split the global lock into multiple ones, which isn't
really a solution, but it would help other controllers not to be
affected by the high frequency of flushing from the memory controller
(which has its own thresholding).

For updates, cpu and memory would use separate percpu locks as well,
which may help slightly.

Outside of this, I think it helps us add controller-specific
optimizations. For example, I tried to generalize the thresholding
code in the memory controller and put it in the rstat code, but I
couldn't really have a single value representing the "pending stats"
from all controllers. It's impossible to compare memory stats (mostly
in pages or bytes) to cpu time stats for instance.

Similarly, with this proposal from Jesper (which I am not saying I am
agreeing with :P), instead of having time-based ratelimiting in both
the rstat code and the memcg code to support different thresholds, we
could have the memory controller set a different threshold for itself.

So perhaps the lock breakdowns are not enough motivation, but if we
start generalizing optimizations in some controllers, we may want to
split the tree for flexibility.