Re: Expensive memory.stat + cpu.stat reads

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Aug 11, 2023 at 05:01:08PM -0700, Yosry Ahmed wrote:
> There have been a lot of problems coming from this global rstat lock:
> hard lockups (when we used to flush atomically), unified flushing
> being expensive, skipping flushing being inaccurate, etc.
> 
> I wonder if it's time to rethink this lock and break it down into
> granular locks. Perhaps a per-cgroup lock, and develop a locking
> scheme where you always lock a parent then a child, then flush the
> child and unlock it and move to the next child, etc. This will allow
> concurrent flushing of non-root cgroups. Even when flushing the root,
> if we flush all its children first without locking the root, then only
> lock the root when flushing the top-level children, then some level of
> concurrency can be achieved.
> 
> Maybe this is too complicated, I never tried to implement it, but I
> have been bouncing around this idea in my head for a while now.
> 
> We can also split the update tree per controller. As far as I can tell
> there is no reason to flush cpu stats for example when someone wants
> to read memory stats.

There's another thread. Let's continue there but I'm a bit skeptical whether
splitting the lock is a good solution here. Regardless of locking, we don't
want to run in an atomic context for that long anwyay.

Thanks.

-- 
tejun