Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Aug 18, 2023 at 2:40 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Wed, Aug 16, 2023 at 3:35 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> > On Wed, Aug 16, 2023 at 12:08 PM Tejun Heo <tj@kernel.org> wrote:
> > >
> > > Hello,
> > >
> > > On Wed, Aug 16, 2023 at 10:11:20AM -0700, Shakeel Butt wrote:
> > > > These options are not white and black and there can be something in
> > > > between but let me be very clear on what I don't want and would NACK.
> > >
> > > I'm not a big fan of interfaces with hidden states. What you're proposing
> > > isn't strictly that but it's still a bit nasty. So, if we can get by without
> > > doing that, that'd be great.
> >
> > Agreed. I will try to send patches soon implementing option (2) above,
> > basically removing unified flushing. I will try to document any
> > potential regressions that may happen and how we may fix them. Ideally
> > we see no regressions.
> >
> > >
> > > > I don't want a global sleepable lock which can be taken by potentially
> > > > any application running on the system. We have seen similar global
> > > > locks causing isolation and priority inversion issues in production.
> > > > So, not another lock which needs to be taken under extreme condition
> > > > (reading stats under OOM) by a high priority task (node controller)
> > > > and might be held by a low priority task.
> > >
> > > Yeah, this is a real concern. Those priority inversions do occur and can be
> > > serious but causing serious problems under memory pressure usually requires
> > > involving memory allocations and IOs. Here, it's just all CPU. So, at least
> > > in OOM conditions, this shouldn't be in the way (the system wouldn't have
> > > anything else to do anyway).
> > >
> > > It is true that this still can lead to priority through CPU competition tho.
> > > However, that problem isn't necessarily solved by what you're suggesting
> > > either unless you want to restrict explicit flushing based on permissions
> > > which is another can of worms.
> >
> > Right. Also in the case of a mutex, if we disable preemption while
> > holding the mutex, this makes sure that whoever holding the mutex does
> > not starve waiters. Essentially the difference would be that waiters
> > will sleep with the mutex instead of spinning, but the mutex holder
> > itself wouldn't sleep.
> >
> > I will make this a separate patch, just in case it's too
> > controversial. Switching the spinlock to a mutex should not block
> > removing unified flushing.
> >
> > >
> > > My preference is not exposing this in user interface. This is mostly arising
> > > from internal implementation details and isn't what users necessarily care
> > > about. There are many things we can do on the kernel side to make trade-offs
> > > among overhead, staleness and priority inversions. If we make this an
> > > explicit userland interface behavior, we get locked into that semantics
> > > which we'll likely regret in some future.
> > >
> >
> > Yeah that's what I am trying to do here as well.  I will try to follow
> > up on this discussion with patches soon.
> >
> > Thanks everyone!
>
> So status update. I tried implementing removing unified flushing. I
> can send the patches if it helps visualize things, but essentially
> mem_cgroup_flush_stats{_ratelimited} takes a memcg argument that it
> passes to cgroup_flush_stats(). No skipping if someone else is
> flushing (stats_flush_ongoing) and no threshold at which we start
> flushing (stats_flush_threshold).
>
> I tested this by some synthetic reclaim stress test that I wrote,
> because reclaim is the easiest way to stress in-kernel flushing. I
> basically create 10s or 100s cgroups and run workers in them that keep
> allocating memory beyond the limit. I also run workers that
> periodically read the stats.
>
> Removing unified flushing makes such a synthetic benchmark 2-3 times
> slower. This is not surprising, as all these concurrent reclaimers
> used to just skip flushing, regardless of whether or not they get
> accurate stats. Now I don't know how realistic such a benchmark is,
> but if there's a workload somewhere that runs into such conditions it
> will surely regress.
>
> Sorry if the above is difficult to visualize. I can send the patches
> and the test script if it makes things easier, I just didn't want to
> overwhelm the thread.
>
> I think there are 2 options going forward:
> (a) If we believe removing unified flushing is the best way forward
> for most use cases, then we need to find a better way of testing this
> approach practically. Any suggestions here are useful.
>
> (b) Decide that it's too expensive to remove unified flushing
> completely, at least for in-kernel flushers that can see a lot of
> concurrency. We can only remove unified flushing for userspace reads.
> Only flush the memcg being read and never skip (essentially what Ivan
> tried). There are already some flushing contexts that do this (e.g.
> zswap memcg charging code). I believe as long as there isn't a lot of
> concurrency in these paths it should be fine to skip unified flushing
> for them, and keep it only for in-kernel flushers.

I sent a short series implementing (b) above:
https://lore.kernel.org/lkml/20230821205458.1764662-1-yosryahmed@google.com/

Basic testing shows that it works well.