Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim stats

[Yosry Ahmed <yosryahmed@google.com>]

On Fri, Oct 28, 2022 at 7:39 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Oct 27, 2022 at 01:43:24PM -0700, Yosry Ahmed wrote:
> > On Thu, Oct 27, 2022 at 7:15 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > On Wed, Oct 26, 2022 at 07:41:21PM -0700, Yosry Ahmed wrote:
> > > > My 2c, if we care about direct reclaim as in reclaim that may stall
> > > > user space application allocations, then there are other reclaim
> > > > contexts that may pollute the direct reclaim stats. For instance,
> > > > proactive reclaim, or reclaim done by writing a limit lower than the
> > > > current usage to memory.max or memory.high, as they are not done in
> > > > the context of the application allocating memory.
> > > >
> > > > At Google, we have some internal direct reclaim memcg statistics, and
> > > > the way we handle this is by passing a flag from such contexts to
> > > > try_to_free_mem_cgroup_pages() in the reclaim_options arg. This flag
> > > > is echod into a scan_struct bit, which we then use to filter out
> > > > direct reclaim operations that actually cause latencies in user space
> > > > allocations.
> > > >
> > > > Perhaps something similar might be more generic here? I am not sure
> > > > what context khugepaged reclaims memory from, but I think it's not a
> > > > memcg context, so maybe we want to generalize the reclaim_options arg
> > > > to try_to_free_pages() or whatever interface khugepaged uses to free
> > > > memory.
> > >
> > > So at the /proc/vmstat level, I'm not sure it matters much because it
> > > doesn't count any cgroup_reclaim() activity.
> > >
> > > But at the cgroup level, it sure would be nice to split out proactive
> > > reclaim churn. Both in terms of not polluting direct reclaim counts,
> > > but also for *knowing* how much proactive reclaim is doing.
> > >
> > > Do you have separate counters for this?
> >
> > Not yet. Currently we only have the first part, not polluting direct
> > reclaim counts.
> >
> > We basically exclude reclaim coming from memory.reclaim, setting
> > memory.max/memory.limit_in_bytes, memory.high (on write, not hitting
> > the high limit), and memory.force_empty from direct reclaim stats.
> >
> > As for having a separate counter for proactive reclaim, do you think
> > it should be limited to reclaim coming from memory.reclaim (and
> > potentially memory.force_empty), or should it include reclaim coming
> > from limit-setting as well?
>
> A combined counter seems reasonable to me. We *have* used the limit
> knobs to drive proactive reclaim in production in the past, so it's
> not a stretch. And I can't think of a scenario where you'd like them
> to be separate.
>
> I could think of two ways of describing it:
>
> pgscan_user: User-requested reclaim. Could be confusing if we ever
> have an in-kernel proactive reclaim driver - unless that would then go
> to another counter (new or kswapd).
>
> pgscan_ext: Reclaim activity from extraordinary/external
> requests. External as in: outside the allocation context.

I imagine if the kernel is doing proactive reclaim on its own, we
might want a separate counter for that anyway to monitor what the
kernel is doing. So maybe pgscan_user sounds nice for now, but I also
like that the latter explicitly says "this is external to the
allocation context". But we can just go with pgscan_user and document
it properly.

How would khugepaged fit in this story? Seems like it would be part of
pgscan_ext but not pgscan_user. I imagine we also don't want to
pollute proactive reclaim counters with khugepaged reclaim (or other
non-direct reclaim).

Maybe pgscan_user and pgscan_kernel/pgscan_indirect for things like khugepaged?
The problem with pgscan_kernel/indirect is that if we add a proactive
reclaim kthread in the future it would technically fit there but we
would want a separate counter for it.

I am honestly not sure where to put khugepaged. The reasons I don't
like a dedicated counter for khugepaged are:
- What if other kthreads like khugepaged start doing the same, do we
add one counter per-thread?
- What if we deprecate khugepaged (or such threads)? Seems more likely
than deprecating kswapd.

Looks like we want a stat that would group all of this reclaim coming
from non-direct kthreads, but would not include a future proactive
reclaim kthread.