Re: [PATCH] mm: memcontrol: asynchronous reclaim for memory.high

[Shakeel Butt <shakeelb@google.com>]

On Wed, Feb 26, 2020 at 2:26 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Wed, Feb 26, 2020 at 12:25:33PM -0800, Shakeel Butt wrote:
> > On Wed, Feb 19, 2020 at 10:12 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > We have received regression reports from users whose workloads moved
> > > into containers and subsequently encountered new latencies. For some
> > > users these were a nuisance, but for some it meant missing their SLA
> > > response times. We tracked those delays down to cgroup limits, which
> > > inject direct reclaim stalls into the workload where previously all
> > > reclaim was handled my kswapd.
> > >
> > > This patch adds asynchronous reclaim to the memory.high cgroup limit
> > > while keeping direct reclaim as a fallback. In our testing, this
> > > eliminated all direct reclaim from the affected workload.
> > >
> > > memory.high has a grace buffer of about 4% between when it becomes
> > > exceeded and when allocating threads get throttled. We can use the
> > > same buffer for the async reclaimer to operate in. If the worker
> > > cannot keep up and the grace buffer is exceeded, allocating threads
> > > will fall back to direct reclaim before getting throttled.
> > >
> > > For irq-context, there's already async memory.high enforcement. Re-use
> > > that work item for all allocating contexts, but switch it to the
> > > unbound workqueue so reclaim work doesn't compete with the workload.
> > > The work item is per cgroup, which means the workqueue infrastructure
> > > will create at maximum one worker thread per reclaiming cgroup.
> > >
> > > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> > > ---
> > >  mm/memcontrol.c | 60 +++++++++++++++++++++++++++++++++++++------------
> > >  mm/vmscan.c     | 10 +++++++--
> >
> > This reminds me of the per-memcg kswapd proposal from LSFMM 2018
> > (https://lwn.net/Articles/753162/).
>
> Ah yes, I remember those discussions. :)
>
> One thing that has changed since we tried to implement this last was
> the workqueue concurrency code. We don't have to worry about a single
> thread or fixed threads per cgroup, because the workqueue code has
> improved significantly to handle concurrency demands, and having one
> work item per cgroup makes sure we have anywhere between 0 threads and
> one thread per cgroup doing this reclaim work, completely on-demand.
>
> Also, with cgroup2, memory and cpu always have overlapping control
> domains, so the question who to account the work to becomes a much
> easier one to answer.
>
> > If I understand this correctly, the use-case is that the job instead
> > of direct reclaiming (potentially in latency sensitive tasks), prefers
> > a background non-latency sensitive task to do the reclaim. I am
> > wondering if we can use the memory.high notification along with a new
> > memcg interface (like memory.try_to_free_pages) to implement a user
> > space background reclaimer. That would resolve the cpu accounting
> > concerns as the user space background reclaimer can share the cpu cost
> > with the task.
>
> The idea is not necessarily that the background reclaimer is lower
> priority work, but that it can execute in parallel on a separate CPU
> instead of being forced into the execution stream of the main work.
>
> So we should be able to fully resolve this problem inside the kernel,
> without going through userspace, by accounting CPU cycles used by the
> background reclaim worker to the cgroup that is being reclaimed.
>
> > One concern with this approach will be that the memory.high
> > notification is too late and the latency sensitive task has faced the
> > stall. We can either introduce a threshold notification or another
> > notification only limit like memory.near_high which can be set based
> > on the job's rate of allocations and when the usage hits this limit
> > just notify the user space.
>
> Yeah, I think it would be a pretty drastic expansion of the memory
> controller's interface.

I understand the concern of expanding the interface and resolving the
problem within kernel but there are genuine use-cases which can be
fulfilled by these interfaces. We have a distributed caching service
which manages the caches in anon pages and their hotness. It is
preferable to drop a cold cache known to the application in the user
space on near stall/oom/memory_pressure then let the kernel swap it
out and face a stall on fault as the caches are replicated and other
nodes can serve it. For such workloads kernel reclaim does not help.
What would be your recommendation for such a workload. I can envision
memory.high + PSI notification but note that these are based on stalls
which the application wants to avoid.

Shakeel