Re: [PATCH v7 00/13] fold per-CPU vmstats remotely

Marcelo Tosatti
2023-03-22T13:10:08Z

On Wed, Mar 22, 2023 at 11:13:02AM +0100, Michal Hocko wrote:
> On Mon 20-03-23 16:07:29, Marcelo Tosatti wrote:
> > On Mon, Mar 20, 2023 at 07:25:55PM +0100, Michal Hocko wrote:
> > > On Mon 20-03-23 15:03:32, Marcelo Tosatti wrote:
> > > > This patch series addresses the following two problems:
> > > >
> > > > 1. A customer provided evidence indicating that a process
> > > > was stalled in direct reclaim:
> > > >
> > > This is addressed by the trivial patch 1.
> > >
> > > [...]
> > > > 2. With a task that busy loops on a given CPU,
> > > > the kworker interruption to execute vmstat_update
> > > > is undesired and may exceed latency thresholds
> > > > for certain applications.
> > >
> > > Yes it can but why does that matter?
> >
> > It matters for the application that is executing and expects
> > not to be interrupted.
>
> Those workloads shouldn't enter the kernel in the first place, no?

It depends on the latency requirements and individual system calls.

> Otherwise the in kernel execution with all the direct or indirect
> dependencies (e.g. via locks) can throw any latency expectations off the
> window.
>
> > > > By having vmstat_shepherd flush the per-CPU counters to the
> > > > global counters from remote CPUs.
> > > >
> > > > This is done using cmpxchg to manipulate the counters,
> > > > both CPU locally (via the account functions),
> > > > and remotely (via cpu_vm_stats_fold).
> > > >
> > > > Thanks to Aaron Tomlin for diagnosing issue 1 and writing
> > > > the initial patch series.
> > > >
> > > >
> > > > Performance details for the kworker interruption:
> > > >
> > > > oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
> > > > oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ...
> > > > oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
> > > > kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...
> > > >
> > > > The example above shows an additional 7us for the
> > > >
> > > > oslat -> kworker -> oslat
> > > >
> > > > switches. In the case of a virtualized CPU, and the vmstat_update
> > > > interruption in the host (of a qemu-kvm vcpu), the latency penalty
> > > > observed in the guest is higher than 50us, violating the acceptable
> > > > latency threshold for certain applications.
> > >
> > > I do not think we have ever promissed any specific latency guarantees
> > > for vmstat. These are statistics have been mostly used for debugging
> > > purposes AFAIK. I am not aware of any specific user space use case that
> > > would be latency sensitive. Your changelog doesn't go into details there
> > > either.
> >
> > There is a class of workloads for which response time can be
> > of interest. MAC scheduler is an example:
> >
> > https://par.nsf.gov/servlets/purl/10090368
>
> Yes, I am not disputing low latency workloads in general. I am just
> saying that you haven't really established a very sound justification
> here.

The -v7 cover letter was updated with additional details,
as you requested (perhaps you missed it):

"Performance details for the kworker interruption:

oslat 1094.456862: sys_mlock(start: 7f7ed0000b60, len: 1000)
oslat 1094.456971: workqueue_queue_work: ... function=vmstat_update ...
oslat 1094.456974: sched_switch: prev_comm=oslat ... ==> next_comm=kworker/5:1 ...
kworker 1094.456978: sched_switch: prev_comm=kworker/5:1 ==> next_comm=oslat ...

The example above shows an additional 7us for the

oslat -> kworker -> oslat

switches. In the case of a virtualized CPU, and the vmstat_update
interruption in the host (of a qemu-kvm vcpu), the latency penalty
observed in the guest is higher than 50us, violating the acceptable
latency threshold for certain applications."

> Of course there are workloads which do not want to conflict with
> any in kernel house keeping. Those have to be configured and implemented
> very carefully though. Vmstat as such should not collide with those
> workloads as long as they do not interact with the kernel in a way
> counters are updated. Is this hard or impossible to avoid?

The practical problem we have been seeing is -RT app initialization.
For example:

1) mlock();
2) enter loop without system calls

> I can imagine that those workloads have an start up sequence where the
> kernel is involved and counters updated so that deferred flushing could
> interfere with the later and latency sensitive phase. Is that a real
> problem in practice? Please tell us much more why we need to make the
> vmstat code more complex.

Yes, it is. I have attached traces and performance numbers above.