Re: [PATCH v2 3/3] mm/vmstat: do not refresh stats for nohz_full CPUs

[Michal Hocko]

On Mon 05-06-23 12:43:24, Marcelo Tosatti wrote:
> On Mon, Jun 05, 2023 at 09:59:57AM +0200, Michal Hocko wrote:
> > On Fri 02-06-23 15:58:00, Marcelo Tosatti wrote:
> > > The interruption caused by queueing work on nohz_full CPUs 
> > > is undesirable for certain aplications.
> > 
> > This is not a proper changelog. I am not going to write a changelog for
> > you this time. Please explain why this is really needed and why this
> > approach is desired. 
> > E.g. why don't you prevent userspace from
> > refreshing stats if interference is not desirable.
> 
> Michal,
> 
> Can you please check if the following looks better, as
> a changelog? thanks
> 
> ---
> 
> schedule_work_on API uses the workqueue mechanism to
> queue a work item on a queue. A kernel thread, which
> runs on the target CPU, executes those work items.
> 
> Therefore, when using the schedule_work_on API,
> it is necessary for the kworker kernel thread to
> be scheduled in, for the work function to be executed.
> 
> Time sensitive applications such as SoftPLCs
> (https://tum-esi.github.io/publications-list/PDF/2022-ETFA-How_Real_Time_Are_Virtual_PLCs.pdf),
> have their response times affected by such interruptions.
> 
> The /proc/sys/vm/stat_refresh file was originally introduced by
> 
> commit 52b6f46bc163eef17ecba4cd552beeafe2b24453
> Author: Hugh Dickins <hughd@xxxxxxxxxx>
> Date:   Thu May 19 17:12:50 2016 -0700
> 
>     mm: /proc/sys/vm/stat_refresh to force vmstat update
> 
>     Provide /proc/sys/vm/stat_refresh to force an immediate update of
>     per-cpu into global vmstats: useful to avoid a sleep(2) or whatever
>     before checking counts when testing.  Originally added to work around a
>     bug which left counts stranded indefinitely on a cpu going idle (an
>     inaccuracy magnified when small below-batch numbers represent "huge"
>     amounts of memory), but I believe that bug is now fixed: nonetheless,
>     this is still a useful knob.

No need to quote the full changelog.
 
> Other than the potential interruption to a time sensitive application,
> if using SCHED_FIFO or SCHED_RR priority on the isolated CPU, then
> system hangs can occur:
> 
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=978688

Confused... This report says that accessing the file (i.e. to force the
refresh) can get stalled because high priority tasks will not allow
kworkers to run. No?

There is simply no way around that unless those kworkers inherit the
priority. It certainly is unfortunate that the call is not killable
but being stuck behind real time busy looping processes is nothing
really uncommong. One has to be really careful when using real time
priorities.

> To avoid the problems above, do not schedule the work to synchronize
> per-CPU mm counters on isolated CPUs. Given the possibility for
> breaking existing userspace applications, avoid changing
> behaviour of access to /proc/sys/vm/stat_refresh, such as
> returning errors to userspace.

You are changing the behavior. The preexisting behavior was to flush
everything. This is clearly changing that.

> ---
> 
> > Also would it make some sense to reduce flushing to cpumask 
> > of the calling process? (certainly a daring thought but have
> > you even considered it?)
> 
> Fail to see the point here ?

I mean that, if you already want to change the semantic of the call then
it would likely be safer to change it in a more robust way and only
flush pcp vmstat caches that are in the process effective cpu mask. This
way one can control which pcp caches to flush (e.g. those that are not
on isolated CPUs or contrary those that are isolated but you can afford 
to flush at the specific moment). See?

Now I am not saying this is the right way to go because there is still a
slim chance this will break userspace expectations. Therefore I have
asked why you simply do not stop any random application accessing
stat_refresh in the first place. These highly specialized setups with
isolated resources shouldn't run arbitrary crap, should that? What if I
just start allocating memory and get the system close to OOM. I am
pretty sure a small latency induced by the vmstat refreshes is the least
problem you will have.

So please step back and try to think whether this is actually fixing
anything real before trying to change a user visible interface.

-- 
Michal Hocko
SUSE Labs