Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation_mask file

[Marcelo Tosatti <mtosatti-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Mon, Jul 19, 2021 at 03:26:49PM +0200, Frederic Weisbecker wrote:
> On Wed, Jul 14, 2021 at 01:31:57PM -0300, Marcelo Tosatti wrote:
> > On Wed, Jul 14, 2021 at 03:54:20PM +0200, Frederic Weisbecker wrote:
> > > Add a new cpuset.isolation_mask file in order to be able to modify the
> > > housekeeping cpumask for each individual isolation feature on runtime.
> > > In the future this will include nohz_full, unbound timers,
> > > unbound workqueues, unbound kthreads, managed irqs, etc...
> > > 
> > > Start with supporting domain exclusion and CPUs passed through
> > > "isolcpus=".
> > 
> > It is possible to just add return -ENOTSUPPORTED for the features 
> > whose support is not present?
> 
> Maybe, although that looks like a specialized error for corner cases.

Well, are you going to implement runtime enablement for all features,
including nohz_full, in the first patch set?

From my POV returning -ENOTSUPPORTED would allow for a gradual 
implementation of the features.

> > > CHECKME: Should we have individual cpuset.isolation.$feature files for
> > >          each isolation feature instead of a single mask file?
> > 
> > Yes, guess that is useful, for example due to the -ENOTSUPPORTED
> > comment above.
> > 
> > 
> > Guarantees on updates
> > =====================
> > 
> > Perhaps start with a document with:
> > 
> > On return to the write to the cpumask file, what are the guarantees?
> > 
> > For example, for kthread it is that any kernel threads from that point
> > on should start with the new mask. Therefore userspace should 
> > respect the order:
> > 
> > 1) Change kthread mask.
> > 2) Move threads.
> > 
> 
> Yep.
> 
> > Updates to interface
> > ====================
> > 
> > Also, thinking about updates to the interface (which today are one
> > cpumask per isolation feature) might be useful. What can happen:
> > 
> > 1) New isolation feature is added, feature name added to the interface.
> > 
> > Userspace must support new filename. If not there, then thats an 
> > old kernel without support for it.
> > 
> > 2) If an isolation feature is removed, a file will be gone. What should
> > be the behaviour there? Remove the file? (userspace should probably 
> > ignore the failure in that case?) (then features names should not be
> > reused, as that can confuse #1 above).
> 
> Heh, yeah that's complicated. I guess we should use one flag per file as that
> fits well within the current cpuset design. But we must carefully choose the new
> files to make sure they have the least chances to be useless in the long term.
> 
> > Or maybe have a versioned scheme?
> 
> I suspect we should avoid that at all costs :-)
> 
> Thanks!

Makes sense.