Re: [PATCH 02/27] sched/isolation: Introduce housekeeping per-cpu rwsem

[Tejun Heo]

Hello,

On Mon, Jun 23, 2025 at 01:34:58PM -0400, Waiman Long wrote:
> On 6/20/25 11:22 AM, Frederic Weisbecker wrote:
> > The HK_TYPE_DOMAIN isolation cpumask, and further the
> > HK_TYPE_KERNEL_NOISE cpumask will be made modifiable at runtime in the
> > future.
> > 
> > The affected subsystems will need to synchronize against those cpumask
> > changes so that:
> > 
> > * The reader get a coherent snapshot
> > * The housekeeping subsystem can safely propagate a cpumask update to
> >    the susbsytems after it has been published.
> > 
> > Protect against readsides that can sleep with per-cpu rwsem. Updates are
> > expected to be very rare given that CPU isolation is a niche usecase and
> > related cpuset setup happen only in preparation work. On the other hand
> > read sides can occur in more frequent paths.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> 
> Thanks for the patch series and it certainly has some good ideas. However I
> am a bit concern about the overhead of using percpu-rwsem for
> synchronization especially when the readers have to wait for the completion
> on the writer side. From my point of view, during the transition period when
> new isolated CPUs are being added or old ones being removed, the reader will
> either get the old CPU data or the new one depending on the exact timing.
> The effect the CPU selection may persist for a while after the end of the
> critical section.
> 
> Can we just rely on RCU to make sure that it either get the new one or the
> old one but nothing in between without the additional overhead?

So, I had a similar thought - ie. does this need full interlocking so that
when the modification operation can wait for existing users to drain? It'd
be nice to explain that part a bit more. That said, percpu_rwsem read path
is pretty cheap, so if that is a requirement, I doubt the overhead
difference between RCU access and percpu read locking would make meaningful
difference.

Thanks.

-- 
tejun