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

[Frederic Weisbecker]

Le Mon, Jun 23, 2025 at 08:03:46AM -1000, Tejun Heo a écrit :
> Hello,
> 
> On Mon, Jun 23, 2025 at 01:57:17PM -0400, Waiman Long wrote:
> > The percpu-rwsem does have a cheaper read side compared with rwsem for
> > typical use case where writer update happens sparingly. However, when the
> > writer has successful acquired the write lock, the readers do have to wait
> > until the writer issues a percpu_up_write() call before they can proceed. It
> > is the delay introduced by this wait that I am worry about. Isolated
> > partitions are typically set up to run RT applications that have a strict
> > latency requirement. So any possible latency spike should be avoided.
> 
> I see. Hmm... this being the mechanism that establishes the isolation, it
> doesn't seem too broken if things stutter a bit when isolation is being
> updated. Let's see what Frederic says why the strong interlocking is needed.

I should be able to work around that.
I think only PCI requires that rwsem because it relies on work_on_cpu().

I can create a dedicated workqueue for it that housekeeping can flush after
the cpumask update.

-- 
Frederic Weisbecker
SUSE Labs