Re: NOHZ interaction between IPI-less kick_ilb() and nohz_csd_func().

[Joel Fernandes]

+Frederic Weisbecker

On Wed, Sep 13, 2023 at 10:32 AM Suleiman Souhlal <suleiman@xxxxxxxxxx> wrote:
>
> (I forgot to also add Vincent...)
>
> On Wed, Sep 13, 2023 at 9:49 PM Suleiman Souhlal <suleiman@xxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > I noticed that on x86 machines that have MWAIT, with NOHZ, when the
> > kernel decides to kick the idle load balance on another CPU in
> > kick_ilb(), there's an optimization that makes it avoid using an IPI
> > and instead exploit the fact that the remote CPU is MWAITing on the
> > thread_info flags, by just setting TIF_NEED_RESCHED, in
> > call_function_single_prep_ipi().
> > However, on the remote CPU, in nohz_csd_func(), we end up not raising
> > the sched softirq due to NEED_RESCHED being set, so the ILB doesn't
> > end up getting done.
> >
> > Is this intended?
> >
> > Thanks,
> > -- Suleiman