Re: [PATCH 3/3] rcu/exp: Remove needless CPU up quiescent state report

Paul E. McKenney
2025-03-18T17:07:16Z

On Fri, Mar 14, 2025 at 03:39:05PM +0100, Frederic Weisbecker wrote:
> Le Mon, Mar 03, 2025 at 12:10:50PM -0800, Paul E. McKenney a écrit :
> > On Fri, Feb 14, 2025 at 12:25:59AM +0100, Frederic Weisbecker wrote:
> > > A CPU coming online checks for an ongoing grace period and reports
> > > a quiescent state accordingly if needed. This special treatment that
> > > shortcuts the expedited IPI finds its origin as an optimization purpose
> > > on the following commit:
> > >
> > > 338b0f760e84 (rcu: Better hotplug handling for synchronize_sched_expedited()
> > >
> > > The point is to avoid an IPI while waiting for a CPU to become online
> > > or failing to become offline.
> > >
> > > However this is pointless and even error prone for several reasons:
> > >
> > > * If the CPU has been seen offline in the first round scanning offline
> > > and idle CPUs, no IPI is even tried and the quiescent state is
> > > reported on behalf of the CPU.
> > >
> > > * This means that if the IPI fails, the CPU just became offline. So
> > > it's unlikely to become online right away, unless the cpu hotplug
> > > operation failed and rolled back, which is a rare event that can
> > > wait a jiffy for a new IPI to be issued.
> > >
> > > * But then the "optimization" applying on failing CPU hotplug down only
> > > applies to !PREEMPT_RCU.
> > >
> > > * This force reports a quiescent state even if ->cpu_no_qs.b.exp is not
> > > set. As a result it can race with remote QS reports on the same rdp.
> > > Fortunately it happens to be OK but an accident is waiting to happen.
> > >
> > > For all those reasons, remove this optimization that doesn't look worthy
> > > to keep around.
> > >
> > > Reported-by: Paul E. McKenney <paulmck@kernel.org>
> > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> >
> > Based on discussions:
> >
> > Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> >
> > If it was still just me doing RCU, I would skip this one, just out of an
> > abundance of caution. But you break it, you buy it! ;-)
>
> I'm trying to be optimistic... ;-))

So am I. ;-)

Thanx, Paul