Re: [PATCH RFC tip/core/rcu 0/16] Prototype RCU usable from idle, exception, offline

Paul E. McKenney
2020-03-15T17:59:24Z

On Sun, Mar 15, 2020 at 01:45:05PM -0400, Mathieu Desnoyers wrote:
> ----- On Mar 13, 2020, at 11:42 AM, paulmck paulmck@kernel.org wrote:
>
> > On Fri, Mar 13, 2020 at 03:41:46PM +0100, Frederic Weisbecker wrote:
> >> On Thu, Mar 12, 2020 at 11:16:18AM -0700, Paul E. McKenney wrote:
> >> > Hello!
> >> >
> >> > This series provides two variants of Tasks RCU, a rude variant inspired
> >> > by Steven Rostedt's use of schedule_on_each_cpu(), and a tracing variant
> >> > requested by the BPF folks and perhaps also of use for other tracing
> >> > use cases.
> >> >
> >> > The tracing variant has explicit read-side markers to permit finite grace
> >> > periods even given in-kernel loops in PREEMPT=n builds It also protects
> >> > code in the idle loop, on exception entry/exit paths, and on the various
> >> > CPU-hotplug online/offline code paths, thus having protection properties
> >> > similar to SRCU. However, unlike SRCU, this variant avoids expensive
> >> > instructions in the read-side primitives, thus having read-side overhead
> >> > similar to that of preemptible RCU.
> >> >
> >> > There are of course downsides. The grace-period code can send IPIs to
> >> > CPUs, even when those CPUs are in the idle loop or in nohz_full userspace.
> >> > It is necessary to scan the full tasklist, much as for Tasks RCU. There
> >> > is a single callback queue guarded by a single lock, again, much as for
> >> > Tasks RCU. If needed, these downsides can be at least partially remedied
> >>
> >> So what we trade to fix the issues we are having with tracing against extended
> >> grace periods, we lose in CPU isolation. That worries me a bit as tracing can
> >> be thoroughly used with nohz_full and CPU isolation.
> >
> > First, disturbing nohz_full CPUs can be avoided by the sysadm simply
> > refusing to remove tracepoints while sensitive applications are running
> > on nohz_full CPUs.
>
> I doubt this approach will survive real-life.

Nothing survives real life, at least not indefinitely. ;-)

> > Second, for non-CPU-bound real-time programs with mostly-idle CPUs,
> > I should be able to decrease the likelihood of sending IPIs pretty much
> > to zero.
> >
> > Or am I missing something here?
>
> I would recommend considering the following alternative for this tracing-rcu
> flavor:
>
> - For all CPUs which are not nohz_full:
> - Implement fast RCU read-side which only requires compiler barriers,
> - Use IPIs to each of those CPUs when doing a grace period.
>
> - For all nohz_full CPUS:
> - Dynamically detect CPUs which are nohz_full,
> - Implement slower RCU read-side with memory barriers,
> - No need to issue any IPI to those CPUs when doing the grace period.
>
> This should cover all use-cases: staying fast for the common case, without
> disturbing RT workloads.
>
> Thoughts ?

I will certainly add this to my list of potential solutions, and thank
you for pointing me at it!

Thanx, Paul