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SubjectRe: [PATCH rcu 0/18] RCU no-CBs CPU updates for v5.17
On Wed, Dec 01, 2021 at 06:03:47PM -0800, Yury Norov wrote:
> On Wed, Dec 01, 2021 at 04:28:48PM -0800, Paul E. McKenney wrote:
> > Hello!
>
> ...
>
> > 17. Allow empty "rcu_nocbs" kernel parameter, courtesy of Frederic
> > Weisbecker.
>
> ...
>
> > Note that #17 might be updated given some ongoing work by Yury Norov
> > to support "none" for bitmaps, including the cpumask taken by the
> > rcu_nocbs kernel-boot parameter.
>
> Hi Paul,
>
> This is the only work that is needed to support 'none':
>
> https://lkml.org/lkml/2021/11/24/2642
>
> >From the last discussion, it's not clear is this 'none' needed or not.
> If needed, I'll submit the patch.

The question is whether rcu_nocbs= semantics like this are reasonable.
In other words, will systems administrators be OK with the idea of
differentiation between "rcu_nocbs=none" on the one hand and not
specifying rcu_nocbs at all on the other.

Thoughts?

Thanx, Paul

------------------------------------------------------------------------

rcu_nocbs= [KNL]
The argument is a cpu list, as described above.

In kernels built with CONFIG_RCU_NOCB_CPU=y,
enable the no-callback CPU mode, which prevents
such CPUs' callbacks from being invoked in
softirq context. Invocation of such CPUs' RCU
callbacks will instead be offloaded to "rcuox/N"
kthreads created for that purpose, where "x" is
"p" for RCU-preempt, "s" for RCU-sched, and "g"
for the kthreads that mediate grace periods; and
"N" is the CPU number. This reduces OS jitter on
the offloaded CPUs, which can be useful for HPC
and real-time workloads. It can also improve
energy efficiency, especially for asymmetric
multiprocessors.

The cpulist specifies which are to be set to
no-callback mode from boot. In particular,
"rcu_nocbs=none" specifies that no CPUs are in
no-callback mode at boot, but that any of them
can be offloaded at runtime. In contrast, in
kernels that boot with no rcu_nocbs kernel boot
parameter, CPUs are not only in no-callback mode
at boot, but they remain that way throughout.

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