Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root partitions

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, May 30, 2022 at 04:29:56PM +0200, nicolas saenz julienne wrote:
> On Mon, 2022-05-30 at 02:40 +0200, Frederic Weisbecker wrote:
> > On Sat, May 28, 2022 at 04:24:50PM +0200, Peter Zijlstra wrote:
> > > On Fri, May 27, 2022 at 10:30:18AM +0200, Juri Lelli wrote:
> > > > Hi,
> > > > 
> > > > On 26/05/22 14:37, Tejun Heo wrote:
> > > > > On Thu, May 26, 2022 at 08:28:43PM -0400, Waiman Long wrote:
> > > > > > I am thinking along the line that it will not be hierarchical. However,
> > > > > > cpuset can be useful if we want to have multiple isolated partitions
> > > > > > underneath the top cpuset with different isolation attributes, but no more
> > > > > > sub-isolated partition with sub-attributes underneath them. IOW, we can only
> > > > > > set them at the first level under top_cpuset. Will that be useful?
> > > > > 
> > > > > At that point, I'd just prefer to have it under /proc or /sys.
> > > > 
> > > > FWIW, I was under the impression that this would nicely fit along the
> > > > side of other feaures towards implenting dynamic isolation of CPUs (say
> > > > https://lore.kernel.org/lkml/20220510153413.400020-1-longman@redhat.com/
> > > > for example). Wouldn't be awkward to have to poke different places to
> > > > achieve isolation at runtime?
> > > 
> > > This, that's what I was thinking.
> > > 
> > > My main objection to the whole thing is that it's an RCU_NOCB specific
> > > interface. *That* I think is daft.
> > > 
> > > I was thinking a partition would be able to designate a house-keeping
> > > sub-partition/mask, but who cares about all the various different
> > > housekeeping parties.
> > 
> > It's time for the isolation users to step up here! I very rarely hear from them
> > and I just can't figure out by myself all the variants of uses for each of the
> > isolation features. May be some people are only interested in nocb for some
> > specific uses, or may be it never makes sense without nohz full and all the rest
> > of the isolation features. So for now I take the very cautious path to split the
> > interface.
> 
> OK, my 2 cents. I personally deal with virtualisation setups that involve RT
> and CPU isolation on both host and guests.
> 
> The main use-case ATM is running DPDK-like workloads. We want to achieve
> latencies in the order of tens of microseconds, so it's essential to avoid
> entering the kernel at all cost. So, no HW interrupts, sched tick, RCU
> callbacks, clocksource watchdogs, softlockup, intel_pstate, timers, etc...
> Everything is deferred onto housekeeping CPUs or disabled.
> 
> Then we have setups that need to deal with HW on the host, exposed to the guest
> through emulation or VirtIO. The same rules apply really, except for some IRQ
> affinity tweaks and sched priority magic.
> 
> I find it hard to see how running RCU callback locally could be useful to any
> latency sensitive workload.
> 
> Frederic, out of curiosity, do you have a use-case in mind that might benefit
> from nohz_full but not rcu_nocb? Maybe HPC?

Would users looking for millisecond-scale latencies want rcu_nocbs but
not nohz_full, that is, the other way around?

							Thanx, Paul