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

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Date:   Mon, 30 May 2022 15:36:01 -0700
From:   Alison Chaiken <achaiken@...ora.tech>
To:     paulmck@...nel.org
Cc:     nicolas saenz julienne <nsaenz@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>, Tejun Heo <tj@...nel.org>,
        Waiman Long <longman@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        rcu@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root partitions

> 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?

On Mon, May 30, 2022 at 8:42 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> Would users looking for millisecond-scale latencies want rcu_nocbs but
> not nohz_full, that is, the other way around?

On Intel processors running 5.15 with the timersd patches from Siewior
backported and Weisbecker's bug-fix, choosing CONFIG_HZ_PERIODIC
prevents cores from entering deeper C-states when hrtimers are
pending.   With CONFIG_NO_HZ_COMMON, the cores do not service
non-blocking timer callbacks until another thread wakes them.  Since
low latency is critical, this is a use case where NO_HZ_FULL will not
work but rcu_nocbs is needed.

-- Alison Chaiken, Aurora Innovation