Re: [RFC PATCH v2 1/2] tick/nohz: Add a policy dependency for full dynticks

[Frederic Weisbecker <frederic@kernel.org>]

Le Sat, Oct 24, 2026 at 12:32:38AM +0800, Peng Wang a écrit :
> On Fri, Oct 23, 2026 at 05:00:22PM +0200, Frederic Weisbecker wrote:
> > Le Sat, Oct 17, 2026 at 09:13:41PM +0800, Peng Wang a 殂rit :
> > > Cloud hosts may pin vCPU threads one-to-one to host CPUs. At full
> > > allocation this includes the housekeeping CPUs, which always retain
> > > their non-idle tick. Operators want comparable performance for otherwise
> > > equivalent vCPUs. Retaining the non-idle tick on nohz_full CPUs as well
> > > avoids one placement-dependent difference in host tick interference.
> > > With spare capacity, selected CPUs may stop the tick to reduce host
> > > scheduler-tick interference with vCPUs.
> > > 
> > > Allocation changes as VMs are created or destroyed while other VMs
> > > remain running. A vCPU thread may be the only runnable task in either
> > > state, so existing tick dependencies do not express this policy.
> > 
> > I'm not sure I understand correctly what you are trying to solve.
> > Are you saying that you want to keep the tick running when there
> > is more than one vCPU per CPU? Because the scheduler already takes
> > care of that.
> 
> Hi Frederic,
> 
> No. We pin vCPU threads one-to-one to host CPUs, without
> oversubscription. By "full allocation" I meant that all allocatable
> host CPUs are in use, including CPUs outside the nohz_full set.
> 
> For example, on an eight-CPU host booted with isolcpus=nohz,1-7,
> CPU0 is outside the nohz_full set:
> 
> - With eight pinned vCPUs, CPU0 retains its non-idle tick while
>   CPUs1-7 may stop theirs. Retaining the tick on CPUs1-7 too avoids
>   this placement-dependent difference in tick interference between
>   otherwise equivalent vCPUs.
> 
> - With seven or fewer pinned vCPUs, we can leave CPU0 without a
>   vCPU and keep them on CPUs1-7. We can then allow tick suppression
>   on CPUs1-7, subject to the existing dependencies. This gives
>   these vCPUs a common tick policy while reducing host tick
>   interference.
> 
> If the vCPU on CPU0 is released while the other seven remain
> running, we allow CPUs1-7 to suppress their ticks. If a new vCPU
> is later assigned to CPU0, we retain the ticks on CPUs1-7 again
> to avoid the difference in tick interference.
> 
> Throughout these transitions, CPU1 can still have its original
> pinned vCPU thread as its only runnable task. Its runnable task
> count does not change merely because CPU0 gains or loses a vCPU,
> so the scheduler's existing tick dependency cannot express this
> allocation policy.

Ok now I understand what you want to do, I just don't understand why.
Why it matters to turn off nohz_full on all CPUs when vCPUs run on housekeeping?

> > > Add a per-CPU POLICY dependency within the boot-time nohz_full set.
> > > Permission defaults to enabled. The policy can be changed without
> > > taking CPUs offline. Revoking permission uses the existing asynchronous
> > > kick; granting it leaves all other dependencies effective. Preserve
> > > POLICY across CPU hotplug and report it in tick_stop tracing.
> > > 
> > > Idle tick suppression, nohz_full membership, context tracking, RCU
> > > offloading and housekeeping configuration are unchanged.
> > 
> > This still keeps the overhead of nohz_full even if the tick stays on, right?
> > (RCU nocb, kernel <-> user overhead, etc...). Isn't it a problem for you?
> 
> Yes. Our setup uses only the nohz flag of isolcpus, without
> additional housekeeping affinity tuning. Context tracking and
> RCU callback offloading remain enabled when the tick is kept
> running.
> 
> We have observed a performance benefit from this nohz_full setup
> with our database workload running on pinned vCPUs.

Do you mean that your database runs better when hosts CPUs are in nohz_full?
But how does this new interface help? Ie: why is it worse to have all CPUs in
nohz_full except the housekeeping rather than shutting down nohz_full
altogether?

Or to put it another way: why is it better to have either:

- all vCPUs on nohz_full host CPUs
or
- all vCPUs on ticking CPUs

And why does it hurt to have a single vCPU running on a ticking CPU.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs