Re: [PATCH] sched/nohz: Fix NOHZ imbalance by adding options for ILB CPU

[Valentin Schneider]

On 20/08/25 10:31, Christoph Lameter (Ampere) wrote:
> On Wed, 20 Aug 2025, Valentin Schneider wrote:
>
>> My first question would be: is NOHZ_FULL really right for your workload?
>
> Yes performance is improved. AI workloads are like HPC workloads in that
> they need to do compute and then rendezvous for data exchange. Variations
> in the runtime due to timer ticks cause idle periods where the rendezvous
> cannot be completed because some cpus are delayed.
>
> The more frequent rendezvous can be performed the better the performance
> numbers will be.

[...]

> The benchmarks show a regression of 10-20% if the tick is operational.
> The context switch overhead is negligible since the cpus are doing compute
> and not system calls.
>

Ah good, that's useful information, thanks!

>> As for the actual balancing, yeah if you have idle NOHZ_FULL CPUs they
>> won't do the periodic balance; the residual 1Hz remote tick doesn't do that
>> either. But they should still do the newidle balance to pull work before
>> going tickless idle, and wakeup balance should help as well, albeit that
>> also depends on your topology.
>
> That should work in general and not depend on any hardware topology. In
> this case we have a linear sched domain including all processors.

Wakeup balance not so much, select_idle_sibling() won't move tasks outside
of the waker's LLC - but AIUI in your case you have just a single node and
I'm assuming one big LLC.