Re: [RFC PATCH 0/3] srcu: KVM: Add, export and use call_srcu_expedited()

[Kunwu Chan]

[...]
> I'm all for a general solution, but that's far, far beyond my SRCU knowledge level.
> I was quite proud of myself for piecing together the incurred-delay. :-)
> 
> FYI, I'm going to be unavailable for ~2 weeks. Nikita (Cc'd) can likely help test
> potential fixes.
> 
Hi Sean, hi Nikita,

Some data points on the RFC scenario.

The setup follows the pattern discussed in the RFC:
a call_srcu()-like path that can start a non-expedited GP,
followed by a synchronize_srcu_expedited()-like wait on
the same SRCU instance.

KVM selftest setup, BPF tracing.Single VM, single vCPU.
Guest holds a long measured window (~8s, fixed in selftest).

Baseline:
  In 5 recent 10-run batches, the no-preemption case produced
  a single >1ms sample in total.

With same-CPU preemption:
  >1ms reproduces consistently and shows up in most runs.

For >1ms samples, sched_switch/off-CPU attribution lines up
with switch-out in the same measured window.

In this setup, >1ms is mostly off-CPU time,not continuous in-CPU execution.

Question on the original report:
when synchronize_srcu_expedited() delay was observed,
was the host mostly idle/bursty, or under steady load?

Can share repro details if needed.

Thanks,
Kunwu Chan