Re: [patch 0/5] cpuidle haltpoll driver and governor (v6)

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Jul 4, 2019 at 1:59 AM Marcelo Tosatti <mtosatti@redhat.com> wrote:
>
> (rebased against queue branch of kvm.git tree)
>
> The cpuidle-haltpoll driver with haltpoll governor allows the guest
> vcpus to poll for a specified amount of time before halting.
> This provides the following benefits to host side polling:
>
>          1) The POLL flag is set while polling is performed, which allows
>             a remote vCPU to avoid sending an IPI (and the associated
>             cost of handling the IPI) when performing a wakeup.
>
>          2) The VM-exit cost can be avoided.
>
> The downside of guest side polling is that polling is performed
> even with other runnable tasks in the host.
>
> Results comparing halt_poll_ns and server/client application
> where a small packet is ping-ponged:
>
> host                                        --> 31.33
> halt_poll_ns=300000 / no guest busy spin    --> 33.40   (93.8%)
> halt_poll_ns=0 / guest_halt_poll_ns=300000  --> 32.73   (95.7%)
>
> For the SAP HANA benchmarks (where idle_spin is a parameter
> of the previous version of the patch, results should be the
> same):
>
> hpns == halt_poll_ns
>
>                            idle_spin=0/   idle_spin=800/    idle_spin=0/
>                            hpns=200000    hpns=0            hpns=800000
> DeleteC06T03 (100 thread) 1.76           1.71 (-3%)        1.78   (+1%)
> InsertC16T02 (100 thread) 2.14           2.07 (-3%)        2.18   (+1.8%)
> DeleteC00T01 (1 thread)   1.34           1.28 (-4.5%)      1.29   (-3.7%)
> UpdateC00T03 (1 thread)   4.72           4.18 (-12%)       4.53   (-5%)
>
> V2:
>
> - Move from x86 to generic code (Paolo/Christian)
> - Add auto-tuning logic (Paolo)
> - Add MSR to disable host side polling (Paolo)
>
> V3:
>
> - Do not be specific about HLT VM-exit in the documentation (Ankur Arora)
> - Mark tuning parameters static and __read_mostly (Andrea Arcangeli)
> - Add WARN_ON if host does not support poll control (Joao Martins)
> - Use sched_clock and cleanup haltpoll_enter_idle (Peter Zijlstra)
> - Mark certain functions in kvm.c as static (kernel test robot)
> - Remove tracepoints as they use RCU from extended quiescent state (kernel
> test robot)
>
> V4:
> - Use a haltpoll governor, use poll_state.c poll code (Rafael J. Wysocki)
>
> V5:
> - Take latency requirement into consideration (Rafael J. Wysocki)
> - Set target_residency/exit_latency to 1 (Rafael J. Wysocki)
> - Do not load cpuidle driver if not virtualized (Rafael J. Wysocki)
>
> V6:
> - Switch from callback to poll_limit_ns variable in cpuidle device structure
> (Rafael J. Wysocki)
> - Move last_used_idx to cpuidle device structure (Rafael J. Wysocki)
> - Drop per-cpu device structure in haltpoll governor (Rafael J. Wysocki)

It looks good to me now, but I have some cpuidle changes in the work
that will clash in some changes in this series if not rebased on top
of it, so IMO it would make sense for me to get patches [1-4/5] at
least into my queue.  I can expose an immutable branch with them for
the KVM tree to consume.  I can take the last patch in the series as
well if I get an ACK for it.

Would that work for everybody?