Re: [patch 0/3] cpuidle-haltpoll driver (v2)

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Jun 07, 2019 at 11:49:51AM +0200, Rafael J. Wysocki wrote:
> On 6/4/2019 12:52 AM, Marcelo Tosatti wrote:
> >The cpuidle-haltpoll driver 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 HLT 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).
> >
> >
> >
> First of all, please CC power management patches (including cpuidle,
> cpufreq etc) to linux-pm@vger.kernel.org (there are people on that
> list who may want to see your changes before they go in) and CC
> cpuidle material (in particular) to Peter Zijlstra.
> 
> Second, I'm not a big fan of this approach to be honest, as it kind
> of is a driver trying to play the role of a governor.
> 
> We have a "polling state" already that could be used here in
> principle so I wonder what would be wrong with that.  Also note that
> there seems to be at least some code duplication between your code
> and the "polling state" implementation, so maybe it would be
> possible to do some things in a common way?

Hi Rafael,

After modifying poll_state.c to use a generic "poll time" driver 
callback [1] (since using a variable "target_residency" for that 
looks really ugly), would need a governor which does:

haltpoll_governor_select_next_state()
	if (prev_state was poll and evt happened on prev poll window) -> POLL.
	if (prev_state == HLT)	-> POLL
	otherwise 		-> HLT

And a "default_idle" cpuidle driver that:

defaultidle_idle()
	if (current_clr_polling_and_test()) {
		local_irq_enable();
		return index;
	}
	default_idle(); 
	return

Using such governor with any other cpuidle driver would 
be pointless (since it would enter the first state only
and therefore not save power).

Not certain about using the default_idle driver with 
other governors: one would rather use a driver that 
supports all states on a given machine.

This combination of governor/driver pair, for the sake
of sharing the idle loop, seems awkward to me.
And fails the governor/driver separation: one will use the
pair in practice.

But i have no problem with it, so i'll proceed with that.

Let me know otherwise.

Thanks.