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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jun 11, 2019 at 12:03:26AM +0200, Rafael J. Wysocki wrote:
> On Mon, Jun 10, 2019 at 5:00 PM Marcelo Tosatti <mtosatti@redhat.com> wrote:
> >
> > 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.
> 
> If my understanding of your argumentation is correct, it is only
> necessary to take the default_idle_call() branch of
> cpuidle_idle_call() in the VM case, so it should be sufficient to
> provide a suitable default_idle_call() which is what you seem to be
> trying to do.

In the VM case, we need to poll before actually halting (this is because
its tricky to implement MWAIT in guests, so polling for some amount 
of time allows the IPI avoidance optimization,
see trace_sched_wake_idle_without_ipi, to take place). 

The amount of time we poll is variable and adjusted (see adjust_haltpoll_ns 
in the patchset).

> I might have been confused by the terminology used in the patch series
> if that's the case.
> 
> Also, if that's the case, this is not cpuidle matter really.  It is a
> matter of providing a better default_idle_call() for the arch at hand.

Peter Zijlstra suggested a cpuidle driver for this. 

Also, other architectures will use the same "poll before exiting to VM"
logic (so we'd rather avoid duplicating this code): PPC, x86, S/390,
MIPS... So in my POV it makes sense to unify this.

So, back to your initial suggestion: 

Q) "Can you unify code with poll_state.c?" 
A) Yes, but it requires a new governor, which seems overkill and unfit
for the purpose.

Moreover, the logic in menu to decide whether its necessary or not
to stop sched tick is useful for us (so a default_idle_call is not
sufficient), because the cost of enabling/disabling the sched tick is
high on VMs.

So i'll fix the comments of the cpuidle driver (which everyone seems
to agree with, except your understandable distate for it) and repost.