Re: [PATCH] sched: introduce configurable delay before entering idle

[Marcelo Tosatti]

On Wed, May 15, 2019 at 11:42:56AM -0700, Ankur Arora wrote:
> On 5/14/19 6:50 AM, Marcelo Tosatti wrote:
> >On Mon, May 13, 2019 at 05:20:37PM +0800, Wanpeng Li wrote:
> >>On Wed, 8 May 2019 at 02:57, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> >>>
> >>>
> >>>Certain workloads perform poorly on KVM compared to baremetal
> >>>due to baremetal's ability to perform mwait on NEED_RESCHED
> >>>bit of task flags (therefore skipping the IPI).
> >>
> >>KVM supports expose mwait to the guest, if it can solve this?
> >>
> >>Regards,
> >>Wanpeng Li
> >
> >Unfortunately mwait in guest is not feasible (uncompatible with multiple
> >guests). Checking whether a paravirt solution is possible.

Hi Ankur,

> 
> Hi Marcelo,
> 
> I was also looking at making MWAIT available to guests in a safe manner:
> whether through emulation or a PV-MWAIT. My (unsolicited) thoughts

What use-case are you interested in? 

> 
> We basically want to handle this sequence:
> 
>     monitor(monitor_address);
>     if (*monitor_address == base_value)
>          mwaitx(max_delay);
> 
> Emulation seems problematic because, AFAICS this would happen:
> 
>     guest                                   hypervisor
>     =====                                   ====
> 
>     monitor(monitor_address);
>         vmexit  ===>                        monitor(monitor_address)
>     if (*monitor_address == base_value)
>          mwait();
>               vmexit    ====>               mwait()
> 
> There's a context switch back to the guest in this sequence which seems
> problematic. Both the AMD and Intel specs list system calls and
> far calls as events which would lead to the MWAIT being woken up:
> "Voluntary transitions due to fast system call and far calls
> (occurring prior to issuing MWAIT but after setting the monitor)".
> 
> 
> We could do this instead:
> 
>     guest                                   hypervisor
>     =====                                   ====
> 
>     monitor(monitor_address);
>         vmexit  ===>                        cache monitor_address
>     if (*monitor_address == base_value)
>          mwait();
>               vmexit    ====>              monitor(monitor_address)
>                                            mwait()
> 
> But, this would miss the "if (*monitor_address == base_value)" check in
> the host which is problematic if *monitor_address changed simultaneously
> when monitor was executed.
> (Similar problem if we cache both the monitor_address and
> *monitor_address.)
> 
> 
> So, AFAICS, the only thing that would work is the guest offloading the
> whole PV-MWAIT operation.
> 
> AFAICS, that could be a paravirt operation which needs three parameters:
> (monitor_address, base_value, max_delay.)
> 
> This would allow the guest to offload this whole operation to
> the host:
>     monitor(monitor_address);
>     if (*monitor_address == base_value)
>          mwaitx(max_delay);
> 
> I'm guessing you are thinking on similar lines?

Sort of: only trying to avoid the IPI to wake a remote vCPU.

Problem is that MWAIT works only on a contiguous range 
of bits in memory (512 bits max on current CPUs).

So if you execute mwait on the host on behalf of the guest,
the region of memory monitored must include both host
and guest bits.

> 
> 
> High level semantics: If the CPU doesn't have any runnable threads, then
> we actually do this version of PV-MWAIT -- arming a timer if necessary
> so we only sleep until the time-slice expires or the MWAIT max_delay does.

That would kill the sched_wake_idle_without_ipi optimization for the
host.

> If the CPU has any runnable threads then this could still finish its
> time-quanta or we could just do a schedule-out.
> 
> 
> So the semantics guaranteed to the host would be that PV-MWAIT
> returns after >= max_delay OR with the *monitor_address changed.
> 
> 
> 
> Ankur