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Date:   Thu, 16 May 2019 19:06:30 -0700
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Marcelo Tosatti <mtosatti@...hat.com>,
        kvm-devel <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Bandan Das <bsd@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] sched: introduce configurable delay before entering idle

On 2019-05-15 6:07 p.m., Wanpeng Li wrote:
> On Thu, 16 May 2019 at 02:42, Ankur Arora <ankur.a.arora@...cle.com> 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@...hat.com> 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 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
> 
> MWAIT emulation is not simple, here is a research
> https://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/mwait.html
Agreed. I had outlined my attempt to do that below and come
to the conclusion that we would need a PV-MWAIT.

Ankur

> 
> Regards,
> Wanpeng Li
> 
>> follow.
>>
>> 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?
>>
>>
>> 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.
>>
>> 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

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