linux-kernel - Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle

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Date:   Tue, 14 Nov 2017 15:02:41 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Juergen Gross <jgross@...e.com>, Quan Xu <quan.xu03@...il.com>,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, x86@...nel.org,
        xen-devel@...ts.xenproject.org
Cc:     Yang Zhang <yang.zhang.wz@...il.com>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops



On 2017/11/13 18:53, Juergen Gross wrote:
> On 13/11/17 11:06, Quan Xu wrote:
>> From: Quan Xu <quan.xu0@...il.com>
>>
>> So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called
>> in idle path which will poll for a while before we enter the real idle
>> state.
>>
>> In virtualization, idle path includes several heavy operations
>> includes timer access(LAPIC timer or TSC deadline timer) which will
>> hurt performance especially for latency intensive workload like message
>> passing task. The cost is mainly from the vmexit which is a hardware
>> context switch between virtual machine and hypervisor. Our solution is
>> to poll for a while and do not enter real idle path if we can get the
>> schedule event during polling.
>>
>> Poll may cause the CPU waste so we adopt a smart polling mechanism to
>> reduce the useless poll.
>>
>> Signed-off-by: Yang Zhang <yang.zhang.wz@...il.com>
>> Signed-off-by: Quan Xu <quan.xu0@...il.com>
>> Cc: Juergen Gross <jgross@...e.com>
>> Cc: Alok Kataria <akataria@...are.com>
>> Cc: Rusty Russell <rusty@...tcorp.com.au>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: "H. Peter Anvin" <hpa@...or.com>
>> Cc: x86@...nel.org
>> Cc: virtualization@...ts.linux-foundation.org
>> Cc: linux-kernel@...r.kernel.org
>> Cc: xen-devel@...ts.xenproject.org
> Hmm, is the idle entry path really so critical to performance that a new
> pvops function is necessary?
Juergen, Here is the data we get when running benchmark netperf:
  1. w/o patch and disable kvm dynamic poll (halt_poll_ns=0):
     29031.6 bit/s -- 76.1 %CPU

  2. w/ patch and disable kvm dynamic poll (halt_poll_ns=0):
     35787.7 bit/s -- 129.4 %CPU

  3. w/ kvm dynamic poll:
     35735.6 bit/s -- 200.0 %CPU

  4. w/patch and w/ kvm dynamic poll:
     42225.3 bit/s -- 198.7 %CPU

  5. idle=poll
     37081.7 bit/s -- 998.1 %CPU



  w/ this patch, we will improve performance by 23%.. even we could improve
  performance by 45.4%, if we use w/patch and w/ kvm dynamic poll. also the
  cost of CPU is much lower than 'idle=poll' case..

> Wouldn't a function pointer, maybe guarded
> by a static key, be enough? A further advantage would be that this would
> work on other architectures, too.

I assume this feature will be ported to other archs.. a new pvops makes code
clean and easy to maintain. also I tried to add it into existed pvops, 
but it
doesn't match.



Quan
Alibaba Cloud
>
> Juergen
>