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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Mon, 13 Nov 2017 19:09:09 +0800
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Subject: Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops
To:     Juergen Gross <jgross@suse.com>
Cc:     Quan Xu <quan.xu03@gmail.com>, kvm <kvm@vger.kernel.org>,
        linux-doc@vger.kernel.org,
        "open list:FILESYSTEMS (VFS and infrastructure)" 
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        "the arch/x86 maintainers" <x86@kernel.org>,
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        Quan Xu <quan.xu0@gmail.com>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Alok Kataria <akataria@vmware.com>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>
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2017-11-13 18:53 GMT+08:00 Juergen Gross <jgross@suse.com>:
> On 13/11/17 11:06, Quan Xu wrote:
>> From: Quan Xu <quan.xu0@gmail.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@gmail.com>
>> Signed-off-by: Quan Xu <quan.xu0@gmail.com>
>> Cc: Juergen Gross <jgross@suse.com>
>> Cc: Alok Kataria <akataria@vmware.com>
>> Cc: Rusty Russell <rusty@rustcorp.com.au>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: x86@kernel.org
>> Cc: virtualization@lists.linux-foundation.org
>> Cc: linux-kernel@vger.kernel.org
>> Cc: xen-devel@lists.xenproject.org
>
> Hmm, is the idle entry path really so critical to performance that a new
> pvops function is necessary? 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.

There is a "Adaptive halt-polling" which are merged to upstream more
than two years ago avoids to thread the critical path and has already
been ported to other architectures. https://lkml.org/lkml/2015/9/3/615

Regards,
Wanpeng Li

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