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Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops




2017-11-13 18:53 GMT+08:00 Juergen Gross <jgross@xxxxxxxx>:
> On 13/11/17 11:06, Quan Xu wrote:
>> From: Quan Xu <quan.xu0@xxxxxxxxx>
>>
>> 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@xxxxxxxxx>
>> Signed-off-by: Quan Xu <quan.xu0@xxxxxxxxx>
>> Cc: Juergen Gross <jgross@xxxxxxxx>
>> Cc: Alok Kataria <akataria@xxxxxxxxxx>
>> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: x86@xxxxxxxxxx
>> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
>
> 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