linux-kernel - Re: [RFC PATCH v2 7/7] sched/idle: update poll time when wakeup from idle

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Date:   Fri, 29 Sep 2017 18:29:21 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Yang Zhang <yang.zhang.wz@...il.com>
Cc:     quan.xu0@...il.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, wanpeng.li@...mail.com, mst@...hat.com,
        pbonzini@...hat.com, tglx@...utronix.de, rkrcmar@...hat.com,
        dmatlack@...gle.com, agraf@...e.de, linux-doc@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH v2 7/7] sched/idle: update poll time when wakeup from
 idle



On 2017/8/29 20:46, Peter Zijlstra wrote:
> On Tue, Aug 29, 2017 at 11:46:41AM +0000, Yang Zhang wrote:
>> In ttwu_do_wakeup, it will update avg_idle when wakeup from idle. Here
>> we just reuse this logic to update the poll time. It may be a little
>> late to update the poll in ttwu_do_wakeup, but the test result shows no
>> obvious performance gap compare with updating poll in irq handler.
>>
>> one problem is that idle_stamp only used when using CFS scheduler. But
>> it is ok since it is the default policy for scheduler and only consider
>> it should enough.
>>
>> Signed-off-by: Yang Zhang <yang.zhang.wz@...il.com>
>> Signed-off-by: Quan Xu <quan.xu0@...il.com>
> Same broken SoB chain, and not a useful word on why you need to adjust
> this crap to begin with. What you want that poll duration to be related
> to is the cost of a VMEXIT/VMENTER cycle, not however long we happened
> to be idle.
>
> So no.

Peter,

I think you are right..

IIUC, the time we happened to be idle may contain a chain of 
VMEXIT/VMENTER cycles,
which would be mainly (except the last VMEXIT/VMENTER cycles) for just 
idle loops. right?

as you mentioned, poll duration to be related to is the cost of __a__ 
VMEXIT/VMENTER cycle.
howerver it is very difficult to measure a VMEXIT/VMENTER cycle 
accurately from
kvm guest, we could find out an approximate one -- dropping the idle 
loops from the
time we happened to be idle.. make sense?

Quan