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

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Message-ID: <1d7251cf-e04f-5216-b622-e1864557935e@gmail.com>
Date:   Fri, 1 Sep 2017 15:30:08 +0800
From:   Yang Zhang <yang.zhang.wz@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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,
        Quan Xu <quan.xu0@...il.com>, 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.

Actually, we should compare the cost of VMEXIT/VMENTER with the real 
duration in idle. We have a rough number of the cost for one 
VMEXIT/VMENTER(it is about 2k~4k cycles depends on the underlying CPU) 
and it introduces 4~5 VMENTER/VMEXITs in idle path which may increase 
about 7us latency in average. So we set the poll duration to 10us by 
default.

Another problem is there is no good way to measure the duration in idle. 
avg_idle is the only way i find so far. Do you have any suggestion to do 
it better? Thanks.

-- 
Yang
Alibaba Cloud Computing