linux-kernel - Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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Date:   Thu, 13 Jul 2017 22:48:55 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     Frederic Weisbecker <fweisbec@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        len.brown@...el.com, rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, paulmck@...ux.vnet.ibm.com,
        yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On 2017/7/13 16:36, Peter Zijlstra wrote:
> On Wed, Jul 12, 2017 at 02:32:40PM -0700, Andi Kleen wrote:
> 
>>> It uses the normal idle path, it just makes the NOHZ enter fail.
>>
>> Which is only a small part of the problem.
> 
> Given the data so far provided it was by far the biggest problem. If you
> want more things changed, you really have to give more data.
> 

I have a data between arch_cpu_idle_enter and arch_cpu_idle_exit, this already
excluded HW sleep.

- totally from arch_cpu_idle_enter entry to arch_cpu_idle_exit return costs
  9122ns - 15318ns.
---- In this period(arch idle), rcu_idle_enter costs 1985ns - 2262ns, rcu_idle_exit
     costs 1813ns - 3507ns

Besides RCU, the period includes c-state selection on X86, a few timestamp updates
and a few computations in menu governor. Also, deep HW-cstate latency can be up
to 100+ microseconds, even if the system is very busy, CPU still has chance to enter
deep cstate, which I guess some outburst workloads are not happy with it.

That's my major concern without a fast idle path.

Thanks,
-Aubrey