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

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Message-ID: <20170713145311.z4zxlyd2dospeoqg@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Jul 2017 16:53:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        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 Thu, Jul 13, 2017 at 10:48:55PM +0800, Li, Aubrey wrote:

> - 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,

So Paul wants more details on where RCU hurts so we can try to fix.

> 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.

Fixing C-state selection by creating an alternative idle path sounds so
very wrong.