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

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Message-ID: <df9cd535-3739-ebd2-f27d-e73c1806f0a1@linux.intel.com>
Date:   Wed, 19 Jul 2017 13:44:06 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Aubrey Li <aubrey.li@...el.com>, len.brown@...el.com,
        rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On 2017/7/18 23:20, Paul E. McKenney wrote:

>> 2) for rcu idle enter/exit, I measured the details which Paul provided, and
>> the result matches with what I have measured before, nothing notable found.
>> But it still makes more sense if we can make rcu idle enter/exit hooked with
>> tick off. (it's possible other workloads behave differently)
> 
> Again, assuming that RCU is informed of CPUs in the kernel, regardless
> of whether or not the tick is on that that point in time.
> 
Yeah, I see, no problem for a normal idle.

But for a short idle, we want to return to the task ASAP. Even though RCU cost
is not notable, it would still be better for me if we can save some cycles in
idle entry and idle exit.

Do we have any problem if we skip RCU idle enter/exit under a fast idle scenario?
My understanding is, if tick is not stopped, then we don't need inform RCU in
idle path, it can be informed in irq exit.

Thanks,
-Aubrey