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

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Message-ID: <b4e37c96-75af-ad3d-19ca-88d5d82e08c4@linux.intel.com>
Date:   Mon, 17 Jul 2017 22:02:49 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     paulmck@...ux.vnet.ibm.com, 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, 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 2017/7/17 21:58, Peter Zijlstra wrote:
> On Mon, Jul 17, 2017 at 09:24:51PM +0800, Li, Aubrey wrote:
>> On 2017/7/14 12:05, Paul E. McKenney wrote:
>>>
>>> More specifically: rcu_needs_cpu(), rcu_prepare_for_idle(),
>>> rcu_cleanup_after_idle(), rcu_eqs_enter(), rcu_eqs_enter_common(),
>>> rcu_dynticks_eqs_enter(), do_nocb_deferred_wakeup(),
>>> rcu_dynticks_task_enter(), rcu_eqs_exit(), rcu_eqs_exit_common(),
>>> rcu_dynticks_task_exit(), rcu_dynticks_eqs_exit().
>>>
>>> The first three (rcu_needs_cpu(), rcu_prepare_for_idle(), and
>>> rcu_cleanup_after_idle()) should not be significant unless you have
>>> CONFIG_RCU_FAST_NO_HZ=y.  If you do, it would be interesting to learn
>>> how often invoke_rcu_core() is invoked from rcu_prepare_for_idle()
>>> and rcu_cleanup_after_idle(), as this can raise softirq.  Also
>>> rcu_accelerate_cbs() and rcu_try_advance_all_cbs().
>>>
>>> Knowing which of these is causing the most trouble might help me
>>> reduce the overhead in the current idle path.
>>
>> I measured two cases, nothing notable found.
> 
> So skipping rcu_idle_{enter,exit}() is not in fact needed at all?
> 

I think it would make more sense if we still put them under the case
where tick is really stopped.

Thanks,
-Aubrey