Date: Mon, 17 Jul 2017 15:58:51 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Li, Aubrey" <aubrey.li@linux.intel.com>
Cc: paulmck@linux.vnet.ibm.com, Andi Kleen <ak@linux.intel.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Christoph Lameter <cl@linux.com>, Aubrey Li <aubrey.li@intel.com>,
        tglx@linutronix.de, len.brown@intel.com, rjw@rjwysocki.net,
        tim.c.chen@linux.intel.com, arjan@linux.intel.com,
        yang.zhang.wz@gmail.com, x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods
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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?