Date: Wed, 12 Jul 2017 19:57:32 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Christoph Lameter <cl@linux.com>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Andi Kleen <ak@linux.intel.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 Wed, Jul 12, 2017 at 07:17:56PM +0200, Peter Zijlstra wrote:
> Could be I'm just not remembering how all that works.. But I was
> wondering if we can do the expensive bits if we've decided to actually
> go NOHZ and avoid doing it on every idle entry.
> 
> IIRC the RCU fast NOHZ bits try and flush the callback list (or paw it
> off to another CPU?) such that we can go NOHZ sooner. Having a !empty
> callback list avoid NOHZ from happening.
> 
> Now if we've already decided we can't in fact go NOHZ due to other
> concerns, flushing the callback list is pointless work. So I'm thinking
> we can find a better place to do this.

I'm a wee bit confused by the split between rcu_prepare_for_idle() and
rcu_needs_cpu().

There's a fair amount overlap there.. that said, I'm thinking we should
be calling rcu_needs_cpu() as the very last test, not the very first,
such that we can bail out of tick_nohz_stop_sched_tick() without having
to incur the penalty of flushing callbacks.