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

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Message-Id: <20170719144827.GB3730@linux.vnet.ibm.com>
Date:   Wed, 19 Jul 2017 07:48:27 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.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 Wed, Jul 19, 2017 at 01:44:06PM +0800, Li, Aubrey wrote:
> 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.

Indeed, the problem arises when the tick is stopped.

							Thanx, Paul