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

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Date:   Wed, 19 Jul 2017 10:03:22 -0500 (CDT)
From:   Christopher Lameter <cl@...ux.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.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, 19 Jul 2017, Paul E. McKenney wrote:

> > 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.

Well is there a boundary when you would want the notification calls? I
would think that even an idle period of a couple of seconds does not
necessarily require a callback to rcu. Had some brokenness here where RCU
calls did not occur for hours or so. At some point the system ran out of
memory but thats far off.