linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Message-Id: <20180709204248.GF3593@linux.vnet.ibm.com>
Date:   Mon, 9 Jul 2018 13:42:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Mon, Jul 09, 2018 at 09:35:38PM +0100, David Woodhouse wrote:
> 
> 
> On Mon, 2018-07-09 at 13:34 -0700, Paul E. McKenney wrote:
> > 
> > So here are the possible code paths when .rcu_urgent_qs is set to true:
> > 
> > 1.      A context switch will record the quiescent state and clear
> >         .rcu_urgent_qs.  (The failure to do the clearing in current -rcu
> >         for PREEMPT builds is a performance bug that I need to fix.)
> 
> What if there's nothing else runnable and there is no actual context
> switch?

The scheduler invokes rcu_note_context_switch() before looking to see
if there really will or won't be a context switch.

I am sure that Peter will correct me if I am confused on this point.  ;-)

							Thanx, Paul

> > 2.      A cond_resched() will cause rcu_all_qs() to be invoked, which
> >         will record a quiescent state and clear .rcu_urgent_qs.
> > 
> > 3.      With the patch below, a scheduling-clock interrupt of a
> >         non-idle non-userspace task will force a reschedule, which
> >         will result in #1 above happening.