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

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Date:   Tue, 10 Jul 2018 11:24:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     David Woodhouse <dwmw2@...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 01:42:48PM -0700, Paul E. McKenney wrote:
> 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.

Correct. Just getting to __schedule() means we can schedule and thus is
a valid point for RCU to progress. Even if we then end up selecting the
very same task and not switching at all.