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

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Message-ID: <1531169145.26547.8.camel@infradead.org>
Date:   Mon, 09 Jul 2018 21:45:45 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     paulmck@...ux.vnet.ibm.com
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, 2018-07-09 at 13:42 -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.
> 
> I am sure that Peter will correct me if I am confused on this point.  ;-)

Ah, OK. Yes, that looks correct. Thanks.

I'll give your patch a spin tomorrow, unless Marius beats me to it.
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