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

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Message-ID: <1531942865.3414.35.camel@infradead.org>
Date:   Wed, 18 Jul 2018 21:41:05 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested



On Wed, 2018-07-18 at 09:37 -0700, Paul E. McKenney wrote:
> On Wed, Jul 18, 2018 at 06:01:51PM +0200, David Woodhouse wrote:
> > 
> > On Wed, 2018-07-18 at 08:36 -0700, Paul E. McKenney wrote:
> > > 
> > > And I finally did get some near misses from an earlier commit, so we
> > > should consider your patch to be officially off the hook.
> >
> > Yay, I like it when it's not my fault. I'll redo it with the ifdef
> > CONFIG_NO_HZ_FULL.
>
> Hey, I didn't say it wasn't your fault, only that it -officially- wasn't
> your fault.  ;-)

I can live with being innocent until proven guilty.

> > 
> > What should it do for the !CONFIG_NO_HZ_FULL case? The existing call in
> > guest_enter_irqoff() clearly wasn't actually doing the right thing
> > anyway, hence the need for the need_resched() patch in $SUBJECT... so
> > should I just leave it doing nothing in guest_enter_irqoff()? 
>
> One starting point would be the combination of your patch and my
> patch, with -rcu commit IDs and diff below.  But yes, it needs to be
> !CONFIG_NO_HZ_FULL.  And no, I am not at all confident that I actually
> found all the places needing change in the core code, so this needs some
> serious review both by the KVM guys and the NO_HZ_FULL guys.

Right, that looks fairly much like the version I'd ended up with. So my
question was...

> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -118,12 +118,12 @@ static inline void guest_enter_irqoff(void)
>  	 * one time slice). Lets treat guest mode as quiescent state, just like
>  	 * we do with user-mode execution.
>  	 */

...if we change this to something like...

#ifdef CONFIG_NO_HZ_FULL
> +	rcu_kvm_enter();
#else
> 	if (!context_tracking_cpu_is_enabled())
> 		rcu_virt_note_context_switch(smp_processor_id());
#endif

... do you actually want me to keep the #else case there? It blatantly
wasn't working anyway for us, perhaps because the condition was false?
That's why I started fixing need_resched() in the first place, and that
fix ought to cover whatever this call to rcu_virt_note_context_switch()
was supposed to be doing?

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