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

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Date:   Wed, 11 Jul 2018 14:08:28 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>
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, Jul 11, 2018 at 09:19:44PM +0100, David Woodhouse wrote:
> On Wed, 2018-07-11 at 13:17 -0700, Paul E. McKenney wrote:
> > As I understand it, they would like to have their guest run uninterrupted
> > for extended times.  Because rcu_virt_note_context_switch() is a
> > point-in-time quiescent state, it cannot tell RCU about the extended
> > quiescent state.
> > 
> > Should we replace the current calls to rcu_virt_note_context_switch()
> > with rcu_kvm_enter() and rcu_kvm_exit()?  Would that be better
> > than the below architecture-by-architecture approach?
> 
> Yes it would. I was already starting to mutter about needing the same
> for ARM and POWER. I'll do a v3 (incorporating your fixes too) in the
> morning.
> 
> Thanks.
> 
> Also... why in $DEITY's name was the existing
> rcu_virt_note_context_switch() not actually sufficient? If we had that
> there, why did we need an additional explicit calls to rcu_all_qs() in
> the KVM loop, or the more complex fixes to need_resched() which
> ultimately had the same effect, to avoid ten-second latencies?

My guess is that this was because control passed through the
rcu_virt_note_context_switch() only once, and then subsequent
scheduling-clock interrupts bypassed this code.  But that is just a guess.
I need to defer to someone who understands the KVM code better than I do.

							Thanx, Paul