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

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Message-ID: <1531340384.8759.86.camel@infradead.org>
Date:   Wed, 11 Jul 2018 21:19:44 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     paulmck@...ux.vnet.ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>
Cc:     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, 2018-07-11 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?
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