linux-kernel - Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode

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Date:   Thu, 12 Jul 2018 09:31:28 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     paulmck@...ux.vnet.ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>
Cc:     peterz@...radead.org, mhillenb@...zon.de,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when
 entering guest mode



On Wed, 2018-07-11 at 16:47 -0700, Paul E. McKenney wrote:
> 
> > > What changed was RCU's reactions to longish grace periods.  It used to
> > > be very aggressive about forcing the scheduler to do otherwise-unneeded
> > > context switches, which became a problem somewhere between v4.9 and v4.15.
> > > I therefore reduced the number of such context switches, which in turn
> > > caused KVM to tell RCU about quiescent states way too infrequently.
> > 
> > You talk about 
> > commit bcbfdd01dce5556a952fae84ef16fd0f12525e7b
> >     rcu: Make non-preemptive schedule be Tasks RCU quiescent state
> > 
> > correct? In fact, then whatever (properly sent) patch comes up should contain
> > a fixes tag.
> 
> Not that one, but this one is at least part of the "team":
> 
> 28053bc72c0e5 ("rcu: Add long-term CPU kicking").  I might need to use
> "git bisect" to find the most relevant commit...  :-/

Whichever commit we blame, I suspect the Fixes: tag wants to go on
Paul's earlier 'Make need_resched() respond to urgent RCU-QS needs'
patch.

The rcu_kvm_enter() thing we're talking about now is actually
addressing a problem which has existed for much longer — that with
NO_HZ_FULL, a CPU might end up in guest mode for an indefinite period
of time, with RCU waiting for it to return.
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