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

David Woodhouse
2018-07-12T08:31:35Z

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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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