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

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Date:   Thu, 19 Jul 2018 09:20:33 +0200
From:   David Woodhouse <dwmw2@...radead.org>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        paulmck@...ux.vnet.ibm.com
Cc:     Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Thu, 2018-07-19 at 08:45 +0200, Christian Borntraeger wrote:
> 
> > My thought would be something like this:
> > 
> >       if (context_tracking_cpu_is_enabled())
> >               rcu_kvm_enter();
> >       else
> >               rcu_virt_note_context_switch(smp_processor_id());
> 
> In the past we needed that (when we introduced that). At least with every
> host interrupt we called this making an rcu event at least every HZ.
> Will the changes in need_resched make this part unnecessary?

Yes, the change in need_resched() should make this part unnecessary.
Unless your architecture's version of the vcpu_run() loop just loops
forever even when TIF_NEED_RESCHED is set? :)

I'm not sure about the context tracking condition in the code snippet
cited above, though. I think that's what caused my problem in the first
place — I have CONTEXT_TRACKING_FORCE && !NO_HZ_FULL. So in 4.15, that
means rcu_user_enter() did nothing and rcu_virt_note_context_switch()
wasn't called. Hence the observed stalls.

Should rcu_user_enter() itself be conditional on CONTEXT_TRACKING not
NO_HZ_FULL? 
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