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

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Date:   Thu, 19 Jul 2018 02:32:06 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     paulmck@...ux.vnet.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 06:03:42PM +0100, David Woodhouse wrote:
> On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote:
> > And here is an updated v4.15 patch with Marius's Reported-by and David's
> > fix to my lost exclamation point.
> 
> Thanks. Are you sending the original version of that to Linus? It'd be
> useful to have the commit ID so that we can watch for it landing, and
> chase this one up to Greg.
> 
> As discussed on IRC, this patch reduces synchronize_sched() latency for
> us from ~4600s to ~160ms, which is nice.
> 
> However, it isn't going to be sufficient in the NO_HZ_FULL case. For
> that you want a patch like the one below, which happily reduces the
> latency in our (!NO_HZ_FULL) case still further to ~40ms.

That is interesting. As I replied to Paul, we are already calling
rcu_user_enter/exit() on guest_enter/exit_irqsoff(). So I'm wondering why
you're seeing such an optimization by repeating those calls.

Perhaps the rcu_user_* somehow aren't actually called from
__context_tracking_enter()...? Some bug in context tracking?
Otherwise it's a curious side effect.

Thanks.