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

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Message-Id: <20180719031152.GR12945@linux.vnet.ibm.com>
Date:   Wed, 18 Jul 2018 20:11:52 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        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 Thu, Jul 19, 2018 at 02:32:06AM +0200, Frederic Weisbecker wrote:
> 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.

David is working with v4.15.  Is this maybe something that has changed
since then?

							Thanx, Paul