linux-kernel - Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via expedited GPs

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Message-ID: <20150702120329.GO3644@twins.programming.kicks-ass.net>
Date:	Thu, 2 Jul 2015 14:03:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 2/5] rcu: Short-circuit normal GPs via
 expedited GPs

On Wed, Jul 01, 2015 at 07:03:13AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 01, 2015 at 03:48:06PM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 01, 2015 at 06:41:44AM -0700, Paul E. McKenney wrote:
> > > On Wed, Jul 01, 2015 at 12:05:21PM +0200, Peter Zijlstra wrote:
> > > > On Tue, Jun 30, 2015 at 02:48:27PM -0700, Paul E. McKenney wrote:
> > > > >  			rsp->gp_state = RCU_GP_WAIT_FQS;
> > > > >  			ret = wait_event_interruptible_timeout(rsp->gp_wq,
> > > > > +				((gf = READ_ONCE(rsp->gp_flags)) &
> > > > > +				 RCU_GP_FLAG_FQS) ||
> > > > > +				(!READ_ONCE(rnp->qsmask) &&
> > > > > +				 !rcu_preempt_blocked_readers_cgp(rnp)) ||
> > > > > +				rcu_exp_gp_seq_done(rsp->exp_rsp,
> > > > > +						    rsp->gp_exp_snap),
> > > > > +				j);
> > > > >  			rsp->gp_state = RCU_GP_DONE_FQS;
> > > > 
> > > > How can the GP be done if we timed out or got interrupted?
> > > 
> > > If all the CPUs still blocking the grace period went idle, or in a
> > > NO_HZ_FULL kernel, entered nohz_full userspace execution.  Or, if
> > > certain low-probability races happen, went offline.
> > 
> > But what if none of those are true and we still timed out? You
> > unconditionally grant the GP.
> 
> Say what???
> 
> I recheck the conditions and break out of the loop only if one or more
> of the grace-period-end conditions is satisfied.  If not, I invoke
> rcu_gp_fqs() to do the scan to see if all remaining CPUs are idle,
> in nohz_full userspace execution, or offline.
> 
> What am I mising here?

The whole wait_event_interruptible_timeout() thing can end without @cond
being true, after which you unconditionally set ->gp_state =
RCU_GP_DONE_FQS.
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