linux-kernel - Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used

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Message-ID: <20140613211034.GA10651@jtriplet-mobl1>
Date:	Fri, 13 Jun 2014 14:10:35 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually
 used

On Fri, Jun 13, 2014 at 01:48:22PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 13, 2014 at 09:44:41AM -0700, Josh Triplett wrote:
> > On Fri, Jun 13, 2014 at 06:21:32PM +0200, Frederic Weisbecker wrote:
> > > On Fri, Jun 13, 2014 at 09:16:30AM -0700, Paul E. McKenney wrote:
> > > > > Is it because we have dynticks CPUs staying too long in the kernel without
> > > > > taking any quiescent states? Are we perhaps missing some rcu_user_enter() or
> > > > > things?
> > > > 
> > > > Sort of the former, but combined with the fact that in-kernel CPUs still
> > > > need scheduling-clock interrupts for RCU to make progress.  I could
> > > > move this to RCU's context-switch hook, but that could be very bad for
> > > > workloads that do lots of context switching.
> > > 
> > > Or I can restart the tick if the CPU stays in the kernel for too long without
> > > a tick. I think that's what we were doing before but we removed that because
> > > we never implemented it correctly (we sent scheduler IPI that did nothing...)
> > 
> > I wonder if timer slack would make sense here: when you have at least
> > one RCU callback pending, set a timer with a huge amount of timer slack,
> > and cancel it if you end up handling the callback via a trip through the
> > scheduler.
> 
> But in this case, we need the tick even if the current CPU has no callbacks
> because it might be in an RCU read-side critical section.

Don't we handle that case via the slowpath of rcu_read_unlock, and a
flag set via IPI?  ("Oh, that CPU has taken too long to note a quiescent
state; send it an IPI to set the special flag that makes unlock do the
work.")

- Josh Triplett
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