Date: Sat, 21 Feb 2015 17:43:43 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>, linux-kernel@vger.kernel.org,
        mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com,
        fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods
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On Sat, Feb 21, 2015 at 05:08:52PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 20, 2015 at 09:45:39AM -0800, Arjan van de Ven wrote:
> > On 2/20/2015 9:43 AM, Peter Zijlstra wrote:
> > >On Fri, Feb 20, 2015 at 09:32:39AM -0800, Arjan van de Ven wrote:
> > >>there's a few others as well that I'm chasing down...
> > >>.. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
> > >
> > >So my objections are twofold:
> > >
> > >  - I object to fast expedites in principle; they spray IPIs across the
> > >    system, so ideally we'd not have them at all, therefore also not at
> > >    boot.
> > >
> > >    Because as soon as the option exists, people will use it for other
> > >    things too.
> > 
> > the option exists today in sysfs and kernel parameter...
> 
> Yeah, Paul and me have been having this argument for a while now ;-)

Indeed we have.  ;-)

And if expedited grace periods start causing latency issues in real-world
workloads, I will address those issues.

In the meantime, one of the nice things about NO_HZ_FULL is that
synchronize_sched_expedited() avoids IPIing CPUs having a single runnable
task that is running in nohz_full mode.  ;-)

								Thanx, Paul

> > >And esp. in bootup code you can special case a lot of stuff; there's
> > >limited concurrency esp. because userspace it not there yet. So we might
> > >not actually need those sync calls.
> > 
> > yeah I am going down that angle as well absolutely.
> > but there are cases that may well be legit (or are 5 function calls deep into common code)
> 
> Good ;-)
> 

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