linux-kernel - Re: [GIT RFC PULL rcu/urgent] Prevent Kconfig from asking pointless questions

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Message-ID: <20150420211504.GW5561@linux.vnet.ibm.com>
Date:	Mon, 20 Apr 2015 14:15:04 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Clark Williams <williams@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, a.p.zijlstra@...llo.nl,
	akpm@...ux-foundation.org, linux-rt-users@...r.kernel.org
Subject: Re: [GIT RFC PULL rcu/urgent] Prevent Kconfig from asking pointless
 questions

On Mon, Apr 20, 2015 at 04:40:49PM -0400, Steven Rostedt wrote:
> On Mon, Apr 20, 2015 at 10:09:03AM -0700, Paul E. McKenney wrote:
> > 
> > The sysfs knob might be nice, but as far as I know nobody has been
> > complaining about it.
> > 
> > Besides, we already have the rcutree.kthread_prio= kernel-boot parameter.
> > So how about if the Kconfig parameter selects either SCHED_OTHER
> > (the default) or SCHED_FIFO:1, and then the boot parameter can be used
> > to select other values.
> 
> Hmm, what priority is this for anyway. To change the priority of the boost
> value at run time, do we only need to change the priority of the rcub threads?
> 
> And the priority of the other rcu threads can change as well with a simple
> chrt?
> 
> If that's the case, then we don't need a sysctl knob at all.

For the grace-period kthreads and the boost kthread, that is the case.
It is also the case for the per-CPU kthreads that invoke RCU callbacks
for the non-offloaded RCU_BOOST configuration (and that replace all
softirq RCU work in -rt).

So, should I just ditch all of the priority-setting within RCU and tell
users to just use chrt?

							Thanx, Paul

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