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

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Message-ID: <20150420165007.295f3fe0@sluggy>
Date:	Mon, 20 Apr 2015 16:50:07 -0500
From:	Clark Williams <williams@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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, 20 Apr 2015 14:15:04 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> 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?

Looks to me like all we need to do is tell people if they need a boost
higher than the compiled in default (RCU_KTHREAD_PRIO), then chrt the
priority of the rcub thread to the desired priority. 

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