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

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Message-ID: <20150421150122.GC5561@linux.vnet.ibm.com>
Date:	Tue, 21 Apr 2015 08:01:22 -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 Tue, Apr 21, 2015 at 09:12:32AM -0400, Steven Rostedt wrote:
> On Mon, 20 Apr 2015 18:22:58 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Mon, Apr 20, 2015 at 04:50:07PM -0500, Clark Williams wrote:
> > > 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. 
> > 
> > There's the rub.  They also need to chrt the RCU grace-period kthreads
> > as well as the per-CPU kthreads (rcuc).  Which is a pain and easy to
> > get wrong.
> > 
> > So at this point, I am leaning towards keeping RCU_KTHREAD_PRIO, but
> > hiding it behind RCU_EXPERT.  Someone in an emergency situation can use
> > chrt to get RCU going, at least assuming that they had the foresight to
> > leave a prio-99 shell running somewhere and assuming that they do the
> > chrt before the system hits OOM.  But they have to do all that anyway
> > if they were to use a sysfs or similar interface.  And it is easy to
> > tell when you have boosted all the necessary kthreads because RCU
> > grace periods start advancing once again.  You don't get that feedback
> > when you set things up at boot time.  ;-)
> > 
> > So again, at least for the moment, I believe that RCU need not provide
> > a run-time interface for changing RCU kthread priorities, that the
> > RCU_KTHREAD_PRIO Kconfig parameter should remain, except that it needs
> > to be hidden behind RCU_EXPERT, and that the rcutree.kthread_prio=
> > kernel-boot parameter should also remain.
> > 
> > Seem reasonable?
> 
> Does chrt override the kthread_prio at run time? If so, then great.
> Otherwise, the sysadmin should still have a way to control their
> priorities of kernel threads (with few exceptions like the migration
> thread).

Yep, RCU sets the prios only at boot time, so if they are set differently
at runtime, they should stay set differently.  Unless chrt refuses to
work on kthreads or something. ;-)

							Thanx, Paul

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