linux-kernel - Re: [PATCH 1/2] sched: Add default dynamic preempt mode Kconfig

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Date:   Wed, 9 Jun 2021 13:27:50 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched: Add default dynamic preempt mode Kconfig

On Tue, Jun 08, 2021 at 04:04:47PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 08, 2021 at 03:55:09PM +0200, Peter Zijlstra wrote:
> > Urgh, would something like this work?
> > 
> > ---
> >  kernel/Kconfig.preempt | 64 ++++++++++++++++++++++++++++++++++++++++----------
> >  1 file changed, 51 insertions(+), 13 deletions(-)
> > 
> > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> > index bd7c4147b9a8..43c68a806e4e 100644
> > --- a/kernel/Kconfig.preempt
> > +++ b/kernel/Kconfig.preempt
> > @@ -1,11 +1,25 @@
> >  # SPDX-License-Identifier: GPL-2.0-only
> >  
> > +config PREEMPT_COUNT
> > +       bool
> > +
> > +config PREEMPTION
> > +       bool
> > +       select PREEMPT_COUNT
> > +
> > +config PREEMPT_DYNAMIC_OPTION
> > +	bool
> > +	help
> > +	  Symbol that gates availablility of PREEMPT_DYNAMIC, selected
> > +	  by preemption models that can be dynamically selected.
> > +
> >  choice
> >  	prompt "Preemption Model"
> > +	default PREEMPT_TYPE_NONE
> >  
> > +config PREEMPT_TYPE_NONE
> >  	bool "No Forced Preemption (Server)"
> > +	select PREEMPT_DYNAMIC_OPTION if HAVE_PREEMPT_DYNAMIC
> >  	help
> >  	  This is the traditional Linux preemption model, geared towards
> >  	  throughput. It will still provide good latencies most of the
> > @@ -17,9 +31,10 @@ config PREEMPT_NONE
> >  	  raw processing power of the kernel, irrespective of scheduling
> >  	  latencies.
> >  
> > +config PREEMPT_TYPE_VOLUNTARY
> >  	bool "Voluntary Kernel Preemption (Desktop)"
> >  	depends on !ARCH_NO_PREEMPT
> > +	select PREEMPT_DYNAMIC_OPTION if HAVE_PREEMPT_DYNAMIC
> >  	help
> >  	  This option reduces the latency of the kernel by adding more
> >  	  "explicit preemption points" to the kernel code. These new
> > @@ -35,12 +50,10 @@ config PREEMPT_VOLUNTARY
> >  
> >  	  Select this if you are building a kernel for a desktop system.
> >  
> > -config PREEMPT
> > +config PREEMPT_TYPE_FULL
> >  	bool "Preemptible Kernel (Low-Latency Desktop)"
> >  	depends on !ARCH_NO_PREEMPT
> > +	select PREEMPT_DYNAMIC_OPTION if HAVE_PREEMPT_DYNAMIC
> >  	help
> >  	  This option reduces the latency of the kernel by making
> >  	  all kernel code (that is not executing in a critical section)
> > @@ -75,15 +88,24 @@ config PREEMPT_RT
> >  
> >  endchoice
> >  
> > +# default model for PREEMPT_DYNAMIC
> >  
> > +config PREEMPT_DYNAMIC_NONE
> > +	bool
> >  
> > +config PREEMPT_DYNAMIC_VOLUNTARY
> > +	bool
> > +
> > +config PREEMPT_DYNAMIC_FULL
> >  	bool
> > +
> > +config PREEMPT_DYNAMIC
> > +	bool "Dynamic Preemption Mode"
> > +	depends on PREEMPT_DYNAMIC_OPTION
> > +	select PREEMPT
> > +	select PREEMPT_DYNAMIC_NONE if PREEMPT_TYPE_NONE
> > +	select PREEMPT_DYNAMIC_VOLUNTARY if PREEMPT_TYPE_VOLUNTARY
> > +	select PREEMPT_DYNAMIC_FULL if PREEMPT_TYPE_FULL
> 
> Arguably PREEMPT_DYNAMIC_{NONE,VOLUNTARY,FULL} are superfluous and the
> default selection can use PREEMPT_TYPE_*.

Right, I'll try that.

Thanks.