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Date: Tue, 8 Jun 2021 15:55:09 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Frederic Weisbecker <frederic@...nel.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 02:04:41PM +0200, Frederic Weisbecker wrote: > Currently the default behaviour for CONFIG_PREEMPT_DYNAMIC is > preempt=full. So distros always have to override that with the boot > option if it's not their default choice. > > Make things more convenient for them with providing that choice at > Kconfig time. > > This should also encourage automatic testing robots relying on randconfig > to run through all the various preempt dynamic flavours. > > (Unfortunately this involved copy-pasting help text for static PREEMPT > Kconfig entries. Perhaps referring to them would be enough?) > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org> > --- > kernel/Kconfig.preempt | 53 ++++++++++++++++++++++++++++++++++++++++++ > kernel/sched/core.c | 23 +++++++++++++++++- > 2 files changed, 75 insertions(+), 1 deletion(-) > > diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt > index bd7c4147b9a8..384110d1a215 100644 > --- a/kernel/Kconfig.preempt > +++ b/kernel/Kconfig.preempt > @@ -100,6 +100,59 @@ config PREEMPT_DYNAMIC > Interesting if you want the same pre-built kernel should be used for > both Server and Desktop workloads. > > +choice > + prompt "Preemption dynamic default boot mode" > + default PREEMPT_DYNAMIC_FULL > + depends on PREEMPT_DYNAMIC > + > +config PREEMPT_DYNAMIC_NONE > + bool "Default boot with no Forced Preemption (Server)" > + help > + This is the traditional Linux preemption model, geared towards > + throughput. It will still provide good latencies most of the > + time, but there are no guarantees and occasional longer delays > + are possible. > + > + Select this option if you are building a kernel for a server or > + scientific/computation system, or if you want to maximize the > + raw processing power of the kernel, irrespective of scheduling > + latencies. > + > +config PREEMPT_DYNAMIC_VOLUNTARY > + bool "Default boot with Voluntary Kernel Preemption (Desktop)" > + help > + This option reduces the latency of the kernel by adding more > + "explicit preemption points" to the kernel code. These new > + preemption points have been selected to reduce the maximum > + latency of rescheduling, providing faster application reactions, > + at the cost of slightly lower throughput. > + > + This allows reaction to interactive events by allowing a > + low priority process to voluntarily preempt itself even if it > + is in kernel mode executing a system call. This allows > + applications to run more 'smoothly' even when the system is > + under load. > + > + Select this if you are building a kernel for a desktop system. > + > +config PREEMPT_DYNAMIC_FULL > + bool "Default boot with Preemptible Kernel (Low-Latency Desktop)" > + help > + This option reduces the latency of the kernel by making > + all kernel code (that is not executing in a critical section) > + preemptible. This allows reaction to interactive events by > + permitting a low priority process to be preempted involuntarily > + even if it is in kernel mode executing a system call and would > + otherwise not be about to reach a natural preemption point. > + This allows applications to run more 'smoothly' even when the > + system is under load, at the cost of slightly lower throughput > + and a slight runtime overhead to kernel code. > + > + Select this if you are building a kernel for a desktop or > + embedded system with latency requirements in the milliseconds > + range. > +endchoice > + > config SCHED_CORE > bool "Core Scheduling for SMT" > default y 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_NONE + default PREEMPT_TYPE_NONE -config PREEMPT_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_VOLUNTARY +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 PREEMPTION - select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK - select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC + 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 -config PREEMPT_COUNT - bool +# default model for PREEMPT_DYNAMIC -config PREEMPTION - bool - select PREEMPT_COUNT +config PREEMPT_DYNAMIC_NONE + bool -config PREEMPT_DYNAMIC +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 help This option allows to define the preemption model on the kernel command line parameter and thus override the default preemption @@ -100,6 +122,22 @@ config PREEMPT_DYNAMIC Interesting if you want the same pre-built kernel should be used for both Server and Desktop workloads. +# actual preemption model + +config PREEMPT_NONE + bool + default y if !PREEMPT_DYNAMIC && PREEMPT_TYPE_NONE + +config PREEMPT_VOLUNTARY + bool + default y if !PREEMPT_DYNAMIC && PREEMPT_TYPE_VOLUNTARY + +config PREEMPT + bool + default y if !PREEMPT_DYNAMIC && PREEMPT_TYPE_FULL + select PREEMPTION + select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK + config SCHED_CORE bool "Core Scheduling for SMT" default y
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