Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched: Tweak default dynamic preempt mode selection | Date | Tue, 09 Nov 2021 09:52:54 +0000 |
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On 09/11/21 06:30, Mike Galbraith wrote: > Not seeing your v2 land yet, I grabbed my mallet and had a go at goal > reconciliation over morning java. Non-lovely result seems to work. >
Yeah so I went down a debatable path, gave up on that and started something different, and gave up on that because it was late :-)
Now interestingly my second attempt is pretty close to what you have below.
> sched, Kconfig: Fix preemption model selection > > Switch PREEMPT_DYNAMIC/PREEMPT_RT dependency around so PREEMPT_RT > can be selected during the initial preemption model selection. > Further, since PREEMPT_DYNAMIC requires PREEMPT, make it depend > upon it instead of selecting it, and add a menu to allow selection > of the boot time behavior, this to allow arches that do not support > PREEMPT_DYNAMIC to retain their various configs untouched. >
Have some nits below, but otherwise where I stand right now I think it's the least ugly way of tackling this :)
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Mike Galbraith <efault@gmx.de> > --- > kernel/Kconfig.preempt | 46 +++++++++++++++++++++++++--------------- > ------ > 1 file changed, 25 insertions(+), 21 deletions(-) > > --- a/kernel/Kconfig.preempt > +++ b/kernel/Kconfig.preempt > @@ -2,11 +2,10 @@ > > choice > prompt "Preemption Model" > - default PREEMPT_NONE_BEHAVIOUR > + default PREEMPT_NONE > > -config PREEMPT_NONE_BEHAVIOUR > +config PREEMPT_NONE > bool "No Forced Preemption (Server)" > - select PREEMPT_NONE if !PREEMPT_DYNAMIC > help > This is the traditional Linux preemption model, geared > towards > throughput. It will still provide good latencies most of the > @@ -18,10 +17,9 @@ config PREEMPT_NONE_BEHAVIOUR > raw processing power of the kernel, irrespective of > scheduling > latencies. > > -config PREEMPT_VOLUNTARY_BEHAVIOUR > +config PREEMPT_VOLUNTARY > bool "Voluntary Kernel Preemption (Desktop)" > depends on !ARCH_NO_PREEMPT > - select PREEMPT_VOLUNTARY if !PREEMPT_DYNAMIC > help > This option reduces the latency of the kernel by adding more > "explicit preemption points" to the kernel code. These new > @@ -37,10 +35,11 @@ config PREEMPT_VOLUNTARY_BEHAVIOUR > > Select this if you are building a kernel for a desktop > system. > > -config PREEMPT_BEHAVIOUR > +config PREEMPT > bool "Preemptible Kernel (Low-Latency Desktop)" > depends on !ARCH_NO_PREEMPT > - select PREEMPT > + select PREEMPTION > + select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK > help > This option reduces the latency of the kernel by making > all kernel code (that is not executing in a critical > section) > @@ -58,7 +57,7 @@ config PREEMPT_BEHAVIOUR > > config PREEMPT_RT > bool "Fully Preemptible Kernel (Real-Time)" > - depends on EXPERT && ARCH_SUPPORTS_RT && !PREEMPT_DYNAMIC > + depends on EXPERT && ARCH_SUPPORTS_RT > select PREEMPTION > help > This option turns the kernel into a real-time kernel by > replacing
This we can't get away from IMO - it's pretty much the same issue as
b8d3349803ba ("sched/rt, Kconfig: Unbreak def/oldconfig with CONFIG_PREEMPT=y")
IOW we can't really touch those preempt configs, but rather add stuff around them.
> @@ -75,17 +74,6 @@ config PREEMPT_RT > > endchoice > > -config PREEMPT_NONE > - bool > - > -config PREEMPT_VOLUNTARY > - bool > - > -config PREEMPT > - bool > - select PREEMPTION > - select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK > - > config PREEMPT_COUNT > bool > > @@ -95,8 +83,7 @@ config PREEMPTION > > config PREEMPT_DYNAMIC > bool "Preemption behaviour defined on boot" > - depends on HAVE_PREEMPT_DYNAMIC > - select PREEMPT > + depends on HAVE_PREEMPT_DYNAMIC && PREEMPT
I got there too. If we look at x86 defconfig, it selects PREEMPT_VOLUNTARY. I initially tried making it so PREEMPT_VOLUNTARY means both:
- VOLUNTARY model if !PREEMPT_DYNAMIC - VOLUNTARY model as default boot-time mode if PREEMPT_DYNAMIC (& similar for other preemption modes)
which, now that I've played with it, is a sad wreck. Having PREEMPT_DYNAMIC depend on PREEMPT at least makes things clear, and defconfig choices remain respected (they just won't get PREEMPT_DYNAMIC if they didn't pick PREEMPT).
> default y > help > This option allows to define the preemption model on the > kernel > @@ -114,6 +101,23 @@ config PREEMPT_DYNAMIC > Interesting if you want the same pre-built kernel should be > used for > both Server and Desktop workloads. > > +if PREEMPT_DYNAMIC
AFAICT that should be
depends on PREEMPT_DYNAMIC
within the choice itself. See e.g. "Kernel compression mode" choice in init/Kconfig.
> +choice > + prompt "Boot Time Preemption Model"
Default boot-time [...]? This is only used if the cmdline parameter is missing. Also a help-text wouldn't hurt. I had: This option defines the default preemption model of the kernel if it hasn't been specified by the command line parameter.
> + default PREEMPT_NONE_BEHAVIOR > + > +config PREEMPT_NONE_BEHAVIOR > + bool "No Forced Preemption (Server)" > + > +config PREEMPT_VOLUNTARY_BEHAVIOR > + bool "Voluntary Kernel Preemption (Desktop)" > + > +config PREEMPT_BEHAVIOR > + bool "Preemptible Kernel (Low-Latency Desktop)" > + > +endchoice > +endif # PREEMPT_DYNAMIC > + > config SCHED_CORE > bool "Core Scheduling for SMT" > depends on SCHED_SMT
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