6.9-stable review patch. If anyone has any objections, please let me know.
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From: Linus Torvalds torvalds@linux-foundation.org
[ Upstream commit 5d272dd1b3430bb31fa30042490fa081512424e4 ]
Hardcoding the number of CPUs at compile time does improve code generation, but if you get it wrong the result will be confusion.
We already limited this earlier to only "experts" (see commit fe5759d5bfda "cpumask: limit visibility of FORCE_NR_CPUS"), but with distro kernel configs often having EXPERT enabled, that turns out to not be much of a limit.
To quote the philosophers at Disney: "Everyone can be an expert. And when everyone's an expert, no one will be".
There's a runtime warning if you then set nr_cpus to anything but the forced number, but apparently that can be ignored too [1] and by then it's pretty much too late anyway.
If we had some real way to limit this to "embedded only", maybe it would be worth it, but let's see if anybody even notices that the option is gone. We need to simplify kernel configuration anyway.
Link: https://lore.kernel.org/all/20240618105036.208a8860@rorschach.local.home/ [1] Reported-by: Steven Rostedt rostedt@goodmis.org Cc: Masami Hiramatsu mhiramat@kernel.org Cc: Mark Rutland mark.rutland@arm.com Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: Paul McKenney paulmck@kernel.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Peter Zijlstra peterz@infradead.org Cc: Yury Norov yury.norov@gmail.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- lib/Kconfig | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-)
diff --git a/lib/Kconfig b/lib/Kconfig index 4557bb8a52565..c98e11c7330ec 100644 --- a/lib/Kconfig +++ b/lib/Kconfig @@ -539,13 +539,7 @@ config CPUMASK_OFFSTACK stack overflow.
config FORCE_NR_CPUS - bool "Set number of CPUs at compile time" - depends on SMP && EXPERT && !COMPILE_TEST - help - Say Yes if you have NR_CPUS set to an actual number of possible - CPUs in your system, not to a default value. This forces the core - code to rely on compile-time value and optimize kernel routines - better. + def_bool !SMP
config CPU_RMAP bool