Commit 1b2d3451 authored by Mark Rutland's avatar Mark Rutland Committed by Peter Zijlstra
Browse files

arm64: Support PREEMPT_DYNAMIC 



This patch enables support for PREEMPT_DYNAMIC on arm64, allowing the
preemption model to be chosen at boot time.

Specifically, this patch selects HAVE_PREEMPT_DYNAMIC_KEY, so that each
preemption function is an out-of-line call with an early return
depending upon a static key. This leaves almost all the codegen up to
the compiler, and side-steps a number of pain points with static calls
(e.g. interaction with CFI schemes). This should have no worse overhead
than using non-inline static calls, as those use out-of-line trampolines
with early returns.

For example, the dynamic_cond_resched() wrapper looks as follows when
enabled. When disabled, the first `B` is replaced with a `NOP`,
resulting in an early return.

| <dynamic_cond_resched>:
|        bti     c
|        b       <dynamic_cond_resched+0x10>     // or `nop`
|        mov     w0, #0x0
|        ret
|        mrs     x0, sp_el0
|        ldr     x0, [x0, #8]
|        cbnz    x0, <dynamic_cond_resched+0x8>
|        paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      <preempt_schedule_common>
|        mov     w0, #0x1
|        ldp     x29, x30, [sp], #16
|        autiasp
|        ret

... compared to the regular form of the function:

| <__cond_resched>:
|        bti     c
|        mrs     x0, sp_el0
|        ldr     x1, [x0, #8]
|        cbz     x1, <__cond_resched+0x18>
|        mov     w0, #0x0
|        ret
|        paciasp
|        stp     x29, x30, [sp, #-16]!
|        mov     x29, sp
|        bl      <preempt_schedule_common>
|        mov     w0, #0x1
|        ldp     x29, x30, [sp], #16
|        autiasp
|        ret

Since arm64 does not yet use the generic entry code, we must define our
own `sk_dynamic_irqentry_exit_cond_resched`, which will be
enabled/disabled by the common code in kernel/sched/core.c. All other
preemption functions and associated static keys are defined there.

Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarArd Biesheuvel <ardb@kernel.org>
Acked-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Acked-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20220214165216.2231574-8-mark.rutland@arm.com
parent 8e12ab7c

arch/arm64/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -192,6 +192,7 @@ config ARM64 
	select HAVE_PERF_EVENTS

	select HAVE_PERF_REGS

	select HAVE_PERF_USER_STACK_DUMP

	select HAVE_PREEMPT_DYNAMIC_KEY

	select HAVE_REGS_AND_STACK_ACCESS_API

	select HAVE_POSIX_CPU_TIMERS_TASK_WORK

	select HAVE_FUNCTION_ARG_ACCESS_API

arch/arm64/include/asm/preempt.h

+17 −2
Original line number Diff line number Diff line
@@ -2,6 +2,7 @@ 
#ifndef __ASM_PREEMPT_H

#define __ASM_PREEMPT_H



#include <linux/jump_label.h>

#include <linux/thread_info.h>



#define PREEMPT_NEED_RESCHED	BIT(32)
@@ -80,10 +81,24 @@ static inline bool should_resched(int preempt_offset) 
}



#ifdef CONFIG_PREEMPTION



void preempt_schedule(void);

#define __preempt_schedule() preempt_schedule()

void preempt_schedule_notrace(void);



#ifdef CONFIG_PREEMPT_DYNAMIC



DECLARE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);

void dynamic_preempt_schedule(void);

#define __preempt_schedule()		dynamic_preempt_schedule()

void dynamic_preempt_schedule_notrace(void);

#define __preempt_schedule_notrace()	dynamic_preempt_schedule_notrace()



#else /* CONFIG_PREEMPT_DYNAMIC */



#define __preempt_schedule()		preempt_schedule()

#define __preempt_schedule_notrace()	preempt_schedule_notrace()



#endif /* CONFIG_PREEMPT_DYNAMIC */

#endif /* CONFIG_PREEMPTION */



#endif /* __ASM_PREEMPT_H */

arch/arm64/kernel/entry-common.c

+9 −1
Original line number Diff line number Diff line
@@ -220,9 +220,17 @@ static void noinstr arm64_exit_el1_dbg(struct pt_regs *regs) 
		lockdep_hardirqs_on(CALLER_ADDR0);

}



#ifdef CONFIG_PREEMPT_DYNAMIC

DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);

#define need_irq_preemption() \

	(static_branch_unlikely(&sk_dynamic_irqentry_exit_cond_resched))

#else

#define need_irq_preemption()	(IS_ENABLED(CONFIG_PREEMPTION))

#endif



static void __sched arm64_preempt_schedule_irq(void)

{

	if (!IS_ENABLED(CONFIG_PREEMPTION))

	if (!need_irq_preemption())

		return;



	/*