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arm64: Support PREEMPT_DYNAMIC

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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: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20220214165216.2231574-8-mark.rutland@arm.com
This commit is contained in:
Mark Rutland 2022-02-14 16:52:16 +00:00 committed by Peter Zijlstra
parent 8e12ab7c0e
commit 1b2d3451ee
3 changed files with 27 additions and 3 deletions
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1
arch/arm64/Kconfig
View File

@ -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

19
arch/arm64/include/asm/preempt.h
View File

@ -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);
#define __preempt_schedule_notrace() preempt_schedule_notrace()
#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 */

10
arch/arm64/kernel/entry-common.c
View File

@ -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;
/*