Re: [PATCH v2 7/7] arm64: support PREEMPT_DYNAMIC

Catalin Marinas
2022-02-04T18:48:26Z

On Fri, Feb 04, 2022 at 03:05:57PM +0000, Mark Rutland wrote:
> 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 (with
> the first `B` being replaced with a `NOP` when the function is
> disabled):
>
> | <dynamic_cond_resched>:
> | bti c
> | b <dynamic_cond_resched+0x10>
> | mov w0, #0x0 // #0
> | 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 // #1
> | 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 // #0
> | ret
> | paciasp
> | stp x29, x30, [sp, #-16]!
> | mov x29, sp
> | bl <preempt_schedule_common>
> | mov w0, #0x1 // #1
> | 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.
>
> Note that PREEMPT_DYNAMIC is `def bool y`, so this will default to
> enabled.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Joey Gouly <joey.gouly@arm.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Will Deacon <will@kernel.org>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>