[PATCH 1/9] arm64: assembler: add cond_yield macro

[Will Deacon]

On Thu, Jan 28, 2021 at 02:06:17PM +0100, Ard Biesheuvel wrote:
> Add a macro cond_yield that branches to a specified label when called if
> the TIF_NEED_RESCHED flag is set and decreasing the preempt count would
> make the task preemptible again, resulting in a schedule to occur. This
> can be used by kernel mode SIMD code that keeps a lot of state in SIMD
> registers, which would make chunking the input in order to perform the
> cond_resched() check from C code disproportionately costly.
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> ---
>  arch/arm64/include/asm/assembler.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> index bf125c591116..5f977a7c6b43 100644
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -745,6 +745,22 @@ USER(\label, ic	ivau, \tmp2)			// invalidate I line PoU
>  .Lyield_out_\@ :
>  	.endm
>  
> +	/*
> +	 * Check whether preempt-disabled code should yield as soon as it
> +	 * is able. This is the case if re-enabling preemption a single
> +	 * time results in a preempt count of zero, and the TIF_NEED_RESCHED
> +	 * flag is set. (Note that the latter is stored negated in the
> +	 * top word of the thread_info::preempt_count field)
> +	 */
> +	.macro		cond_yield, lbl:req, tmp:req
> +#ifdef CONFIG_PREEMPTION
> +	get_current_task \tmp
> +	ldr		\tmp, [\tmp, #TSK_TI_PREEMPT]
> +	cmp		\tmp, #PREEMPT_DISABLE_OFFSET
> +	beq		\lbl

Fancy that, I didn't know the '.' was optional in "b.eq"!

Anyway, a very similar code sequence exists inside if_will_cond_yield_neon,
only it doesn't touch the flags. Can we use that sequence instead, and then
use the new macro from there?

Will