Commit d13c613f authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Will Deacon
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arm64: assembler: add cond_yield macro 



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: default avatarArd Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210203113626.220151-2-ardb@kernel.org


Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent 5c8fe583

arch/arm64/include/asm/assembler.h

+16 −0
Original line number Diff line number Diff line
@@ -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]

	sub		\tmp, \tmp, #PREEMPT_DISABLE_OFFSET

	cbz		\tmp, \lbl

#endif

	.endm



/*

 * This macro emits a program property note section identifying

 * architecture features which require special handling, mainly for