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arm64: assembler: add cond_yield macro

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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@kernel.org>
Link: https://lore.kernel.org/r/20210203113626.220151-2-ardb@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Ard Biesheuvel 2021-02-03 12:36:18 +01:00 committed by Will Deacon
parent 5c8fe583cc
commit d13c613f13
1 changed files with 16 additions and 0 deletions
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16
arch/arm64/include/asm/assembler.h
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@ -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