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 <a...@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
+#endif
+ .endm
+
/*
* This macro emits a program property note section identifying
* architecture features which require special handling, mainly for
--
2.29.2
