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

On Thu, 28 Jan 2021 at 21:25, Will Deacon [off-list ref] wrote:

On Thu, Jan 28, 2021 at 08:24:01PM +0000, Will Deacon wrote:

quoted

On Thu, Jan 28, 2021 at 02:06:17PM +0100, Ard Biesheuvel wrote:

quoted

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>
---
 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?

... and now I noticed the last patch :)

But it would still be nice not to clobber the flags inside the macro.

Yeah, that's a good point - I did not consider that. I'll fix that for v2.

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