Re: [PATCH 1/9] arm64: assembler: add cond_yield macro
From: Will Deacon <will@kernel.org>
Date: 2021-01-28 20:25:33
Also in:
linux-arm-kernel
On Thu, Jan 28, 2021 at 02:06:17PM +0100, Ard Biesheuvel wrote:
quoted hunk ↗ jump to hunk
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? Will