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

[Will Deacon]

On Thu, Jan 28, 2021 at 08:24:01PM +0000, Will Deacon wrote:
> On Thu, Jan 28, 2021 at 02:06:17PM +0100, Ard Biesheuvel wrote:
> > 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 at 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.

Will