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

[Ard Biesheuvel]

On Thu, 28 Jan 2021 at 21:25, Will Deacon <will at kernel.org> wrote:
>
> 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.
>

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