[PATCH v2 11/19] arm64: assembler: add macro to conditionally yield the NEON under PREEMPT

Wed Dec 6 04:12:51 PST 2017

On Wed, Dec 06, 2017 at 11:57:12AM +0000, Ard Biesheuvel wrote:
> On 6 December 2017 at 11:51, Dave Martin <Dave.Martin at arm.com> wrote:
> > On Tue, Dec 05, 2017 at 06:04:34PM +0000, Ard Biesheuvel wrote:
> >> On 5 December 2017 at 12:45, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> >> >
> >> >
> >> >> On 5 Dec 2017, at 12:28, Dave Martin <Dave.Martin at arm.com> wrote:
> >> >>
> >> >>> On Mon, Dec 04, 2017 at 12:26:37PM +0000, Ard Biesheuvel wrote:
> >> >>> Add a support macro to conditionally yield the NEON (and thus the CPU)
> >> >>> that may be called from the assembler code. Given that especially the
> >> >>> instruction based accelerated crypto code may use very tight loops, add
> >> >>> some parametrization so that the TIF_NEED_RESCHED flag test is only
> >> >>> executed every so many loop iterations.
> >> >>>
> >> >>> In some cases, yielding the NEON involves saving and restoring a non
> >> >>> trivial amount of context (especially in the CRC folding algorithms),
> >> >>> and so the macro is split into two, and the code in between is only
> >> >>> executed when the yield path is taken, allowing the contex to be preserved.
> >> >>> The second macro takes a label argument that marks the resume-from-yield
> >> >>> path, which should restore the preserved context again.
> >> >>>
> >> >>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> >> >>> ---
> >> >>> arch/arm64/include/asm/assembler.h | 50 ++++++++++++++++++++
> >> >>> 1 file changed, 50 insertions(+)
> >> >>>
> >> >>> diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> >> >>> index aef72d886677..917b026d3e00 100644
> >> >>> --- a/arch/arm64/include/asm/assembler.h
> >> >>> +++ b/arch/arm64/include/asm/assembler.h
> >> >>> @@ -512,4 +512,54 @@ alternative_else_nop_endif
> >> >>> #endif
> >> >>>    .endm
> >> >>>
> >> >>> +/*
> >> >>> + * yield_neon - check whether to yield to another runnable task from
> >> >>> + *        kernel mode NEON code (running with preemption disabled)
> >> >>> + *
> >> >>> + * - Check whether the preempt count is exactly 1, in which case disabling
> >> >>> + *   preemption once will make the task preemptible. If this is not the case,
> >> >>> + *   yielding is pointless.
> >> >>> + * - Check whether TIF_NEED_RESCHED is set, and if so, disable and re-enable
> >> >>> + *   kernel mode NEON (which will trigger a reschedule), and branch to the
> >> >>> + *   yield fixup code at @lbl.
> >> >>> + */
> >> >>> +    .macro        yield_neon, lbl:req, ctr, order, stride, loop
> >> >>> +    yield_neon_pre    \ctr, \order, \stride, \loop
> >> >>> +    yield_neon_post    \lbl
> >> >>> +    .endm
> >> >>> +
> >> >>> +    .macro        yield_neon_pre, ctr, order=0, stride, loop=4444f
> >> >>> +#ifdef CONFIG_PREEMPT
> >> >>> +    /*
> >> >>> +     * With some algorithms, it makes little sense to poll the
> >> >>> +     * TIF_NEED_RESCHED flag after every iteration, so only perform
> >> >>> +     * the check every 2^order strides.
> >> >>> +     */
> >> >>> +    .if        \order > 1
> >> >>> +    .if        (\stride & (\stride - 1)) != 0
> >> >>> +    .error        "stride should be a power of 2"
> >> >>> +    .endif
> >> >>> +    tst        \ctr, #((1 << \order) * \stride - 1) & ~(\stride - 1)
> >> >>> +    b.ne        \loop
> >> >>> +    .endif
> >> >>
> >> >> I'm not sure what baking in this check gives us, and this seems to
> >> >> conflate two rather different things: yielding and defining a
> >> >> "heartbeat" frequency for the calling code.
> >> >>
> >> >> Can we separate out the crypto-loop-helper semantics from the yield-
> >> >> neon stuff?
> >> >>
> >> >
> >> > Fair enough. I incorporated the check here so it disappears from the code entirely when !CONFIG_PREEMPT, because otherwise, you end up with a sequence that is mispredicted every # iterations without any benefit.
> >> > I guess i could macroise that separately though.
> >> >
> >> >> If we had
> >> >>
> >> >>    if_cond_yield_neon
> >> >>    // patchup code
> >> >>    endif_yield_neon
> >> >>
> >>
> >> I like this, but ...
> >>
> >> >> then the caller is free to conditionally branch over that as appropriate
> >> >> like
> >> >>
> >> >> loop:
> >> >>    // crypto stuff
> >> >>    tst x0, #0xf
> >> >>    b.ne    loop
> >> >>
> >> >>    if_cond_yield_neon
> >> >>    // patchup code
> >> >>    endif_cond_yield_neon
> >> >>
> >>
> >> I need to put the post patchup code somewhere too. Please refer to the
> >> CRC patches for the best examples of this.
> >
> > I'm not sure I follow.  If you need to do something different after
> > patchup, can't you either pull that code into the if...endif too, or
> > otherwise put a branch before the endif?
> >
>
> No, not really.
>
> What I currently have is
>
> >    if_cond_yield_neon
> >    // patchup code
> >    endif_cond_yield_neon
>
> which is being turned into
>
>     get_thread_info x0
>     ldr w1, [x0, #TSK_TI_PREEMPT]
>     ldr x0, [x0, #TSK_TI_FLAGS]
>     cmp w1, #1 // == PREEMPT_OFFSET
>     csel x0, x0, xzr, eq
>     tbnz x0, #TIF_NEED_RESCHED, 5555f // needs rescheduling?
> 4444:
>
>     .subsection 1
> 5555:
>     // patchup code
>     bl kernel_neon_end
>     bl kernel_neon_begin
>     // post patchup code goes here
>     b 4444b
>     .subsection
>
> so what I basically need is a place to put the post patchup code,
> unless I open code it and branch to a different label right after
> kernel_neon_begin

Ah, I get you.  I had convinced myself that there was only post-
patchup code.

So you conceptually want something like

if_will_cond_yield_neon
        // pre-yield patchup code
do_cond_yield_neon
        // post-yield patchup code
endif_yield_neon

.macro if_will_cond_yield_neon
        // do preempt_count == 1 && TIF_NEED_RESCHED check
        b.wont_yield 3432f
.endm

.macro do_cond_yield_neon
        bl      kernel_neon_end
        bl      kernel_neon_begin
.endm

.macro endif_yield_neon
3432:
.endm

The main difference I see is that this doesn't get the slow-case
code out of line.

Or have I still missed something?

Cheers
---Dave
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