Re: [PATCH] riscv: Fix build with CONFIG_CC_OPTIMIZE_FOR_SIZE=y

[Guo Ren]

,

On Wed, Oct 5, 2022 at 9:01 AM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>
> On 5 Oct 2022, at 01:38, Guo Ren <guoren@xxxxxxxxxx> wrote:
> >
> > On Wed, Oct 5, 2022 at 1:24 AM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
> >>
> >> On 4 Oct 2022, at 17:52, Atish Patra <atishp@xxxxxxxxxxxxxx> wrote:
> >>>
> >>> On Sat, Oct 1, 2022 at 1:13 PM Conor Dooley <conor@xxxxxxxxxx> wrote:
> >>>>
> >>>> On Wed, Sep 28, 2022 at 08:26:01PM -0700, Atish Patra wrote:
> >>>>> On Wed, Sep 28, 2022 at 2:16 PM Conor Dooley <conor@xxxxxxxxxx> wrote:
> >>>>>>
> >>>>>> On Wed, Sep 28, 2022 at 12:21:55AM -0700, Atish Patra wrote:
> >>>>>>> On Sat, Sep 24, 2022 at 4:15 PM Conor Dooley <conor@xxxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>> On Fri, Sep 23, 2022 at 11:01:28AM -0700, Atish Patra wrote:
> >>>>>>>>> On Fri, Sep 23, 2022 at 12:18 AM Heiko Stuebner <heiko@xxxxxxxxx> wrote:
> >>>>>>>>>>
> >>>>>>>>>> Hi,
> >>>>>>>>>>
> >>>>>>>>>> Am Donnerstag, 22. September 2022, 17:52:46 CEST schrieb Jessica Clarke:
> >>>>>>>>>>> On 22 Sept 2022, at 16:45, Heiko Stuebner <heiko@xxxxxxxxx> wrote:
> >>>>>>>>>>>>
> >>>>>>>>>>>> Am Donnerstag, 22. September 2022, 08:09:58 CEST schrieb Samuel Holland:
> >>>>>>>>>>>>> commit 8eb060e10185 ("arch/riscv: add Zihintpause support") broke
> >>>>>>>>>>>>> building with CONFIG_CC_OPTIMIZE_FOR_SIZE enabled (gcc 11.1.0):
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> CC arch/riscv/kernel/vdso/vgettimeofday.o
> >>>>>>>>>>>>> In file included from <command-line>:
> >>>>>>>>>>>>> ./arch/riscv/include/asm/jump_label.h: In function 'cpu_relax':
> >>>>>>>>>>>>> ././include/linux/compiler_types.h:285:33: warning: 'asm' operand 0 probably does not match constraints
> >>>>>>>>>>>>> 285 | #define asm_volatile_goto(x...) asm goto(x)
> >>>>>>>>>>>>> | ^~~
> >>>>>>>>>>>>> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro 'asm_volatile_goto'
> >>>>>>>>>>>>> 41 | asm_volatile_goto(
> >>>>>>>>>>>>> | ^~~~~~~~~~~~~~~~~
> >>>>>>>>>>>>> ././include/linux/compiler_types.h:285:33: error: impossible constraint in 'asm'
> >>>>>>>>>>>>> 285 | #define asm_volatile_goto(x...) asm goto(x)
> >>>>>>>>>>>>> | ^~~
> >>>>>>>>>>>>> ./arch/riscv/include/asm/jump_label.h:41:9: note: in expansion of macro 'asm_volatile_goto'
> >>>>>>>>>>>>> 41 | asm_volatile_goto(
> >>>>>>>>>>>>> | ^~~~~~~~~~~~~~~~~
> >>>>>>>>>>>>> make[1]: *** [scripts/Makefile.build:249: arch/riscv/kernel/vdso/vgettimeofday.o] Error 1
> >>>>>>>>>>>>> make: *** [arch/riscv/Makefile:128: vdso_prepare] Error 2
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Having a static branch in cpu_relax() is problematic because that
> >>>>>>>>>>>>> function is widely inlined, including in some quite complex functions
> >>>>>>>>>>>>> like in the VDSO. A quick measurement shows this static branch is
> >>>>>>>>>>>>> responsible by itself for around 40% of the jump table.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Drop the static branch, which ends up being the same number of
> >>>>>>>>>>>>> instructions anyway. If Zihintpause is supported, we trade the nop from
> >>>>>>>>>>>>> the static branch for a div. If Zihintpause is unsupported, we trade the
> >>>>>>>>>>>>> jump from the static branch for (what gets interpreted as) a nop.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Fixes: 8eb060e10185 ("arch/riscv: add Zihintpause support")
> >>>>>>>>>>>>> Signed-off-by: Samuel Holland <samuel@xxxxxxxxxxxx>
> >>>>>>>>>>>>> ---
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> arch/riscv/include/asm/hwcap.h | 3 ---
> >>>>>>>>>>>>> arch/riscv/include/asm/vdso/processor.h | 25 ++++++++++---------------
> >>>>>>>>>>>>> 2 files changed, 10 insertions(+), 18 deletions(-)
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> >>>>>>>>>>>>> index 6f59ec64175e..b21d46e68386 100644
> >>>>>>>>>>>>> --- a/arch/riscv/include/asm/hwcap.h
> >>>>>>>>>>>>> +++ b/arch/riscv/include/asm/hwcap.h
> >>>>>>>>>>>>> @@ -68,7 +68,6 @@ enum riscv_isa_ext_id {
> >>>>>>>>>>>>> */
> >>>>>>>>>>>>> enum riscv_isa_ext_key {
> >>>>>>>>>>>>> RISCV_ISA_EXT_KEY_FPU, /* For 'F' and 'D' */
> >>>>>>>>>>>>> - RISCV_ISA_EXT_KEY_ZIHINTPAUSE,
> >>>>>>>>>>>>> RISCV_ISA_EXT_KEY_MAX,
> >>>>>>>>>>>>> };
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> @@ -88,8 +87,6 @@ static __always_inline int riscv_isa_ext2key(int num)
> >>>>>>>>>>>>> return RISCV_ISA_EXT_KEY_FPU;
> >>>>>>>>>>>>> case RISCV_ISA_EXT_d:
> >>>>>>>>>>>>> return RISCV_ISA_EXT_KEY_FPU;
> >>>>>>>>>>>>> - case RISCV_ISA_EXT_ZIHINTPAUSE:
> >>>>>>>>>>>>> - return RISCV_ISA_EXT_KEY_ZIHINTPAUSE;
> >>>>>>>>>>>>> default:
> >>>>>>>>>>>>> return -EINVAL;
> >>>>>>>>>>>>> }
> >>>>>>>>>>>>> diff --git a/arch/riscv/include/asm/vdso/processor.h b/arch/riscv/include/asm/vdso/processor.h
> >>>>>>>>>>>>> index 1e4f8b4aef79..789bdb8211a2 100644
> >>>>>>>>>>>>> --- a/arch/riscv/include/asm/vdso/processor.h
> >>>>>>>>>>>>> +++ b/arch/riscv/include/asm/vdso/processor.h
> >>>>>>>>>>>>> @@ -4,30 +4,25 @@
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> #ifndef __ASSEMBLY__
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> -#include <linux/jump_label.h>
> >>>>>>>>>>>>> #include <asm/barrier.h>
> >>>>>>>>>>>>> -#include <asm/hwcap.h>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> static inline void cpu_relax(void)
> >>>>>>>>>>>>> {
> >>>>>>>>>>>>> - if (!static_branch_likely(&riscv_isa_ext_keys[RISCV_ISA_EXT_KEY_ZIHINTPAUSE])) {
> >>>>>>>>>>>>> #ifdef __riscv_muldiv
> >>>>>>>>>>>>> - int dummy;
> >>>>>>>>>>>>> - /* In lieu of a halt instruction, induce a long-latency stall. */
> >>>>>>>>>>>>> - __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> >>>>>>>>>>>>> + int dummy;
> >>>>>>>>>>>>> + /* In lieu of a halt instruction, induce a long-latency stall. */
> >>>>>>>>>>>>> + __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
> >>>>>>>>>>>>> #endif
> >>>>>>>>>>>>> - } else {
> >>>>>>>>>>>>> - /*
> >>>>>>>>>>>>> - * Reduce instruction retirement.
> >>>>>>>>>>>>> - * This assumes the PC changes.
> >>>>>>>>>>>>> - */
> >>>>>>>>>>>>> + /*
> >>>>>>>>>>>>> + * Reduce instruction retirement.
> >>>>>>>>>>>>> + * This assumes the PC changes.
> >>>>>>>>>>>>> + */
> >>>>>>>>>>>>> #ifdef __riscv_zihintpause
> >>>>>>>>>>>>> - __asm__ __volatile__ ("pause");
> >>>>>>>>>>>>> + __asm__ __volatile__ ("pause");
> >>>>>>>>>>>>> #else
> >>>>>>>>>>>>> - /* Encoding of the pause instruction */
> >>>>>>>>>>>>> - __asm__ __volatile__ (".4byte 0x100000F");
> >>>>>>>>>>>>> + /* Encoding of the pause instruction */
> >>>>>>>>>>>>> + __asm__ __volatile__ (".4byte 0x100000F");
> >>>>>>>>>>>>> #endif
> >>>>>>>>>>>>
> >>>>>>>>>>>> hmm, though before this part of the code was only ever accessed
> >>>>>>>>>>>> when the zhintpause extension was really available on the running
> >>>>>>>>>>>> machine while now the pause instruction is called every time.
> >>>>>>>>>>>>
> >>>>>>>>>>>> So I'm just wondering, can't this run into some "illegal instruction"
> >>>>>>>>>>>> thingy on machines not supporting the extension?
> >>>>>>>>>>>
> >>>>>>>>>>> No. The encoding for pause was deliberately chosen to be one of the
> >>>>>>>>>>> “useless” encodings of fence, with the hope that existing
> >>>>>>>>>>> microarchitectures might take a while to execute it and thus it would
> >>>>>>>>>>> still function as a slow-running instruction. It’s somewhat
> >>>>>>>>>>> questionable whether the div is even needed, the worst that happens is
> >>>>>>>>>>> cpu_relax isn’t very relaxed and you spin a bit faster. Any
> >>>>>>>>>>> implementations where that’s true probably also don’t have fancy
> >>>>>>>>>>> clock/power management anyway, and div isn’t going to be a low-power
> >>>>>>>>>>> operation so the only real effect is likely hammering on contended
> >>>>>>>>>>> atomics a bit more, and who cares about that on the low core count
> >>>>>>>>>>> systems we have today.
> >>>>>>>>>>
> >>>>>>>>>> thanks a lot for that explanation, which made things a lot clearer.
> >>>>>>>>>>
> >>>>>>>>>> So as you said, dropping the div part might make the function even smaller,
> >>>>>>>>>> though somehow part of me would want to add some sort of comment to
> >>>>>>>>>> the function for when the next developer stumbles over the unconditional
> >>>>>>>>>> use of pause :-) .
> >>>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> I agree. If that's what microarch will do, we can drop div altogether.
> >>>>>>>>> Though microarch may be treated as nop even if it is undesirable.
> >>>>>>>>> IIRC, the div was introduced for the rocket chip which would induce a
> >>>>>>>>> long latency stall with div instruction (zero as operands).
> >>>>>>>>>
> >>>>>>>>> Does any other core or newer rocket chip actually induce a latency
> >>>>>>>>> stall with div instruction ?
> >>>>>>>>> If not, it is equivalent to NOP as well. We can definitely remove the div.
> >>>>>>>>> The only cores affected will be the older rocket core.
> >>>>>>>>>
> >>>>>>>>> Tagging some folks to understand what their core does.
> >>>>>>>>>
> >>>>>>>>> @Paul Walmsley @Guo Ren @Conor Dooley ?
> >>>>>>>>
> >>>>>>>> I am no microarch expert by _any_ stretch of the imagination, but
> >>>>>>>> from a quick experiment it looks like the u54s on PolarFire SoC behave
> >>>>>>>> in the same way, and div w/ zero operands does in fact take significantly
> >>>>>>>> longer than regular division (looks to be about 3x).
> >>>>>>>>
> >>>>>>>
> >>>>>>> Thanks. Do you have any data on how much the "pause" instruction takes.
> >>>>>>
> >>>>>> So these numbers you may consider as being pulled out of a magic hat
> >>>>>> as all I am doing is reading the counters from userspace and there is
> >>>>>> some variance etc. Plus the fact that I just started hacking at some
> >>>>>> existing code I had lying around as I'm pretty snowed under at the
> >>>>>> moment.
> >>>>>>
> >>>>>> Doing the following takes about 70 cycles on both a PolarFire SoC and an
> >>>>>> unmatched:
> >>>>>> long divisor = 2, dividend = 100000, dest;
> >>>>>> asm("div %0, zero, zero" : "=r" (dest));
> >>>>>> and equates to:
> >>>>>> sd a5,-48(s0)
> >>>>>> div a5,zero,zero
> >>>>>>
> >>>>>> Clocking in at about 40 cycles is some actual divisions, I just did the
> >>>>>> following a dozen times, doing a trivial computation:
> >>>>>> long divisor = 2, dividend = 100000, dest;
> >>>>>> asm("div %0, %1, %2" : "=r" (dividend) : "r" (dividend), "r" (divisor))
> >>>>>>
> >>>>>> ie, a load of the following:
> >>>>>> sd a5,-48(s0)
> >>>>>> ld a5,-48(s0)
> >>>>>> ld a4,-40(s0)
> >>>>>> div a5,a5,a4
> >>>>>>
> >>>>>> So clearly the div w/ zero args makes a difference...
> >>>>>>
> >>>>>> On PolarFire SoC, `0x100000F` takes approx 6 cycles. On my unmatched, it
> >>>>>> takes approx 40. Again, I just had an asm block & called the instruction
> >>>>>> a number times and took the average - here it was 48 times.
> >>>>>>
> >>>>>> Take the actual numbers with a fist full of salt, but at least the
> >>>>>> relative numbers should be of some use to you.
> >>>>>>
> >>>>>> Hope that's somewhat helpful, maybe next week I can do something a
> >>>>>> little more useful for you...
> >>>>>>
> >>>>>
> >>>>> Thanks. It would be good to understand what happens when "pause" is
> >>>>> executed on these boards ?
> >>>>
> >>>> The actual pause instruction? uhh, so with the usual "I don't know what
> >>>> I am doing" disclaimer, I ran each of the .insn and pause instruction 48
> >>>> times in a row and checked the time elapsed via rdcycle & then ran that
> >>>> c program 1000 times in a bash loop. Got the below, the insns were run
> >>>> first and then the pauses.
> >>>> insn pause
> >>>> min 2.3 3.2
> >>>> max 9.5 10.6
> >>>> avg 27.0 29.1
> >>>> 5% 2.9 4.2
> >>>> 95% 18.1 19.1
> >>>>
> >>>> Swapping the pause & insn order around made a minor difference, but not
> >>>> enough to report on. I'd be very wary of drawing any real conclusions
> >>>> from this data, but at least both are roughly similar (and certainly not
> >>>> even close to doing the div w/ zero args.
> >>>>
> >>>
> >>> Yeah. That's what I was expecting. So we can't drop the div for now. Otherwise,
> >>> the existing hardware(don't support Zhintpause) suffers by spinning faster.
> >>
> >> But does that actually matter in practice? If it doesn’t noticeable
> >> affect performance then you don’t need to keep the div. There are a lot
> >> of architectures that even just define cpu_relax() as barrier().
> > Div is not semantic accurate for standard code, it should be in
> > vendors' errata. I agree to leave nop as default and put a pause
> > instead after the feature is detected.
>
> Nobody’s suggesting a literal nop instruction, that would be worse than
> either div or pause. It’s always safe to execute pause, the question is
> just whether on existing systems that don’t implement Zihintpause it
> gets executed too quickly such that performance is degraded due to
> spinning more aggressively.
Why do you ensure pause can't be an illegal instruction in some old machine?
Why do you ensure div could save power for all microarchitectures?

nop (default) -> div/<other instructions> (moved into vendor errata)
-> pause (when ZiHintPause feature detected)

>
> Jess
>
> >>
> >> Jess
> >>
> >>> Thanks for running the experiments.
> >>>
> >>>> Again, hope that is helpful?
> >>>> Conor.
> >>>>
> >>>>>
> >>>>>> Conor.
> >>>>>>
> >>>>>>> I understand that it is not available in these cores. Just wanted to
> >>>>>>> understand if microarchitecture
> >>>>>>> actually takes a while executing the useless encoding as pointed out by Jessica.
> >>>>>>>
> >>>>>>> If that's the case, we can remove the div instruction altogether.
> >>>>>>> Otherwise, this patch will cause some performance regression
> >>>>>>> for existing SoC (HiFive unleashed has the same core. Not sure about
> >>>>>>> unmatched though).
> >>>>>>> This needs to be documented at least.
> >>>>>>>
> >>>>>>>> Hope that's helpful,
> >>>>>>>> Conor.
> >>>>>>>>
> >>>>>>>> (I just did a quick check of what pretty much amounted to a bunch of
> >>>>>>>> div a5,zero,zero in a row versus div a5,a5,a5)
> >>>>>>>>
> >>>>>>>>>
> >>>>>>>>> (Please add anybody who may have an insight to execution flow on
> >>>>>>>>> existing Linux capable cores)
> >>>>>>>>>
> >>>>
> >>>
> >>>
> >>> --
> >>> Regards,
> >>> Atish
> >>
> >
> >
> > --
> > Best Regards
> > Guo Ren
>


-- 
Best Regards
 Guo Ren