Message-ID: <CAJF2gTSwm9UkbvKpWuPTPyNQhApJivwwfJ_A9YA6WoC4HNsxbg@mail.gmail.com>
Date:   Wed, 5 Oct 2022 08:38:01 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Jessica Clarke <jrtc27@...c27.com>
Cc:     Atish Patra <atishp@...shpatra.org>,
        Conor Dooley <conor@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Samuel Holland <samuel@...lland.org>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...osinc.com>, Dao Lu <daolu@...osinc.com>,
        Jisheng Zhang <jszhang@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Fix build with CONFIG_CC_OPTIMIZE_FOR_SIZE=y

On Wed, Oct 5, 2022 at 1:24 AM Jessica Clarke <jrtc27@...c27.com> wrote:
>
> On 4 Oct 2022, at 17:52, Atish Patra <atishp@...shpatra.org> wrote:
> >
> > On Sat, Oct 1, 2022 at 1:13 PM Conor Dooley <conor@...nel.org> 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@...nel.org> 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@...nel.org> 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@...ech.de> wrote:
> >>>>>>>>
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>> Am Donnerstag, 22. September 2022, 17:52:46 CEST schrieb Jessica Clarke:
> >>>>>>>>> On 22 Sept 2022, at 16:45, Heiko Stuebner <heiko@...ech.de> 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@...lland.org>
> >>>>>>>>>>> ---
> >>>>>>>>>>>
> >>>>>>>>>>> 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.

>
> 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

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