Message-Id: <B496688F-28CA-43E7-8D87-0E61AA17E6D5@jrtc27.com>
Date:   Wed, 5 Oct 2022 15:25:32 +0100
From:   Jessica Clarke <jrtc27@...c27.com>
To:     Guo Ren <guoren@...nel.org>
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 5 Oct 2022, at 02:40, Guo Ren <guoren@...nel.org> wrote:
> 
> ,
> 
> On Wed, Oct 5, 2022 at 9:01 AM Jessica Clarke <jrtc27@...c27.com> wrote:
>> 
>> On 5 Oct 2022, at 01:38, Guo Ren <guoren@...nel.org> wrote:
>>> 
>>> 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.
>> 
>> 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?

Because that’s how it’s defined; it uses one of the many hints
(instructions that aren’t a canonical nop but are defined to behave
like one in terms of architectural side-effects) from RV32I/RV64I.

> Why do you ensure div could save power for all microarchitectures?

I don’t. In fact it almost certainly won’t make the core enter a low
power state. It will just help reduce the amount of memory traffic by
taking a while to execute. I would rather not use div at all.

Jess

> 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

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