Message-ID: <bd82d240-34d7-4df9-650b-c039555f05e3@huawei.com>
Date:   Mon, 10 Jan 2022 11:00:43 +0800
From:   He Ying <heying24@...wei.com>
To:     Mark Rutland <mark.rutland@....com>
CC:     <catalin.marinas@....com>, <will@...nel.org>, <marcan@...can.st>,
        <maz@...nel.org>, <joey.gouly@....com>, <pcc@...gle.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: Make CONFIG_ARM64_PSEUDO_NMI macro wrap all the
 pseudo-NMI code

Hi Mark,

I'm just back from the weekend and sorry for the delayed reply.


在 2022/1/7 21:19, Mark Rutland 写道:
> On Fri, Jan 07, 2022 at 03:55:36AM -0500, He Ying wrote:
>> Our product has been updating its kernel from 4.4 to 5.10 recently and
>> found a performance issue. We do a bussiness test called ARP test, which
>> tests the latency for a ping-pong packets traffic with a certain payload.
>> The result is as following.
>>
>>   - 4.4 kernel: avg = ~20s
>>   - 5.10 kernel (CONFIG_ARM64_PSEUDO_NMI is not set): avg = ~40s
> Have you tested with a recent mainline kernel, e.g. v5.15?

Actuallly no, that's because this test is only available for the product 
environment and

we don't have an available 5.15 kernel for it yet.

>
> Is this test publicly available, and can you say which hardrware (e.g. which
> CPU implementation) you're testing with?

Actually no. The test is only available for our product environment now. 
We are testing

with hisilicon 1213 (4 ARM Cortex-A72 cores).

>
>> I have been just learning arm64 pseudo-NMI code and have a question,
>> why is the related code not wrapped by CONFIG_ARM64_PSEUDO_NMI?
> The code in question is all patched via alternatives, and when
> CONFIG_ARM64_PSEUDO_NMI is not selected, the code was expected to only have the
> overhead of the regular DAIF manipulation.
I don't understand alernatives very well and I'll apreciate it if you 
can explain it a bit more.
>
>> I wonder if this brings some performance regression.
>>
>> First, I make this patch and then do the test again. Here's the result.
>>
>>   - 5.10 kernel with this patch not applied: avg = ~40s
>>   - 5.10 kernel with this patch applied: avg = ~23s
>>
>> Amazing! Note that all kernel is built with CONFIG_ARM64_PSEUDO_NMI not
>> set. It seems the pseudo-NMI feature actually brings some overhead to
>> performance event if CONFIG_ARM64_PSEUDO_NMI is not set.
> I'm surprised the overhead is so significant; as above this is all patched in
> and so the overhead when this is disabled is expected to be *extremely* small.
>
> For example, wjen CONFIG_ARM64_PSEUDO_NMI, in arch_local_irq_enable():
>
> * The portion under the system_has_prio_mask_debugging() test will be removed
>    entirely by the compiler, as this internally checks
>    IS_ENABLED(CONFIG_ARM64_PSEUDO_NMI).
>
> * The assembly will be left as a write to DAIFClr. The only additional cost
>    should be that of generating GIC_PRIO_IRQON into a register.
>
> * The pmr_sync() will be removed entirely by the compiler as is defined
>    conditionally dependent on CONFIG_ARM64_PSEUDO_NMI.
>
> I can't spot an obvious issue with that or ther other cases. In the common case
> those add no new instructions, and in the worst case they only add NOPs.

Thanks for your detailed explaination! Actually I can't understand the 
result exactly.

I build two 5.10 kernel images with this patch applied or not and 
objdump them. Indeed,

the disassembles of 'arch_local_irq_restore' are the same. Do you have 
any ideas how

we can find the root cause why this patch improves the performance so much?


However, the test result is trustworthy because we do it many times and 
the result is

always repeatable.

>
>> Furthermore, I find the feature also brings some overhead to vmlinux size.
>> I build 5.10 kernel with this patch applied or not while
>> CONFIG_ARM64_PSEUDO_NMI is not set.
>>
>>   - 5.10 kernel with this patch not applied: vmlinux size is 384060600 Bytes.
>>   - 5.10 kernel with this patch applied: vmlinux size is 383842936 Bytes.
>>
>> That means arm64 pseudo-NMI feature may bring ~200KB overhead to
>> vmlinux size.
> I suspect that's just the (unused) alternatives, and we could improve that by
> passing the config into the alternative blocks.

Do you mean the sections generated by the alternatives? I don't understand

alernatives very well and I'll apreciate it if you can explain it a bit 
more.

>
>> Above all, arm64 pseudo-NMI feature brings some overhead to vmlinux size
>> and performance even if config is not set. To avoid it, add macro control
>> all around the related code.
>>
>> Signed-off-by: He Ying <heying24@...wei.com>
>> ---
>>   arch/arm64/include/asm/irqflags.h | 38 +++++++++++++++++++++++++++++--
>>   arch/arm64/kernel/entry.S         |  4 ++++
>>   2 files changed, 40 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
>> index b57b9b1e4344..82f771b41cf5 100644
>> --- a/arch/arm64/include/asm/irqflags.h
>> +++ b/arch/arm64/include/asm/irqflags.h
>> @@ -26,6 +26,7 @@
>>    */
>>   static inline void arch_local_irq_enable(void)
>>   {
>> +#ifdef CONFIG_ARM64_PSEUDO_NMI
>>   	if (system_has_prio_mask_debugging()) {
>>   		u32 pmr = read_sysreg_s(SYS_ICC_PMR_EL1);
>>   
>> @@ -41,10 +42,18 @@ static inline void arch_local_irq_enable(void)
>>   		: "memory");
>>   
>>   	pmr_sync();
>> +#else
>> +	asm volatile(
>> +		"msr	daifclr, #3		// arch_local_irq_enable"
>> +		:
>> +		:
>> +		: "memory");
>> +#endif
> I'm happy to rework this to improve matters, but I am very much not happy with
> duplicating the logic for the !PSEUDO_NMI case. Adding more ifdeffery and
> copies of that is not acceptable.
I agree. Adding these ifdeffery is a bit ugly. Let's see if there are 
some better ways.
>
> Instead, can you please try changing the alternative to also take the config,
> e.g. here have:
>
> |       asm volatile(ALTERNATIVE(
> |               "msr    daifclr, #3             // arch_local_irq_enable",
> |               __msr_s(SYS_ICC_PMR_EL1, "%0"),
> |               ARM64_HAS_IRQ_PRIO_MASKING,
> |               CONFIG_ARM64_PSEUDO_NMI)
> |               :
> |               : "r" ((unsigned long) GIC_PRIO_IRQON)
> |               : "memory");
>
> ... and see if that makes a significant difference?
>
> Likewise for the other casees.

OK, I'll try it. But I have some questions. Here's the comment of 
ALERNATIVE:

/*
  * Usage: asm(ALTERNATIVE(oldinstr, newinstr, feature));
  *
  * Usage: asm(ALTERNATIVE(oldinstr, newinstr, feature, CONFIG_FOO));
  * N.B. If CONFIG_FOO is specified, but not selected, the whole block
  *      will be omitted, including oldinstr.
  */

If CONFIG_FOO is not selected, the whole block will be omitted including 
oldinstr.

But we still want the oldinstr in this situation. Do I misunderstand 
something?

>
>>   #endif /* __ASM_IRQFLAGS_H */
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index 2f69ae43941d..ffc32d3d909a 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -300,6 +300,7 @@ alternative_else_nop_endif
>>   	str	w21, [sp, #S_SYSCALLNO]
>>   	.endif
>>   
>> +#ifdef CONFIG_ARM64_PSEUDO_NMI
>>   	/* Save pmr */
>>   alternative_if ARM64_HAS_IRQ_PRIO_MASKING
>>   	mrs_s	x20, SYS_ICC_PMR_EL1
>> @@ -307,6 +308,7 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKING
>>   	mov	x20, #GIC_PRIO_IRQON | GIC_PRIO_PSR_I_SET
>>   	msr_s	SYS_ICC_PMR_EL1, x20
>>   alternative_else_nop_endif
>> +#endif
>>   
>>   	/* Re-enable tag checking (TCO set on exception entry) */
>>   #ifdef CONFIG_ARM64_MTE
>> @@ -330,6 +332,7 @@ alternative_else_nop_endif
>>   	disable_daif
>>   	.endif
>>   
>> +#ifdef CONFIG_ARM64_PSEUDO_NMI
>>   	/* Restore pmr */
>>   alternative_if ARM64_HAS_IRQ_PRIO_MASKING
>>   	ldr	x20, [sp, #S_PMR_SAVE]
>> @@ -339,6 +342,7 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKING
>>   	dsb	sy				// Ensure priority change is seen by redistributor
>>   .L__skip_pmr_sync\@:
>>   alternative_else_nop_endif
>> +#endif
> For these two I think the ifdeffery is fine, but I'm surprised this has a
> measureable impact as the alternatives should be initialized to NOPS (and never
> modified).

Yes, these NOPs may bring some overhead. But I can't say how much these 
NOPs contribute to

the result of test.

>
> Thanks,
> Mark.
> .

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