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Subject: Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Aubrey Li <aubrey.li@intel.com>, mingo@redhat.com,
        peterz@infradead.org, hpa@zytor.com, ak@linux.intel.com,
        tim.c.chen@linux.intel.com, dave.hansen@intel.com,
        arjan@linux.intel.com, linux-kernel@vger.kernel.org
References: <20181218042218.8653-1-aubrey.li@intel.com>
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 <alpine.DEB.2.21.1812181630530.14031@nanos.tec.linutronix.de>
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <b14a9c82-3336-0f13-1a27-fba929e6b4fb@linux.intel.com>
Date:   Wed, 19 Dec 2018 00:28:11 +0800
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On 2018/12/18 23:32, Thomas Gleixner wrote:
> On Tue, 18 Dec 2018, Li, Aubrey wrote:
> 
>> On 2018/12/18 22:14, Thomas Gleixner wrote:
>>> On Tue, 18 Dec 2018, Aubrey Li wrote:
>>>> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
>>>> index a38bf5a1e37a..8778ac172255 100644
>>>> --- a/arch/x86/include/asm/fpu/internal.h
>>>> +++ b/arch/x86/include/asm/fpu/internal.h
>>>> @@ -411,6 +411,13 @@ static inline int copy_fpregs_to_fpstate(struct fpu *fpu)
>>>>  {
>>>>  	if (likely(use_xsave())) {
>>>>  		copy_xregs_to_kernel(&fpu->state.xsave);
>>>> +
>>>> +		/*
>>>> +		 * AVX512 state is tracked here because its use is
>>>> +		 * known to slow the max clock speed of the core.
>>>> +		 */
>>>> +		if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
>>>> +			fpu->avx512_timestamp = jiffies_64;
>>>
>>> Even if unlikely this is incorrect when running a 32 bit kernel because
>>> there jiffies_64 cannot be atomically loaded vs. a concurrent update. See
>>> the comment in include/linux/jiffies.h right above the jiffies_64
>>> declaration.
>>>
>> Yeah, I noticed this, because this is under use_xsave() condition, also need
>> valid AVX512 state, so a 32 bit kernel won't enter this branch.
> 
> What exactly prevents a 32bit kernel from having the AVX512 feature bit
> set? And if it cannot be set on 32bit, then why are you compiling that code
> in at all?
> 

I misunderstood, you mean 32bit kernel, not 32bit machine. Theoretically 32bit
kernel can use AVX512, but not sure if anyone use it like this. get_jiffies_64()
includes jiffies_lock ops so not good in context switch. So I want to use raw
jiffies_64 here. jiffies is a good candidate but it has wraparound overflow issue.
Other time source are expensive here.

Should I limit the code only running on 64bit kernel? 

Thanks,
-Aubrey