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Subject: Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks
To:     Thomas Gleixner <tglx@linutronix.de>,
        Aubrey Li <aubrey.li@intel.com>
Cc:     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>
 <alpine.DEB.2.21.1812181448170.12686@nanos.tec.linutronix.de>
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <62997df2-0817-b54d-b11e-00a567f1d67c@linux.intel.com>
Date:   Tue, 18 Dec 2018 23:11:39 +0800
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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.

Thanks,
-Aubrey