Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

[Li, Aubrey]

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