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

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Message-ID: <alpine.DEB.2.21.1812181448170.12686@nanos.tec.linutronix.de>
Date:   Tue, 18 Dec 2018 15:14:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Aubrey Li <aubrey.li@...el.com>
cc:     mingo@...hat.com, peterz@...radead.org, hpa@...or.com,
        ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        dave.hansen@...el.com, arjan@...ux.intel.com,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

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.

Thanks,

	tglx