linux-kernel - Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks

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Message-ID: <e51c6513-3b5d-972c-5d26-d20283057bb8@intel.com>
Date:   Tue, 11 Dec 2018 09:20:56 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        mingo@...hat.com, peterz@...radead.org, hpa@...or.com
Cc:     ak@...ux.intel.com, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, linux-kernel@...r.kernel.org,
        Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks

 to update AVX512 state
> + */
> +static inline void update_avx512_state(struct fpu *fpu)
> +{
> +	/*
> +	 * AVX512 state is tracked here because its use is known to slow
> +	 * the max clock speed of the core.
> +	 *
> +	 * However, AVX512-using tasks are expected to clear this state when
> +	 * not actively using these registers. Thus, this tracking mechanism
> +	 * can miss. To ensure that false-negatives do not immediately show
> +	 * up, decay the usage count over time.
> +	 */
> +	if (fpu->state.xsave.header.xfeatures & XFEATURE_MASK_AVX512)
> +		fpu->avx512_usage = AVX512_STATE_DECAY_COUNT;
> +	else if (fpu->avx512_usage)
> +		fpu->avx512_usage--;
> +}
> +
>  /*
>   * This function is called only during boot time when x86 caps are not set
>   * up and alternative can not be used yet.
> @@ -411,6 +432,7 @@ static inline int copy_fpregs_to_fpstate(struct fpu *fpu)
>  {
>  	if (likely(use_xsave())) {
>  		copy_xregs_to_kernel(&fpu->state.xsave);
> +		update_avx512_state(fpu);
>  		return 1;
>  	}


Is there a reason we shouldn't do:

	if (!cpu_feature_enabled(X86_FEATURE_AVX512F))
		update_avx512_state(fpu);

?