linux-kernel - Re: [PATCH v4 2/2] proc: add AVX-512 usage to /proc/pid/status

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Date:   Tue, 11 Dec 2018 09:57:03 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.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, dave.hansen@...el.com, arjan@...ux.intel.com,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [PATCH v4 2/2] proc: add AVX-512 usage to /proc/pid/status



On 12/10/18 4:24 PM, Aubrey Li wrote:
> AVX-512 components usage could cause core turbo frequency drop. So
> it's useful to expose AVX-512 components usage as a heuristic hint
> for the user space job scheduler to cluster the AVX-512 using tasks
> together.
> 
> Example:
> $ cat /proc/pid/status | grep AVX512_hint
> AVX512_hint:   1
> 
> The hint number '0' indicates the task recently didn't use AVX-512
> components thus unlikely has frequency drop issue. And the number '1'
> indicates the task recently used AVX-512 components thus could cause
> core frequency drop. User space tools may want to further check by:
> 
> $ perf stat --pid <pid> -e core_power.lvl2_turbo_license -- sleep 1
> 
>  Performance counter stats for process id '3558':
> 
>      3,251,565,961      core_power.lvl2_turbo_license
> 
>        1.004031387 seconds time elapsed
> 
> Non-zero counter value confirms that the task causes frequency drop.
> 
> Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Arjan van de Ven <arjan@...ux.intel.com>
> ---
>  arch/x86/kernel/fpu/xstate.c | 19 +++++++++++++++++++
>  fs/proc/array.c              |  5 +++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index 87a57b7642d3..98baa47c97b0 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -7,6 +7,7 @@
>  #include <linux/cpu.h>
>  #include <linux/mman.h>
>  #include <linux/pkeys.h>
> +#include <linux/seq_file.h>
>  
>  #include <asm/fpu/api.h>
>  #include <asm/fpu/internal.h>
> @@ -1245,3 +1246,21 @@ int copy_user_to_xstate(struct xregs_state *xsave, const void __user *ubuf)
>  
>  	return 0;
>  }
> +
> +/*
> + * Report CPU specific thread state
> + */
> +void arch_task_state(struct seq_file *m, struct task_struct *task)
> +{
> +	/*
> +	 * Check the processor and build option if AVX512 is supported.
> +	 */
> +	if (!cpu_feature_enabled(X86_FEATURE_AVX512F))
> +		return;
> +	/*
> +	 * Report AVX-512 components usage:
> +	 */
> +	seq_put_decimal_ull(m, "AVX512_hint:\t",
> +				task->thread.fpu.avx512_usage ? 1 : 0);

I believe you need some kind of documentation of this new interface
in kernel's Documentation.  So an admin doesn't need to look into the
kernel code to figure out what this /proc/pid/status is supposed to do.

Tim