linux-kernel - Re: [PATCH v13 2/3] x86,/proc/pid/status: Add AVX-512 usage elapsed time

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Date:   Fri, 5 Apr 2019 22:27:08 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Aubrey Li <aubrey.li@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        tim.c.chen@...ux.intel.com, Dave Hansen <dave.hansen@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        aubrey.li@...el.com, kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v13 2/3] x86,/proc/pid/status: Add AVX-512 usage elapsed time

On Fri, Apr 5, 2019 at 10:02 PM Aubrey Li <aubrey.li@...ux.intel.com> wrote:
> AVX-512 components use could cause core turbo frequency drop. So
> it's useful to expose AVX-512 usage elapsed time as a heuristic hint
> for the user space job scheduler to cluster the AVX-512 using tasks
> together.
>
> Tensorflow example:
> $ while [ 1 ]; do cat /proc/pid/status | grep AVX; sleep 1; done
> AVX512_elapsed_ms:      4
> AVX512_elapsed_ms:      8
> AVX512_elapsed_ms:      4
>
> This means that 4 milliseconds have elapsed since the AVX512 usage
> of tensorflow task was detected when the task was scheduled out.
>
> Or:
> $ cat /proc/pid/status | grep AVX512_elapsed_ms
> AVX512_elapsed_ms:      -1

(Very nitpicky, feel free to ignore: If you change the /proc/pid to
/proc/tid in the commit message, it becomes clearer that this status
is really per-task/thread, not per-process/threadgroup.)

[...]
> +
> +/*
> + * Report the amount of time elapsed in millisecond since last AVX512
> + * use in the task.
> + */
> +static void avx512_status(struct seq_file *m, struct task_struct *task)
> +{
> +       unsigned long timestamp = task->thread.fpu.avx512_timestamp;

This is theoretically a data race, right? Should this have a READ_ONCE() on it?

Is there something that zeroes out the avx512_timestamp on
fork()/clone(), or will every child inherit the avx512 timestamp? As
far as I can tell, the timestamp is inherited; I think it would be
nicer to zero it out at that point. Either way, It might be worth
documenting this decision.