linux-kernel - Re: [PATCH v12 3/3] Documentation/filesystems/proc.txt: add AVX512_elapsed

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Date:   Sat, 23 Feb 2019 19:17:39 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Aubrey Li <aubrey.li@...ux.intel.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, aubrey.li@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 3/3] Documentation/filesystems/proc.txt: add
 AVX512_elapsed_ms

On Sat, 23 Feb 2019, Thomas Gleixner wrote:
> On Thu, 21 Feb 2019, Aubrey Li wrote:
> Something like this instead of this conglomorate of useful, irrelevant and
> misleading information:
> 
>   The AVX512_elapsed_ms entry shows the milliseconds elapsed since the last
>   time AVX512 usage was recorded. The recording happens on a best effort
>   basis when a task is scheduled out. This means that the value depends on
>   two factors:
> 
>     1) The time which the task spent on the CPU without being scheduled
>        out. With CPU isolation and a single runnable task this can take
>        several seconds.
> 
>     2) The time since the task was scheduled out last. Depending on the
>        reason for being scheduled out (time slice exhausted, syscall ...)
>        this can be arbitrary long time.
> 
>   As a consequence the value cannot be considered precise and authoritive
>   information. The application which uses this information has to be aware
>   of the overall scenario on the system in order to determine whether a
>   task is a real AVX512 user or not.
> 
> See? No jiffies, no code snippets, no absolute numbers and no magic
> recommendation which might be correct for your test scenario, but
> completely bogus for some other scenario.
> 
> Instead it contains the things which a application programmer who wants to
> use that value needs to know. He then has to map it to his scenario and
> build the crystal ball logic which makes it perhaps useful.

And of course the special value -1 needs to be documented as well....

Thanks,

	tglx