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

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Date:   Wed, 12 Dec 2018 08:59:16 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     Arjan van de Ven <arjan@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        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,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] x86/fpu: track AVX-512 usage of tasks

On 2018/12/12 8:14, Arjan van de Ven wrote:
> On 12/11/2018 3:46 PM, Li, Aubrey wrote:
>> On 2018/12/12 1:18, Dave Hansen wrote:
>>> On 12/10/18 4:24 PM, Aubrey Li wrote:
>>>> The tracking turns on the usage flag at the next context switch of
>>>> the task, but requires 3 consecutive context switches with no usage
>>>> to clear it. This decay is required because well-written AVX-512
>>>> applications are expected to clear this state when not actively using
>>>> AVX-512 registers.
>>>
>>> One concern about this:  Given a HZ=1000 system, this means that the
>>> flag needs to get scanned every ~3ms.  That's a pretty good amount of
>>> scanning on a system with hundreds or thousands of tasks running around.
>>>
>>> How many tasks does this scale to until you're eating up an entire CPU
>>> or two just scanning /proc?
>>>
>>
>> Do we have a real requirement to do this in practical environment?
>> AFAIK, 1s or even 5s is good enough in some customers environment.
> 
> maybe instead of a 1/0 bit, it's useful to store the timestamp of the last
> time we found the task to use avx? (need to find a good time unit)
> 
> 
Are you suggesting kernel does not do any translation, just provide a fact
to the user space tool and let user space tool to decide how to use this info?

So how does user space tool use this timestamp in your mind?

Thanks,
-Aubrey