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

From: Li, Aubrey
Date: Tue Dec 11 2018 - 19:59:22 EST

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?