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

[Arjan van de Ven]

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)