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

From: Dave Hansen
Date: Tue Dec 11 2018 - 12:18:29 EST

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?