Re: [PATCH v6 1/3] x86/fpu: track AVX-512 usage of tasks

From: Thomas Gleixner
Date: Wed Dec 19 2018 - 04:46:15 EST

On Wed, 19 Dec 2018, Li, Aubrey wrote:
> On 2018/12/19 5:38, Andi Kleen wrote:
> >> I misunderstood, you mean 32bit kernel, not 32bit machine. Theoretically 32bit
> >> kernel can use AVX512, but not sure if anyone use it like this. get_jiffies_64()
> >> includes jiffies_lock ops so not good in context switch. So I want to use raw
> >> jiffies_64 here. jiffies is a good candidate but it has wraparound overflow issue.
> >> Other time source are expensive here.
> >>
> >> Should I limit the code only running on 64bit kernel?
> >
> > Yes making it 64bit only should be fine.
> >
> > Other alternative would be to use 32bit jiffies on 32bit. I assume
> > wrapping is not that big a problem here.
> >
> Thomas, is this acceptable?

Just do the math. jiffies on 32bit wrap around depending on HZ:

HZ=100 248 days
HZ=1000 24 days

So, yes it takes quite some time, but from then on the information is
bogus. Whether that matters or not is a different question. At least it
needs proper documentation.