Messages in this thread | | | Date | Thu, 6 Aug 2020 10:54:29 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH 1/2] sched/topology: Allow archs to override cpu_smt_mask |
| |
On Thu, Aug 06, 2020 at 03:32:25PM +1000, Michael Ellerman wrote:
> That brings with it a bunch of problems, such as existing software that > has been developed/configured for Power8 and expects to see SMT8. > > We also allow LPARs to be live migrated from Power8 to Power9 (and back), so > maintaining the illusion of SMT8 is considered a requirement to make that work.
So how does that work if the kernel booted on P9 and demuxed the SMT8 into 2xSMT4? If you migrate that state onto a P8 with actual SMT8 you're toast again.
> Yeah I agree the naming is confusing. > > Let's call them "SMT4 cores" and "SMT8 cores"?
Works for me, thanks!
> The problem is we are already lying to userspace, because firmware lies to us. > > ie. the firmware on these systems shows us an SMT8 core, and so current kernels > show SMT8 to userspace. I don't think we can realistically change that fact now, > as these systems are already out in the field. > > What this patch tries to do is undo some of the mess, and at least give the > scheduler the right information.
What a mess... I think it depends on what you do with that P9 to P8 migration case. Does it make sense to have a "p8_compat" boot arg for the case where you want LPAR migration back onto P8 systems -- in which case it simply takes the firmware's word as gospel and doesn't untangle things, because it can actually land on a P8.
| |