Re: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf

[Peter Zijlstra]

On Sat, Sep 09, 2017 at 09:42:12PM -0700, Andy Lutomirski wrote:
> PeterZ and Ingo, would you be okay with adding a define so arches can
> opt out of the task_struct::active_mm field entirely?  That is, with
> the option set, task_struct wouldn't have an active_mm field, the core
> wouldn't call mmgrab and mmdrop, and the arch would be responsible for
> that bookkeeping instead?  x86, and presumably all arches without
> cross-core invalidation, would probably prefer to just shoot down the
> old mm entirely in __mmput() rather than trying to figure out when do
> finish freeing old mms.  After all, exit_mmap() is going to send an
> IPI regardless, so I see no reason to have the scheduler core pin an
> old dead mm just because some random kernel thread's active_mm field
> points to it.

I'm only quickly skimming this thread, but I don't see anything too
worrysome being proposed.

If you're in LA next week we can talk about it in more detail if you
want.