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

[Rik van Riel <riel@redhat.com>]

On Sat, 2017-09-09 at 12:28 -0700, Andy Lutomirski wrote:
> -
> I propose the following fix.  If PCID is on, then, in
> enter_lazy_tlb(), we switch to init_mm with the no-flush flag set.
> (And we give init_mm its own dedicated ASID to keep it simple and
> fast
> -- no need to use the LRU ASID mapping to assign one
> dynamically.)  We
> clear the bit in mm_cpumask.  That is, we more or less just skip the
> whole lazy TLB optimization and rely on PCID CPUs having reasonably
> fast CR3 writes.  No extra IPIs.

Avoiding the IPIs is probably what matters the most, especially
on systems with deep C states, and virtual machines where the
host may be running something else, causing the IPI service time
to go through the roof for idle VCPUs.

> Also, sorry Rik, this means your old increased laziness optimization
> is dead in the water.  It will have exactly the same speculative load
> problem.

Doesn't a memory barrier solve that speculative load
problem?

The memory barrier could be added only to the path
that potentially skips reloading the TLB, under the
assumption that a memory barrier is cheaper than a
TLB reload (even with ASID).