lazy tlb: shoot lazies, a non-refcounting lazy tlb option

lazy tlb: shoot lazies, a non-refcounting lazy tlb option

On big systems, the mm refcount can become highly contented when doing a
lot of context switching with threaded applications (particularly
switching between the idle thread and an application thread).

Abandoning lazy tlb slows switching down quite a bit in the important
user->idle->user cases, so instead implement a non-refcounted scheme that
causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down any
remaining lazy ones.

Shootdown IPIs are some concern, but they have not been observed to be a
big problem with this scheme (the powerpc implementation generated 314
additional interrupts on a 144 CPU system during a kernel compile).  There
are a number of strategies that could be employed to reduce IPIs if they
turn out to be a problem for some workload.

Link: https://lkml.kernel.org/r/20210605014216.446867-4-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>