lazy tlb: introduce lazy mm refcount helper functions

lazy tlb: introduce lazy mm refcount helper functions

Patch series "shoot lazy tlbs", v4.

On a 16-socket 192-core POWER8 system, a context switching benchmark with
as many software threads as CPUs (so each switch will go in and out of
idle), upstream can achieve a rate of about 1 million context switches per
second.  After this series it goes up to 118 million.

This patch (of 4):

Add explicit _lazy_tlb annotated functions for lazy mm refcounting.  This
makes lazy mm references more obvious, and allows explicit refcounting to
be removed if it is not used.

If a kernel thread's current lazy tlb mm happens to be the one it wants to
use, then kthread_use_mm() cleverly transfers the mm refcount from the
lazy tlb mm reference to the returned reference.  If the lazy tlb mm
reference is no longer identical to a normal reference, this trick does
not work, so that is changed to be explicit about the two references.

Link: https://lkml.kernel.org/r/20210605014216.446867-1-npiggin@gmail.com
Link: https://lkml.kernel.org/r/20210605014216.446867-2-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>