On Mon 12-10-20 17:20:08, Jann Horn wrote:
> On Mon, Oct 12, 2020 at 5:07 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > On Mon 12-10-20 13:49:40, Jann Horn wrote:
> > > Since 34e55232e59f7b19050267a05ff1226e5cd122a5 (introduced back in
> > > v2.6.34), Linux uses per-thread RSS counters to reduce cache contention on
> > > the per-mm counters. With a 4K page size, that means that you can end up
> > > with the counters off by up to 252KiB per thread.
> >
> > Do we actually have any strong case to keep this exception to the
> > accounting?
>
> I have no clue. The concept of "concurrently modified cache lines are
> bad" seemed vaguely reasonable to me... but I have no idea how much
> impact this actually has on massively multithreaded processes.
I do remember some discussion when imprecision turned out to be a real
problem (Android?).
Anyway, I have to say that 34e55232e59f ("mm: avoid false sharing of
mm_counter") sounds quite dubious to me and it begs for re-evaluation.
Btw. thanks for trying to document this weird behavior. This is
certainly useful but I am suspecting that dropping it might be even
better.
--
Michal Hocko
SUSE Labs
