Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value

From: Joonsoo Kim
Date: Thu Jan 28 2016 - 00:09:15 EST

On Wed, Jan 27, 2016 at 03:13:12PM -0800, David Rientjes wrote:
> On Thu, 26 Nov 2015, Joonsoo Kim wrote:
> > I understand design decision, but, it is better to get value as much
> > as accurate if there is no performance problem. My patch would not
> > cause much performance degradation because it is just adding one
> > this_cpu_read().
> >
> > Consider about following example. Current implementation returns
> > interesting output if someone do following things.
> >
> > v1 = zone_page_state(XXX);
> > mod_zone_page_state(XXX, 1);
> > v2 = zone_page_state(XXX);
> >
> > v2 would be same with v1 in most of cases even if we already update
> > it.
> >
> > This situation could occurs in page allocation path and others. If
> > some task try to allocate many pages, then watermark check returns
> > same values until updating vmstat even if some freepage are allocated.
> > There are some adjustments for this imprecision but why not do it become
> > accurate? I think that this change is reasonable trade-off.
> >
> I'm not sure that NR_ISOLATED_* should be vmstats in the first place. The
> most important callers that depend on its accuracy is
> zone_reclaimable_pages() and the too_many_isolated() loop in both
> shrink_inactive_list() and memory compaction. If zlc's are updated every
> 1s, the HZ/10 in those loops don't really matter, they may as well be
> HZ/2.
> I think memory compaction updates the counters in the most appropriate
> way, by incrementing a counter and then finally doing
> mod_zone_page_state() for the counter. The other updaters are thp
> collapse and page migration.
> I discount user-visible vmstats here because the trade-off has already
> been made that they may be stale for up to 1s and userspace isn't
> affected.
> So what happens if we simply convert NR_ISOLATED_* into per-zone
> atomic64_t?

Just a small uncomfortable thing is that calculation is done
with different kinds of metric. For example, comparing vmstat values
(NR_INACTIVE_*, NR_ACTIVE_*) with per-zone atomic NR_ISOLATED_*
looks ugly and error-prone because their accuracy is different.