Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim stats

[Yang Shi <shy828301@gmail.com>]

On Tue, Oct 25, 2022 at 1:54 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Tue, Oct 25, 2022 at 12:40:15PM -0700, Yang Shi wrote:
> > On Tue, Oct 25, 2022 at 10:05 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > Direct reclaim stats are useful for identifying a potential source for
> > > application latency, as well as spotting issues with kswapd. However,
> > > khugepaged currently distorts the picture: as a kernel thread it
> > > doesn't impose allocation latencies on userspace, and it explicitly
> > > opts out of kswapd reclaim. Its activity showing up in the direct
> > > reclaim stats is misleading. Counting it as kswapd reclaim could also
> > > cause confusion when trying to understand actual kswapd behavior.
> > >
> > > Break out khugepaged from the direct reclaim counters into new
> > > pgsteal_khugepaged, pgdemote_khugepaged, pgscan_khugepaged counters.
> > >
> > > Test with a huge executable (CONFIG_READ_ONLY_THP_FOR_FS):
> > >
> > > pgsteal_kswapd 1342185
> > > pgsteal_direct 0
> > > pgsteal_khugepaged 3623
> > > pgscan_kswapd 1345025
> > > pgscan_direct 0
> > > pgscan_khugepaged 3623
> >
> > There are other kernel threads or works may allocate memory then
> > trigger memory reclaim, there may be similar problems for them and
> > someone may try to add a new stat. So how's about we make the stats
> > more general, for example, call it "pg{steal|scan}_kthread"?
>
> I'm not convinved that's a good idea.
>
> Can you generally say that userspace isn't indirectly waiting for one
> of those allocating threads? With khugepaged, we know.

AFAIK, ksm may do slab allocation with __GFP_DIRECT_RECLAIM. Some
device mapper drivers may do heavy lift in the work queue, for
example, dm-crypt, particularly for writing.

>
> And those other allocations are usually ___GFP_KSWAPD_RECLAIM, so if
> they do direct reclaim, we'd probably want to know that kswapd is
> failing to keep up (doubly so if userspace is waiting). In a shared
> kthread counter, khugepaged would again muddy the waters.