Re: [PATCH v7 04/12] mm: multigenerational LRU: groundwork

[Yu Zhao]

On Fri, Mar 11, 2022 at 3:16 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Tue, Feb 15, 2022 at 10:43 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
> >
> > On Thu, Feb 10, 2022 at 03:41:57PM -0500, Johannes Weiner wrote:
> >
> > Thanks for reviewing.
> >
> > > > +static inline bool lru_gen_is_active(struct lruvec *lruvec, int gen)
> > > > +{
> > > > +   unsigned long max_seq = lruvec->lrugen.max_seq;
> > > > +
> > > > +   VM_BUG_ON(gen >= MAX_NR_GENS);
> > > > +
> > > > +   /* see the comment on MIN_NR_GENS */
> > > > +   return gen == lru_gen_from_seq(max_seq) || gen == lru_gen_from_seq(max_seq - 1);
> > > > +}
> > >
> > > I'm still reading the series, so correct me if I'm wrong: the "active"
> > > set is split into two generations for the sole purpose of the
> > > second-chance policy for fresh faults, right?
> >
> > To be precise, the active/inactive notion on top of generations is
> > just for ABI compatibility, e.g., the counters in /proc/vmstat.
> > Otherwise, this function wouldn't be needed.
>
> Hi Yu,
> I am still quite confused as i am seeing both active/inactive and lru_gen.
> eg:
>
> root@ubuntu:~# cat /proc/vmstat | grep active
> nr_zone_inactive_anon 22797
> nr_zone_active_anon 578405
> nr_zone_inactive_file 0
> nr_zone_active_file 4156
> nr_inactive_anon 22800
> nr_active_anon 578574
> nr_inactive_file 0
> nr_active_file 4215

Yes, this is expected. We have to maintain the ABI, i.e., the
*_active/inactive_* counters.

> and:
>
> root@ubuntu:~# cat /sys//kernel/debug/lru_gen
>
> ...
> memcg    36 /user.slice/user-0.slice/user@0.service
>  node     0
>          20      18820         22           0
>          21       7452          0           0
>          22       7448          0           0
> memcg    33 /user.slice/user-0.slice/user@0.service/app.slice
>  node     0
>           0    2171452          0           0
>           1    2171452          0           0
>           2    2171452          0           0
>           3    2171452          0           0
> memcg    37 /user.slice/user-0.slice/session-1.scope
>  node     0
>          42      51804     102127           0
>          43      18840     275622           0
>          44      16104     216805           1
>
> Does it mean one page could be in both one of the generations and one
> of the active/inactive lists?

In terms of the data structure, evictable pages are either on
lruvec->lists or lrugen->lists.

> Do we have some mapping relationship between active/inactive lists
> with generations?

For the counters, yes -- pages in max_seq and max_seq-1 are counted as
active, and the rest are inactive.

> We used to put a faulted file page in inactive, if we access it a
> second time, it can be promoted
> to active. then in recent years, we have also applied this to anon
> pages while kernel adds
> workingset protection for anon pages. so basically both anon and file
> pages go into the inactive
> list for the 1st time, if we access it for the second time, they go to
> the active list. if we don't access
> it any more, they are likely to be reclaimed as they are inactive.
> we do have some special fastpath for code section, executable file
> pages are kept on active list
> as long as they are accessed.

Yes.

> so all of the above concerns are actually not that correct?

They are valid concerns but I don't know any popular workloads that
care about them.