Re: [PATCH] mm: skip current when memcg reclaim

[Zhaoyang Huang]

On Tue, Oct 19, 2021 at 5:09 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 19-10-21 15:11:30, Zhaoyang Huang wrote:
> > On Mon, Oct 18, 2021 at 8:41 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Mon 18-10-21 17:25:23, Zhaoyang Huang wrote:
> > > > On Mon, Oct 18, 2021 at 4:23 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> [...]
> > > > > I would be really curious about more specifics of the used hierarchy.
> > > > What I am facing is a typical scenario on Android, that is a big
> > > > memory consuming APP(camera etc) launched while background filled by
> > > > other processes. The hierarchy is like what you describe above where B
> > > > represents the APP and memory.low is set to help warm restart. Both of
> > > > kswapd and direct reclaim work together to reclaim pages under this
> > > > scenario, which can cause 20MB file page delete from LRU in several
> > > > second. This change could help to have current process's page escape
> > > > from being reclaimed and cause page thrashing. We observed the result
> > > > via systrace which shows that the Uninterruptible sleep(block on page
> > > > bit) and iowait get smaller than usual.
> > >
> > > I still have hard time to understand the exact setup and why the patch
> > > helps you. If you want to protect B more than the low limit would allow
> > > for by stealiong from C then the same thing can happen from anybody
> > > reclaiming from C so in the end there is no protection. The same would
> > > apply for any global direct memory reclaim done by a 3rd party. So I
> > > suspect that your patch just happens to work by a luck.
> > B and C compete fairly and superior than others. The idea based on
> > assuming NOT all groups will trap into direct reclaim concurrently, so
> > we want to have the groups steal pages from the processes under
> > root(Non-memory sensitive) or other groups with lower thresholds(high
> > memory tolerance) or the one totally sleeping(not busy for the time
> > being, borrow some pages).
>
> I am really confused now. The memcg reclaim cannot really reclaim
> anything from outside of the reclaimed hierarchy. Protected memcgs are
> only considered if the reclaim was not able to reclaim anything during
> the first hierarchy walk. That would imply that the reclaimed hierarchy
> has either all memcgs with memory protected or non-protected memcgs do
> not have any memory to reclaim.
>
> I think it would really help to provide much details about what is going
> on here before we can move forward.
>
> > > Why both B and C have low limit setup and they both cannot be reclaimed?
> > > Isn't that a weird setup where A hard limit is too close to sum of low
> > > limits of B and C?
> > >
> > > In other words could you share a more detailed configuration you are
> > > using and some more details why both B and C have been skipped during
> > > the first pass of the reclaim?
> > My practical scenario is that important processes(vip APP etc) are
> > placed into protected memcg and keep other processes just under root.
> > Current introduces direct reclaim because of alloc_pages(DMA_ALLOC
> > etc), in which the number of allocation would be much larger than low
> > but would NOT be charged to LRU. Whereas, current also wants to keep
> > the pages(.so files to exec) on LRU.
>
> I am sorry but this description makes even less sense to me. If your
> important process runs under a protected memcg and everything else is
> running under root memcg then your memcg will get protected as long as
> there is a reclaimable memory. There should ever be only global memory
> reclaim happening, unless you specify a hard/high limit on your
> important memcg. If you do so then there is no way to reclaim from
> outside of that specific memcg.
>
> I really fail how your patch can help with either of those situations.

please find cgv2 hierarchy on my sys[1], where uid_2000 is a cgroup
under root and trace_printk info[3] from trace_printk embedded in
shrink_node[2]. I don't why you say there should be no reclaim from
groups under root which opposite to[3]

[1]
/sys/fs/cgroup # ls uid_2000
cgroup.controllers  cgroup.max.depth        cgroup.stat
cgroup.type   io.pressure     memory.events.local  memory.max
memory.pressure      memory.swap.events
cgroup.events       cgroup.max.descendants  cgroup.subtree_control
cpu.pressure  memory.current  memory.high          memory.min
memory.stat          memory.swap.max
cgroup.freeze       cgroup.procs            cgroup.threads
cpu.stat      memory.events   memory.low           memory.oom.group
memory.swap.current  pid_275

[2]
@@ -2962,6 +2962,7 @@ static bool shrink_node(pg_data_t *pgdat, struct
scan_control *sc)

                        reclaimed = sc->nr_reclaimed;
                        scanned = sc->nr_scanned;
+                     trace_printk("root %x memcg %x reclaimed
%ld\n",root_mem_cgroup,memcg,sc->nr_reclaimed);
                        shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
                        node_lru_pages += lru_pages;

[3]
 allocator@4.0-s-1034    [005] ....   442.077013: shrink_node: root
ef022800 memcg ef027800 reclaimed 41
   kworker/u16:3-931     [002] ....   442.077019: shrink_node: root
ef022800 memcg c7e54000 reclaimed 17
 allocator@4.0-s-1034    [005] ....   442.077019: shrink_node: root
ef022800 memcg ef025000 reclaimed 41
 allocator@4.0-s-1034    [005] ....   442.077024: shrink_node: root
ef022800 memcg ef023000 reclaimed 41
   kworker/u16:3-931     [002] ....   442.077026: shrink_node: root
ef022800 memcg c7e57800 reclaimed 17
 allocator@4.0-s-1034    [005] ....   442.077028: shrink_node: root
ef022800 memcg ef026800 reclaimed 41


> --
> Michal Hocko
> SUSE Labs