Re: [PATCH] mm: avoid blocking lock_page() in kcompactd

[Cong Wang]

On Thu, Jan 9, 2020 at 4:28 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Thu, Jan 9, 2020 at 2:57 PM Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> >
> > We observed kcompactd hung at __lock_page():
> >
> >  INFO: task kcompactd0:57 blocked for more than 120 seconds.
> >        Not tainted 4.19.56.x86_64 #1
> >  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> >  kcompactd0      D    0    57      2 0x80000000
> >  Call Trace:
> >   ? __schedule+0x236/0x860
> >   schedule+0x28/0x80
> >   io_schedule+0x12/0x40
> >   __lock_page+0xf9/0x120
> >   ? page_cache_tree_insert+0xb0/0xb0
> >   ? update_pageblock_skip+0xb0/0xb0
> >   migrate_pages+0x88c/0xb90
> >   ? isolate_freepages_block+0x3b0/0x3b0
> >   compact_zone+0x5f1/0x870
> >   kcompactd_do_work+0x130/0x2c0
> >   ? __switch_to_asm+0x35/0x70
> >   ? __switch_to_asm+0x41/0x70
> >   ? kcompactd_do_work+0x2c0/0x2c0
> >   ? kcompactd+0x73/0x180
> >   kcompactd+0x73/0x180
> >   ? finish_wait+0x80/0x80
> >   kthread+0x113/0x130
> >   ? kthread_create_worker_on_cpu+0x50/0x50
> >   ret_from_fork+0x35/0x40
> >
> > which faddr2line maps to:
> >
> >   migrate_pages+0x88c/0xb90:
> >   lock_page at include/linux/pagemap.h:483
> >   (inlined by) __unmap_and_move at mm/migrate.c:1024
> >   (inlined by) unmap_and_move at mm/migrate.c:1189
> >   (inlined by) migrate_pages at mm/migrate.c:1419
> >
> > Sometimes kcompactd eventually got out of this situation, sometimes not.
> >
> > I think for memory compaction, it is a best effort to migrate the pages,
> > so it doesn't have to wait for I/O to complete. It is fine to call
> > trylock_page() here, which is pretty much similar to
> > buffer_migrate_lock_buffers().
> >
> > Given MIGRATE_SYNC_LIGHT is used on compaction path, just relax the
> > check for it.
>
> But this changed the semantics of MIGRATE_SYNC_LIGHT which means
> blocking on most operations but not ->writepage. When
> MIGRATE_SYNC_LIGHT is used it means compaction priority is increased
> (the initial priority is ASYNC) due to whatever reason (i.e. not
> enough clean, non-writeback and non-locked pages to migrate). So, it
> has to wait for some pages to try to not backoff pre-maturely.

Thanks for explaining MIGRATE_SYNC_LIGHT. I didn't dig the history
of MIGRATE_SYNC_LIGHT.

>
> If I read the code correctly, buffer_migrate_lock_buffers() also
> blocks on page lock with non-ASYNC mode.
>
> Since v5.1 Mel Gorman improved compaction a lot. So, I'm wondering if
> this happens on the latest upstream or not.

Unfortunately we can't test upstream kernel because:
1) We don't know how to reproduce it.
2) It is not easy to deploy upstream kernel in our environment.


>
> And, did you figure out who is locking the page for such long time? Or
> there might be too many waiters on the list for this page?

I tried to dump the process stacks after we saw the hung task
with /proc/X/stack (apparently sysrq is not an option for production),
but I didn't find anything useful. I didn't see any other process
hung in lock_page() either, all mm-related kernel threads were sleeping
(non-D) at the time I debugged. So, it is possible there was some place
missing a unlock_page() too, which is too late to debug after the hung
task was reported.

Thanks.