Re: [PATCH v3] migrate_pages: Avoid blocking for IO in MIGRATE_SYNC_LIGHT

[Doug Anderson]

Hi,

On Tue, May 2, 2023 at 6:45 PM Hillf Danton <hdanton@xxxxxxxx> wrote:
>
> On 2 May 2023 14:20:54 -0700 Douglas Anderson <dianders@xxxxxxxxxxxx>
> > On Sun, Apr 30, 2023 at 1:53=E2=80=AFAM Hillf Danton <hdanton@xxxxxxxx> wrote:
> > > On 28 Apr 2023 13:54:38 -0700 Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > > The MIGRATE_SYNC_LIGHT mode is intended to block for things that will
> > > > finish quickly but not for things that will take a long time. Exactly
> > > > how long is too long is not well defined, but waits of tens of
> > > > milliseconds is likely non-ideal.
> > > >
> > > > When putting a Chromebook under memory pressure (opening over 90 tabs
> > > > on a 4GB machine) it was fairly easy to see delays waiting for some
> > > > locks in the kcompactd code path of > 100 ms. While the laptop wasn't
> > > > amazingly usable in this state, it was still limping along and this
> > > > state isn't something artificial. Sometimes we simply end up with a
> > > > lot of memory pressure.
> > >
> > > Given longer than 100ms stall, this can not be a correct fix if the
> > > hardware fails to do more than ten IOs a second.
> > >
> > > OTOH given some pages reclaimed for compaction to make forward progress
> > > before kswapd wakes kcompactd up, this can not be a fix without spotting
> > > the cause of the stall.
> >
> > Right that the system is in pretty bad shape when this happens and
> > it's not very effective at doing IO or much of anything because it's
> > under bad memory pressure.
>
> Based on the info in another reply [1]
>
>    | I put some more traces in and reproduced it again. I saw something
>    | that looked like this:
>    |
>    | 1. balance_pgdat() called wakeup_kcompactd() with order=10 and that
>    | caused us to get all the way to the end and wakeup kcompactd (there
>    | were previous calls to wakeup_kcompactd() that returned early).
>    |
>    | 2. kcompactd started and completed kcompactd_do_work() without blocking.
>    |
>    | 3. kcompactd called proactive_compact_node() and there blocked for
>    | ~92ms in one case, ~120ms in another case, ~131ms in another case.
>
> I see fragmentation given order=10 and proactive_compact_node(). Can you
> specify the evidence of bad memory pressure?

What type of evidence are you looking for? When I'm reproducing these
problems, I'm running a test that specifically puts the system under
memory pressure by opening up lots of tabs in the Chrome browser. When
I start seeing these printouts, I can take a look at the system and I
can see that it's pretty much constantly swapping in and swapping out.


> > I guess my first thought is that, when this happens then a process
> > holding the lock gets preempted and doesn't get scheduled back in for
> > a while. That _should_ be possible, right? In the case where I'm
> > reproducing this then all the CPUs would be super busy madly trying to
> > compress / decompress zram, so it doesn't surprise me that a process
> > could get context switched out for a while.
>
> Could switchout turn the below I/O upside down?
>                 /*
>                  * In "light" mode, we can wait for transient locks (eg
>                  * inserting a page into the page table), but it's not
>                  * worth waiting for I/O.
>                  */

I'm not sure I understand what you're asking, sorry!

-Doug