Re: [RFC PATCH] migrate_pages: Never block waiting for the page lock

[Huang, Ying]

Doug Anderson <dianders@xxxxxxxxxxxx> writes:

> Hi,
>
> On Mon, Apr 17, 2023 at 8:18 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Doug Anderson <dianders@xxxxxxxxxxxx> writes:
>>
>> > Hi,
>> >
>> > On Sun, Apr 16, 2023 at 6:15 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >>
>> >> Doug Anderson <dianders@xxxxxxxxxxxx> writes:
>> >>
>> >> > Hi,
>> >> >
>> >> > On Thu, Apr 13, 2023 at 8:10 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >> >>
>> >> >> Douglas Anderson <dianders@xxxxxxxxxxxx> writes:
>> >> >>
>> >> >> > Currently when we try to do page migration and we're in "synchronous"
>> >> >> > mode (and not doing direct compaction) then we'll wait an infinite
>> >> >> > amount of time for a page lock. This does not appear to be a great
>> >> >> > idea.
>> >> >> >
>> >> >> > One issue can be seen when I put a device under extreme memory
>> >> >> > pressure. I took a sc7180-trogdor Chromebook (4GB RAM, 8GB zram
>> >> >> > swap). I ran the browser along with Android (which runs from a
>> >> >> > loopback mounted 128K block-size squashfs "disk"). I then manually ran
>> >> >> > the mmm_donut memory pressure tool [1]. The system is completely
>> >> >> > unusable both with and without this patch since there are 8 processes
>> >> >> > completely thrashing memory, but it was still interesting to look at
>> >> >> > how migration was behaving. I put some timing code in and I could see
>> >> >> > that we sometimes waited over 25 seconds (in the context of
>> >> >> > kcompactd0) for a page lock to become available. Although the 25
>> >> >> > seconds was the high mark, it was easy to see tens, hundreds, or
>> >> >> > thousands of milliseconds spent waiting on the lock.
>> >> >> >
>> >> >> > Instead of waiting, if I bailed out right away (as this patch does), I
>> >> >> > could see kcompactd0 move forward to successfully to migrate other
>> >> >> > pages instead. This seems like a better use of kcompactd's time.
>> >> >> >
>> >> >> > Thus, even though this didn't make the system any more usable in my
>> >> >> > absurd test case, it still seemed to make migration behave better and
>> >> >> > that feels like a win. It also makes the code simpler since we have
>> >> >> > one fewer special case.
>> >> >>
>> >> >> TBH, the test case is too extreme for me.
>> >> >
>> >> > That's fair. That being said, I guess the point I was trying to make
>> >> > is that waiting for this lock could take an unbounded amount of time.
>> >> > Other parts of the system sometimes hold a page lock and then do a
>> >> > blocking operation. At least in the case of kcompactd there are better
>> >> > uses of its time than waiting for any given page.
>> >> >
>> >> >> And, we have multiple "sync" mode to deal with latency requirement, for
>> >> >> example, we use MIGRATE_SYNC_LIGHT for compaction to avoid too long
>> >> >> latency.  If you have latency requirement for some users, you may
>> >> >> consider to add new "sync" mode.
>> >> >
>> >> > Sure. kcompactd_do_work() is currently using MIGRATE_SYNC_LIGHT. I
>> >> > guess my first thought would be to avoid adding a new mode and make
>> >> > MIGRATE_SYNC_LIGHT not block here. Then anyone that truly needs to
>> >> > wait for all the pages to be migrated can use the heavier sync modes.
>> >> > It seems to me like the current users of MIGRATE_SYNC_LIGHT would not
>> >> > want to block for an unbounded amount of time here. What do you think?
>> >>
>> >> It appears that you can just use MIGRATE_ASYNC if you think the correct
>> >> behavior is "NOT block at all".  I found that there are more
>> >> fine-grained controls on this in compaction code, please take a look at
>> >> "enum compact_priority" and its comments.
>> >
>> > Actually, the more I think about it the more I think the right answer
>> > is to keep kcompactd as using MIGRATE_SYNC_LIGHT and make
>> > MIGRATE_SYNC_LIGHT not block on the folio lock.
>>
>> Then, what is the difference between MIGRATE_SYNC_LIGHT and
>> MIGRATE_ASYNC?
>
> Aren't there still some differences even if we remove blocking this
> one lock? ...or maybe your point is that maybe the other differences
> have similar properties?

Sorry for confusing words.  Here, I asked you to list the implementation
difference between MIGRATE_ASYNC and MIGRATE_SYNC_LIGHT after your
proposed changes.  Which are waited in MIGRATE_SYNC_LIGHT but not in
MIGRATE_ASYNC?

> OK, so let's think about just using MIGRATE_ASYNC and either leaving
> MIGRATE_SYNC_LIGHT alone or deleting it (if there are no users left).
> The nice thing is that the only users of MIGRATE_SYNC_LIGHT are in
> "compaction.c" and there are only 3 places where it's specified.
>
> 1. kcompactd_do_work() - This is what I was analyzing and where I
> argued that indefinite blocking is less useful than simply trying to
> compact a different page. So sure, moving this to MIGRATE_ASYNC seems
> like it would be OK?
>
> 2. proactive_compact_node() - Just like kcompactd_do_work(), this is
> called from kcompactd and thus probably should have the same mode.
>
> 3. compact_zone_order() - This explicitly chooses between
> MIGRATE_SYNC_LIGHT and MIGRATE_ASYNC, so I guess I'd keep
> MIGRATE_SYNC_LIGHT just for this use case. It looks as if
> compact_zone_order() is called for direct compaction and thus making
> it synchronous can make sense, especially since it seems to go to the
> synchronous case after it failed with the async case. Ironically,
> though, the exact lock I was proposing to not wait on _isn't_ ever
> waited on in direct reclaim (see the comment in migrate_folio_unmap()
> about deadlock), but the other differences between SYNC_LIGHT and
> ASYNC come into play.
>
> If the above sounds correct then I'm OK w/ moving #1 and #2 to
> MIGRATE_ASYNC and leaving #3 as the sole user or MIGRATE_SYNC_LIGHT.
>
>> > kcompactd can accept some blocking but we don't want long / unbounded
>> > blocking. Reading the comments for MIGRATE_SYNC_LIGHT, this also seems
>> > like it fits pretty well. MIGRATE_SYNC_LIGHT says that the stall time
>> > of writepage() is too much. It's entirely plausible that someone else
>> > holding the lock is doing something as slow as writepage() and thus
>> > waiting on the lock can be just as bad for latency.
>>
>> IIUC, during writepage(), the page/folio will be unlocked.
>>
>> But, during page reading, the page/folio will be locked.  I don't really
>> understand why we can wait for page reading but cannot wait for page
>> writeback.
>
> I'm not sure I totally got your point here. It sorta sounds as if
> you're making the same point that I was?

Yes, kind of.  It is a question, not conclusion.

> IIUC by waiting on the lock
> we may be implicitly waiting for someone to finish reading which seems
> as bad as waiting for writing. That was why I was arguing that with
> MIGRATE_SYNC_LIGHT (which says that waiting for the write was too
> slow) that we shouldn't wait for the lock (which may be blocking on a
> read).

Best Regards,
Huang, Ying