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

[Huang, Ying]

Doug Anderson <dianders@xxxxxxxxxxxx> writes:

> Hi,
>
> On Tue, Apr 18, 2023 at 5:34 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> >> >> >> 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?
>
> Ah, got it! It's not always the easiest to follow all the code paths,
> but let's see what I can find.
>
> I guess to start with, though, I will assert that someone seems to
> have believed that there was an important difference between
> MIGRATE_ASYNC and MIGRATE_SYNC_LIGHT besides waiting on the lock in
> migrate_folio_unmapt() since (as I found in my previous digging) the
> "direct reclaim" path never grabs this lock but explicitly sometimes
> chooses MIGRATE_ASYNC some times and MIGRATE_SYNC_LIGHT other times.
>
> OK, so looking at mainline Linux and comparing differences in behavior
> between SYNC_LIGHT and ASYNC and thoughts about which one should be
> used for kcompactd. Note that I won't go _too_ deep into all the
> differences...
>
> --
>
> In nfs.c:
>
> 1. We will wait for the fscache if SYNC_LIGHT but not ASYNC. No idea
> what would be the most ideal for calls from kcompactd.

This appears like something like disk writing.

> In compaction.c:
>
> 2. We will update the non-async "compact_cached_migrate_pfn" for
> SYNC_LIGHT but not ASYNC since we keep track of sync and async
> progress separately.
>
> 3. compact_lock_irqsave() note contentions for ASYNC but not
> SYNC_LIGHT and cause an earlier abort. Seems like kcompactd would want
> the SYNC_LIGHT behavior since this isn't about things indefinitely
> blocking.
>
> 4. isolate_migratepages_block() will bail if too_many_isolated() for
> ASYNC but not SYNC_LIGHT. My hunch is that kcompactd wants the
> SYNC_LIGHT behavior for kcompact.
>
> 5. If in direct compaction, isolate_migratepages_block() sets
> "skip_on_failure" for ASYNC but not SYNC_LIGHT. My hunch is that
> kcompactd wants the SYNC_LIGHT behavior for kcompact.
>
> 6. suitable_migration_source() considers more things suitable
> migration sources when SYNC_LIGHT but not (ASYNC+direct_compaction).
> Doesn't matter since kcompactd isn't direct compaction and non-direct
> compaction is the same.
>
> 7. fast_isolate_around() does less scanning when SYNC_LIGHT but not
> (ASYNC+direct_compaction). Again, it doesn't matter for kcompactd.
>
> 8. isolate_freepages() uses a different stride with SYNC_LIGHT vs.
> ASYNC. My hunch is that kcompactd wants the SYNC_LIGHT behavior for
> kcompact.
>
> In migrate.c:
>
> 9. buffer_migrate_lock_buffers() will block waiting to lock buffers
> with SYNC_LIGHT but not ASYNC. I don't know for sure, but this feels
> like something we _wouldn't_ want to block on in kcompactd and instead
> should look for easier pickings.

IIUC, this is similar as page lock.

> 10. migrate_folio_unmap() has the case we've already talked about
>
> 11. migrate_folio_unmap() does batch flushes for async because it
> doesn't need to worry about a class of deadlock. Somewhat recent code
> actually ends up running this code first even for sync modes to get
> the batch.
>
> 12. We'll retry a few more times for SYNC_LIGHT than ASYNC. Seems like
> the more retries won't really hurt for kcompactd.
>
> --
>
> So from looking at all the above, I'll say that kcompactd should stick
> with SYNC_LIGHT and we should fix #10. In other words, like my
> original patch except that we keep blocking on the lock in the full
> SYNC modes.
>
> It's possible that we should also change case #9 I listed above. Do
> you know if locking buffers is likely to block on something as slow as
> page reading/writing?

IIUC, this is related to page reading/writing.  Buffer head is used by
ext2/4 to read/write.

Thank you very much for your research.  It looks like ASYNC isn't
appropriate for kcompactd.

>From the comments of SYNC_LIGHT,

 * MIGRATE_SYNC_LIGHT in the current implementation means to allow blocking
 *	on most operations but not ->writepage as the potential stall time
 *	is too significant

To make SYNC_LIGHT block on less operations than before, I guess that
you need to prove the stall time can be long with the operation with
not-so-extreme test cases.

Best Regards,
Huang, Ying