Re: [PATCH 3/4] mm: compaction: Update pageblock skip when first migration candidate is not at the start

[Mel Gorman]

On Tue, Jun 06, 2023 at 03:11:27PM +0200, Vlastimil Babka wrote:
> On 6/2/23 14:48, Mel Gorman wrote:
> > On Fri, Jun 02, 2023 at 02:19:00PM +0200, Vlastimil Babka wrote:
> >> On 6/2/23 13:16, Mel Gorman wrote:
> >> > On Mon, May 29, 2023 at 02:43:48PM +0200, Vlastimil Babka wrote:
> >> >> On 5/29/23 12:33, Mel Gorman wrote:
> >> >> > On Thu, May 25, 2023 at 03:37:43PM +0200, Vlastimil Babka wrote:
> >> >> >> On 5/15/23 13:33, Mel Gorman wrote:
> >> >> >> > isolate_migratepages_block should mark a pageblock as skip if scanning
> >> >> >> > started on an aligned pageblock boundary but it only updates the skip
> >> >> >> > flag if the first migration candidate is also aligned. Tracing during
> >> >> >> > a compaction stress load (mmtests: workload-usemem-stress-numa-compact)
> >> >> >> > that many pageblocks are not marked skip causing excessive scanning of
> >> >> >> > blocks that had been recently checked. Update pageblock skip based on
> >> >> >> > "valid_page" which is set if scanning started on a pageblock boundary.
> >> >> >> > 
> >> >> >> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> >> >> >> 
> >> >> >> I wonder if this has an unintended side-effect that if we resume
> >> >> >> isolate_migratepages_block() of a partially compacted pageblock to finish
> >> >> >> it, test_and_set_skip() will now tell us to abort, because we already set
> >> >> >> the skip bit in the previous call. This would include the
> >> >> >> cc->finish_pageblock rescan cases.
> >> >> >> 
> >> >> >> So unless I miss something that already prevents that, I agree we should not
> >> >> >> tie setting the skip bit to pageblock_aligned(pfn), but maybe if we are not
> >> >> >> pageblock aligned, we should ignore the already-set skip bit, as it was most
> >> >> >> likely being set by us in the previous iteration and should not prevent us
> >> >> >> from finishing the pageblock?
> >> >> >> 
> >> >> > 
> >> >> > Hmm, I think you're right. While it should not hit the original bug,
> >> >> > migration candidates are missed until the next compaction scan which
> >> >> > could be tricky to detect. Something like this as a separate patch?
> >> >> > Build tested only but the intent is for an unaligned start to set the skip
> >> >> > bet if already unset but otherwise complete the scan. Like earlier fixes,
> >> >> > this might overscan some pageblocks in a given context but we are probably
> >> >> > hitting the limits on how compaction can run efficiently in the current
> >> >> > scheme without causing other side-effects :(
> >> >> 
> >> >> Yeah that should work! I think it should be even folded to 3/4 but if you
> >> >> want separate, fine too.
> >> >> 
> >> > 
> >> > I was not happy with the test results so limited the scope of the patch
> >> > which performed much better both in terms of absolute performance and
> >> > compaction activity.
> >> 
> >> That's surprising. Does that mean that if we isolate COMPACT_CLUSTER_MAX
> >> pages, migrate them without failing, but it's not enough to succeed (i.e.
> >> there are more pages we need to migrate to free up a whole pageblock), it's
> >> better to give up on the rest of the pageblock rather than continue?
> > 
> > I don't have precise enough data to answer that with certainty but probably
> > yes, at least in terms of scan activity. The first version had spikes of
> > pages scanned for migration that are not always reproducible and not on
> > all machines.
> 
> Well, that kinda sucks. So for the patch (with adding the missing NOT below).
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> But in raises a question whether we should terminate compaction under the
> right conditions after a successful migration immediately, rather than
> invoke another iteration of isolate_migratepages_block() where we could skip
> over some pages uselessly only to abort at first valid page due to the skip bit.
> It would save some cycles and be much more obvious than now, where anyone
> trying to understand how it works in detail might conclude it's an oversight?
> 

It sounds like a solid idea and would be a good standalone patch with
the usual supporting data. At a quick glance, the check for a page
stored on compact_control happens surprisingly late which makes me think
we probably over-compact in direct compaction in particular. It would
need supporting data because it probably means that compaction cost gets
spread over multiple tasks requiring high-order pages instead of one
unlucky task doing compaction works that unrelated tasks indirectly
benefit from. It's probably more sensible behaviour that tasks requiring
high-order pages pay the cost if kcompactd cannot keep up but supporting
data would tell us one way or the other.

-- 
Mel Gorman
SUSE Labs