Suspicious error for CMA stress test

[Lucas Stach]

Am Freitag, den 18.03.2016, 21:58 +0100 schrieb Vlastimil Babka:
> On 03/18/2016 03:42 PM, Lucas Stach wrote:
> > Am Freitag, den 18.03.2016, 15:10 +0100 schrieb Vlastimil Babka:
> >> On 03/17/2016 04:52 PM, Joonsoo Kim wrote:
> >> > 2016-03-18 0:43 GMT+09:00 Vlastimil Babka <vbabka at suse.cz>:
> >>
> >> OK, here it is. Hanjun can you please retest this, as I'm not sure if you had
> >> the same code due to the followup one-liner patches in the thread. Lucas, see if
> >> it helps with your issue as well. Laura and Joonsoo, please also test and review
> >> and check changelog if my perception of the problem is accurate :)
> >>
> >
> > This doesn't help for my case, as it is still trying to merge pages in
> > isolated ranges. It even tries extra hard at doing so.
> >
> > With concurrent isolation and frees going on this may lead to the start
> > page of the range to be isolated merging into an higher order buddy page
> > if it isn't already pageblock aligned, leading both test_pages_isolated
> > and isolate_freepages to fail on an otherwise perfectly fine range.
> >
> > What I am arguing is that if a page is freed into an isolated range we
> > should not try merge it with it's buddies at all, by setting max_order =
> > order. If the range is isolated because want to isolate freepages from
> > it, the work to do the merging is wasted, as isolate_freepages will
> > split higher order pages into order-0 pages again.
> >
> > If we already finished isolating freepages and are in the process of
> > undoing the isolation, we don't strictly need to do the merging in
> > __free_one_page, but can defer it to unset_migratetype_isolate, allowing
> > to simplify those code paths by disallowing any merging of isolated
> > pages at all.
> 
> Oh, I think understand now. Yeah, skipping merging for pages in isolated 
> pageblocks might be a rather elegant solution. But still, we would have to check 
> buddy's migratetype at order >= pageblock_order like my patch does, which is 
> annoying. Because even without isolated merging, the buddy might have already 
> had order>=pageblock_order when it was isolated.

> So what if isolation also split existing buddies in the pageblock immediately 
> when it sets the MIGRATETYPE_ISOLATE on the pageblock? Then we would have it 
> guaranteed that there's no isolated buddy - a buddy candidate at order >= 
> pageblock_order either has a smaller order (so it's not a buddy) or is not 
> MIGRATE_ISOLATE so it's safe to merge with.
> 
> Does that make sense?
> 
This might increase the the overhead of isolation a lot. CMA is also
used for small order allocations, so the work of splitting a whole
pageblock to allocate a small number of pages out just to merge a lot of
them again on unisolation might make this unattractive.

My feeling is that checking the buddy migratetype for >=pageblock_order
frees might be lower overhead, but I have no hard numbers to back this
claim.

Then on the other hand moving the work to isolation/unisolation affects
only code paths that are expected to be quite slow anyways, doing the
check in _free_one_page will affect everyone.

Regards,
Lucas