Re: Memory hotplug softlock issue

[Michal Hocko <mhocko@kernel.org>]

On Thu 15-11-18 15:53:56, Baoquan He wrote:
> On 11/15/18 at 08:30am, Michal Hocko wrote:
> > On Thu 15-11-18 13:10:34, Baoquan He wrote:
> > > On 11/14/18 at 04:00pm, Michal Hocko wrote:
> > > > On Wed 14-11-18 22:52:50, Baoquan He wrote:
> > > > > On 11/14/18 at 10:01am, Michal Hocko wrote:
> > > > > > I have seen an issue when the migration cannot make a forward progress
> > > > > > because of a glibc page with a reference count bumping up and down. Most
> > > > > > probable explanation is the faultaround code. I am working on this and
> > > > > > will post a patch soon. In any case the migration should converge and if
> > > > > > it doesn't do then there is a bug lurking somewhere.
> > > > > > 
> > > > > > Failing on ENOMEM is a questionable thing. I haven't seen that happening
> > > > > > wildly but if it is a case then I wouldn't be opposed.
> > > > > 
> > > > > Applied your debugging patches, it helps a lot to printing message.
> > > > > 
> > > > > Below is the dmesg log about the migrating failure. It can't pass
> > > > > migrate_pages() and loop forever.
> > > > > 
> > > > > [  +0.083841] migrating pfn 10fff7d0 failed 
> > > > > [  +0.000005] page:ffffea043ffdf400 count:208 mapcount:201 mapping:ffff888dff4bdda8 index:0x2
> > > > > [  +0.012689] xfs_address_space_operations [xfs] 
> > > > > [  +0.000030] name:"stress" 
> > > > > [  +0.004556] flags: 0x5fffffc0000004(uptodate)
> > > > > [  +0.007339] raw: 005fffffc0000004 ffffc900000e3d80 ffffc900000e3d80 ffff888dff4bdda8
> > > > > [  +0.009488] raw: 0000000000000002 0000000000000000 000000cb000000c8 ffff888e7353d000
> > > > > [  +0.007726] page->mem_cgroup:ffff888e7353d000
> > > > > [  +0.084538] migrating pfn 10fff7d0 failed 
> > > > > [  +0.000006] page:ffffea043ffdf400 count:210 mapcount:201 mapping:ffff888dff4bdda8 index:0x2
> > > > > [  +0.012798] xfs_address_space_operations [xfs] 
> > > > > [  +0.000034] name:"stress" 
> > > > > [  +0.004524] flags: 0x5fffffc0000004(uptodate)
> > > > > [  +0.007068] raw: 005fffffc0000004 ffffc900000e3d80 ffffc900000e3d80 ffff888dff4bdda8
> > > > > [  +0.009359] raw: 0000000000000002 0000000000000000 000000cb000000c8 ffff888e7353d000
> > > > > [  +0.007728] page->mem_cgroup:ffff888e7353d000
> > > > 
> > > > I wouldn't be surprised if this was a similar/same issue I've been
> > > > chasing recently. Could you try to disable faultaround to see if that
> > > > helps. It seems that it helped in my particular case but I am still
> > > > waiting for the final good-to-go to post the patch as I do not own the
> > > > workload which triggered that issue.
> > > 
> > > Tried, still stuck in last block sometime. Usually after several times
> > > of hotplug/unplug. If stop stress program, the last block will be
> > > offlined immediately.
> > 
> > Is the pattern still the same? I mean failing over few pages with
> > reference count jumping up and down between attempts?
> 
> ->count jumping up and down, mapcount stays the same value.
> 
> > 
> > > [root@ ~]# cat /sys/kernel/debug/fault_around_bytes 
> > > 4096
> > 
> > Can you make it 0?
> 
> I executed 'echo 0 > fault_around_bytes', value less than one page size
> will round up to one page.

OK, I have missed that. So then there must be a different source of the
page count volatility. Is it always the same file?

I think we can rule out memory reclaim because that depends on the page
lock. Is the stress test hitting on memory compaction? In other words,
are
grep compact /proc/vmstat
counters changing during the offline test heavily? I am asking because I
do not see compaction pfn walkers skipping over MIGRATE_ISOLATE
pageblocks. But I might be missing something easily.

It would be also good to find out whether this is fs specific. E.g. does
it make any difference if you use a different one for your stress
testing?
-- 
Michal Hocko
SUSE Labs