Hi, Honza, Jan Kara <jack@xxxxxxx> writes: > On Fri 17-02-23 13:47:48, Hugh Dickins wrote: >> On Mon, 13 Feb 2023, Huang Ying wrote: >> >> > From: "Huang, Ying" <ying.huang@xxxxxxxxx> >> > >> > Now, migrate_pages() migrate folios one by one, like the fake code as >> > follows, >> > >> > for each folio >> > unmap >> > flush TLB >> > copy >> > restore map >> > >> > If multiple folios are passed to migrate_pages(), there are >> > opportunities to batch the TLB flushing and copying. That is, we can >> > change the code to something as follows, >> > >> > for each folio >> > unmap >> > for each folio >> > flush TLB >> > for each folio >> > copy >> > for each folio >> > restore map >> > >> > The total number of TLB flushing IPI can be reduced considerably. And >> > we may use some hardware accelerator such as DSA to accelerate the >> > folio copying. >> > >> > So in this patch, we refactor the migrate_pages() implementation and >> > implement the TLB flushing batching. Base on this, hardware >> > accelerated folio copying can be implemented. >> > >> > If too many folios are passed to migrate_pages(), in the naive batched >> > implementation, we may unmap too many folios at the same time. The >> > possibility for a task to wait for the migrated folios to be mapped >> > again increases. So the latency may be hurt. To deal with this >> > issue, the max number of folios be unmapped in batch is restricted to >> > no more than HPAGE_PMD_NR in the unit of page. That is, the influence >> > is at the same level of THP migration. >> > >> > We use the following test to measure the performance impact of the >> > patchset, >> > >> > On a 2-socket Intel server, >> > >> > - Run pmbench memory accessing benchmark >> > >> > - Run `migratepages` to migrate pages of pmbench between node 0 and >> > node 1 back and forth. >> > >> > With the patch, the TLB flushing IPI reduces 99.1% during the test and >> > the number of pages migrated successfully per second increases 291.7%. >> > >> > Xin Hao helped to test the patchset on an ARM64 server with 128 cores, >> > 2 NUMA nodes. Test results show that the page migration performance >> > increases up to 78%. >> > >> > This patchset is based on mm-unstable 2023-02-10. >> >> And back in linux-next this week: I tried next-20230217 overnight. >> >> There is a deadlock in this patchset (and in previous versions: sorry >> it's taken me so long to report), but I think one that's easily solved. >> >> I've not bisected to precisely which patch (load can take several hours >> to hit the deadlock), but it doesn't really matter, and I expect that >> you can guess. >> >> My root and home filesystems are ext4 (4kB blocks with 4kB PAGE_SIZE), >> and so is the filesystem I'm testing, ext4 on /dev/loop0 on tmpfs. >> So, plenty of ext4 page cache and buffer_heads. >> >> Again and again, the deadlock is seen with buffer_migrate_folio_norefs(), >> either in kcompactd0 or in khugepaged trying to compact, or in both: >> it ends up calling __lock_buffer(), and that schedules away, waiting >> forever to get BH_lock. I have not identified who is holding BH_lock, >> but I imagine a jbd2 journalling thread, and presume that it wants one >> of the folio locks which migrate_pages_batch() is already holding; or >> maybe it's all more convoluted than that. Other tasks then back up >> waiting on those folio locks held in the batch. >> >> Never a problem with buffer_migrate_folio(), always with the "more >> careful" buffer_migrate_folio_norefs(). And the patch below fixes >> it for me: I've had enough hours with it now, on enough occasions, >> to be confident of that. >> >> Cc'ing Jan Kara, who knows buffer_migrate_folio_norefs() and jbd2 >> very well, and I hope can assure us that there is an understandable >> deadlock here, from holding several random folio locks, then trying >> to lock buffers. Cc'ing fsdevel, because there's a risk that mm >> folk think something is safe, when it's not sufficient to cope with >> the diversity of filesystems. I hope nothing more than the below is >> needed (and I've had no other problems with the patchset: good job), >> but cannot be sure. > > I suspect it can indeed be caused by the presence of the loop device as > Huang Ying has suggested. What filesystems using buffer_heads do is a > pattern like: > > bh = page_buffers(loop device page cache page); > lock_buffer(bh); > submit_bh(bh); > - now on loop dev this ends up doing: > lo_write_bvec() > vfs_iter_write() > ... > folio_lock(backing file folio); > > So if migration code holds "backing file folio" lock and at the same time > waits for 'bh' lock (while trying to migrate loop device page cache page), it > is a deadlock. > > Proposed solution of never waiting for locks in batched mode looks like a > sensible one to me... Thank you very much for detail explanation! Best Regards, Huang, Ying
