On Thu, Jan 06, 2022 at 09:48:29AM +1100, Dave Chinner wrote:
> On Wed, Jan 05, 2022 at 08:45:05PM +0000, Trond Myklebust wrote:
> > On Tue, 2022-01-04 at 21:09 -0500, Trond Myklebust wrote:
> > > On Tue, 2022-01-04 at 12:22 +1100, Dave Chinner wrote:
> > > > On Tue, Jan 04, 2022 at 12:04:23AM +0000, Trond Myklebust wrote:
> > > > > We have different reproducers. The common feature appears to be
> > > > > the
> > > > > need for a decently fast box with fairly large memory (128GB in
> > > > > one
> > > > > case, 400GB in the other). It has been reproduced with HDs, SSDs
> > > > > and
> > > > > NVME systems.
> > > > >
> > > > > On the 128GB box, we had it set up with 10+ disks in a JBOD
> > > > > configuration and were running the AJA system tests.
> > > > >
> > > > > On the 400GB box, we were just serially creating large (> 6GB)
> > > > > files
> > > > > using fio and that was occasionally triggering the issue. However
> > > > > doing
> > > > > an strace of that workload to disk reproduced the problem faster
> > > > > :-
> > > > > ).
> > > >
> > > > Ok, that matches up with the "lots of logically sequential dirty
> > > > data on a single inode in cache" vector that is required to create
> > > > really long bio chains on individual ioends.
> > > >
> > > > Can you try the patch below and see if addresses the issue?
> > > >
> > >
> > > That patch does seem to fix the soft lockups.
> > >
> >
> > Oops... Strike that, apparently our tests just hit the following when
> > running on AWS with that patch.
>
> OK, so there are also large contiguous physical extents being
> allocated in some cases here.
>
> > So it was harder to hit, but we still did eventually.
>
> Yup, that's what I wanted to know - it indicates that both the
> filesystem completion processing and the iomap page processing play
> a role in the CPU usage. More complex patch for you to try below...
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
>
> xfs: limit individual ioend chain length in writeback
>
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> Trond Myklebust reported soft lockups in XFS IO completion such as
> this:
>
> watchdog: BUG: soft lockup - CPU#12 stuck for 23s! [kworker/12:1:3106]
> CPU: 12 PID: 3106 Comm: kworker/12:1 Not tainted 4.18.0-305.10.2.el8_4.x86_64 #1
> Workqueue: xfs-conv/md127 xfs_end_io [xfs]
> RIP: 0010:_raw_spin_unlock_irqrestore+0x11/0x20
> Call Trace:
> wake_up_page_bit+0x8a/0x110
> iomap_finish_ioend+0xd7/0x1c0
> iomap_finish_ioends+0x7f/0xb0
> xfs_end_ioend+0x6b/0x100 [xfs]
> xfs_end_io+0xb9/0xe0 [xfs]
> process_one_work+0x1a7/0x360
> worker_thread+0x1fa/0x390
> kthread+0x116/0x130
> ret_from_fork+0x35/0x40
>
> Ioends are processed as an atomic completion unit when all the
> chained bios in the ioend have completed their IO. Logically
> contiguous ioends can also be merged and completed as a single,
> larger unit. Both of these things can be problematic as both the
> bio chains per ioend and the size of the merged ioends processed as
> a single completion are both unbound.
>
> If we have a large sequential dirty region in the page cache,
> write_cache_pages() will keep feeding us sequential pages and we
> will keep mapping them into ioends and bios until we get a dirty
> page at a non-sequential file offset. These large sequential runs
> can will result in bio and ioend chaining to optimise the io
"can result"?
> patterns. The pages iunder writeback are pinned within these chains
"pages under writeback"
> until the submission chaining is broken, allowing the entire chain
> to be completed. This can result in huge chains being processed
> in IO completion context.
>
> We get deep bio chaining if we have large contiguous physical
> extents. We will keep adding pages to the current bio until it is
> full, then we'll chain a new bio to keep adding pages for writeback.
> Hence we can build bio chains that map millions of pages and tens of
> gigabytes of RAM if the page cache contains big enough contiguous
> dirty file regions. This long bio chain pins those pages until the
> final bio in the chain completes and the ioend can iterate all the
> chained bios and complete them.
>
> OTOH, if we have a physically fragmented file, we end up submitting
> one ioend per physical fragment that each have a small bio or bio
> chain attached to them. We do not chain these at IO submission time,
> but instead we chain them at completion time based on file
> offset via iomap_ioend_try_merge(). Hence we can end up with unbound
> ioend chains being built via completion merging.
>
> XFS can then do COW remapping or unwritten extent conversion on that
> merged chain, which involves walking an extent fragment at a time
> and running a transaction to modify the physical extent information.
> IOWs, we merge all the discontiguous ioends together into a
> contiguous file range, only to then process them individually as
> discontiguous extents.
>
> This extent manipulation is computationally expensive and can run in
> a tight loop, so merging logically contiguous but physically
> discontigous ioends gains us nothing except for hiding the fact the
> fact we broke the ioends up into individual physical extents at
> submission and then need to loop over those individual physical
> extents at completion.
<nod>
> Hence we need to have mechanisms to limit ioend sizes and
> to break up completion processing of large merged ioend chains:
>
> 1. bio chains per ioend need to be bound in length. Pure overwrites
> go straight to iomap_finish_ioend() in softirq context with the
> exact bio chain attached to the ioend by submission. Hence the only
> way to prevent long holdoffs here is to bound ioend submission
> sizes because we can't reschedule in softirq context.
<nod>
> 2. iomap_finish_ioends() has to handle unbound merged ioend chains
> correctly. This relies on any one call to iomap_finish_ioend() being
> bound in runtime so that cond_resched() can be issued regularly as
> the long ioend chain is processed. i.e. this relies on mechanism #1
> to limit individual ioend sizes to work correctly.
<nod>
> 3. filesystems have to loop over the merged ioends to process
> physical extent manipulations. This means they can loop internally,
> and so we break merging at physical extent boundaries so the
> filesystem can easily insert reschedule points between individual
> extent manipulations.
<nod> I think I grok this all now. Just a couple minor questions
more...
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/iomap/buffered-io.c | 47 +++++++++++++++++++++++++++++++++++++++++++----
> fs/xfs/xfs_aops.c | 16 +++++++++++++++-
> include/linux/iomap.h | 1 +
> 3 files changed, 59 insertions(+), 5 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 71a36ae120ee..39214577bc46 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1066,17 +1066,34 @@ iomap_finish_ioend(struct iomap_ioend *ioend, int error)
> }
> }
>
> +/*
> + * Ioend completion routine for merged bios. This can only be called from task
> + * contexts as merged ioends can be of unbound length. Hence we have to break up
> + * the page writeback completion into manageable chunks to avoid long scheduler
> + * holdoffs. We aim to keep scheduler holdoffs down below 10ms so that we get
> + * good batch processing throughput without creating adverse scheduler latency
> + * conditions.
> + */
> void
> iomap_finish_ioends(struct iomap_ioend *ioend, int error)
> {
> struct list_head tmp;
> + int segments;
Nit: io_segments is u32, this should be unsigned int.
> +
> + might_sleep();
>
> list_replace_init(&ioend->io_list, &tmp);
> + segments = ioend->io_segments;
> iomap_finish_ioend(ioend, error);
>
> while (!list_empty(&tmp)) {
> + if (segments > 32768) {
> + cond_resched();
> + segments = 0;
> + }
> ioend = list_first_entry(&tmp, struct iomap_ioend, io_list);
> list_del_init(&ioend->io_list);
> + segments += ioend->io_segments;
> iomap_finish_ioend(ioend, error);
> }
I wonder, should we take one more swing at cond_resched at the end of
the function so that we can return to the caller having given the system
at least one chance to reschedule?
(Don't really care all that strongly; aside from the nits I mentioned, I
think I'm comfy with stuffing this one in after willy's iomap
fiolio^Wfoolio^WFOLIOS conversion goes upstream next week.)
--D
> }
> @@ -1098,6 +1115,15 @@ iomap_ioend_can_merge(struct iomap_ioend *ioend, struct iomap_ioend *next)
> return false;
> if (ioend->io_offset + ioend->io_size != next->io_offset)
> return false;
> + /*
> + * Do not merge physically discontiguous ioends. The filesystem
> + * completion functions will have to iterate the physical
> + * discontiguities even if we merge the ioends at a logical level, so
> + * we don't gain anything by merging physical discontiguities here.
> + */
> + if (ioend->io_inline_bio.bi_iter.bi_sector + (ioend->io_size >> 9) !=
> + next->io_inline_bio.bi_iter.bi_sector)
> + return false;
> return true;
> }
>
> @@ -1175,6 +1201,7 @@ iomap_submit_ioend(struct iomap_writepage_ctx *wpc, struct iomap_ioend *ioend,
> return error;
> }
>
> + ioend->io_segments += bio_segments(ioend->io_bio);
> submit_bio(ioend->io_bio);
> return 0;
> }
> @@ -1199,6 +1226,7 @@ iomap_alloc_ioend(struct inode *inode, struct iomap_writepage_ctx *wpc,
> ioend->io_flags = wpc->iomap.flags;
> ioend->io_inode = inode;
> ioend->io_size = 0;
> + ioend->io_segments = 0;
> ioend->io_offset = offset;
> ioend->io_bio = bio;
> return ioend;
> @@ -1211,11 +1239,14 @@ iomap_alloc_ioend(struct inode *inode, struct iomap_writepage_ctx *wpc,
> * so that the bi_private linkage is set up in the right direction for the
> * traversal in iomap_finish_ioend().
> */
> -static struct bio *
> -iomap_chain_bio(struct bio *prev)
> +static void
> +iomap_chain_bio(struct iomap_ioend *ioend)
> {
> + struct bio *prev = ioend->io_bio;
> struct bio *new;
>
> + ioend->io_segments += bio_segments(prev);
> +
> new = bio_alloc(GFP_NOFS, BIO_MAX_VECS);
> bio_copy_dev(new, prev);/* also copies over blkcg information */
> new->bi_iter.bi_sector = bio_end_sector(prev);
> @@ -1225,7 +1256,8 @@ iomap_chain_bio(struct bio *prev)
> bio_chain(prev, new);
> bio_get(prev); /* for iomap_finish_ioend */
> submit_bio(prev);
> - return new;
> +
> + ioend->io_bio = new;
> }
>
> static bool
> @@ -1241,6 +1273,13 @@ iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t offset,
> return false;
> if (sector != bio_end_sector(wpc->ioend->io_bio))
> return false;
> + /*
> + * Limit ioend bio chain lengths to minimise IO completion latency. This
> + * also prevents long tight loops ending page writeback on all the pages
> + * in the ioend.
> + */
> + if (wpc->ioend->io_segments >= 4096)
> + return false;
> return true;
> }
>
> @@ -1264,7 +1303,7 @@ iomap_add_to_ioend(struct inode *inode, loff_t offset, struct page *page,
> }
>
> if (bio_add_page(wpc->ioend->io_bio, page, len, poff) != len) {
> - wpc->ioend->io_bio = iomap_chain_bio(wpc->ioend->io_bio);
> + iomap_chain_bio(wpc->ioend);
> __bio_add_page(wpc->ioend->io_bio, page, len, poff);
> }
>
> diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> index c8c15c3c3147..148a8fce7029 100644
> --- a/fs/xfs/xfs_aops.c
> +++ b/fs/xfs/xfs_aops.c
> @@ -136,7 +136,20 @@ xfs_end_ioend(
> memalloc_nofs_restore(nofs_flag);
> }
>
> -/* Finish all pending io completions. */
> +/*
> + * Finish all pending IO completions that require transactional modifications.
> + *
> + * We try to merge physical and logically contiguous ioends before completion to
> + * minimise the number of transactions we need to perform during IO completion.
> + * Both unwritten extent conversion and COW remapping need to iterate and modify
> + * one physical extent at a time, so we gain nothing by merging physically
> + * discontiguous extents here.
> + *
> + * The ioend chain length that we can be processing here is largely unbound in
> + * length and we may have to perform significant amounts of work on each ioend
> + * to complete it. Hence we have to be careful about holding the CPU for too
> + * long in this loop.
> + */
> void
> xfs_end_io(
> struct work_struct *work)
> @@ -157,6 +170,7 @@ xfs_end_io(
> list_del_init(&ioend->io_list);
> iomap_ioend_try_merge(ioend, &tmp);
> xfs_end_ioend(ioend);
> + cond_resched();
> }
> }
>
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 6d1b08d0ae93..bfdba72f4e30 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -257,6 +257,7 @@ struct iomap_ioend {
> struct list_head io_list; /* next ioend in chain */
> u16 io_type;
> u16 io_flags; /* IOMAP_F_* */
> + u32 io_segments;
> struct inode *io_inode; /* file being written to */
> size_t io_size; /* size of the extent */
> loff_t io_offset; /* offset in the file */
