Re: [PATCH v5 27/32] netfs: Change the read result collector to only use one work item

Ihor Solodrai
2025-01-24T17:59:54Z

On Monday, December 16th, 2024 at 12:41 PM, David Howells <dhowells@redhat.com> wrote:

> Change the way netfslib collects read results to do all the collection for
> a particular read request using a single work item that walks along the
> subrequest queue as subrequests make progress or complete, unlocking folios
> progressively rather than doing the unlock in parallel as parallel requests
> come in.
>
> The code is remodelled to be more like the write-side code, though only
> using a single stream. This makes it more directly comparable and thus
> easier to duplicate fixes between the two sides.
>
> This has a number of advantages:
>
> (1) It's simpler. There doesn't need to be a complex donation mechanism
> to handle mismatches between the size and alignment of subrequests and
> folios. The collector unlocks folios as the subrequests covering each
> complete.
>
> (2) It should cause less scheduler overhead as there's a single work item
> in play unlocking pages in parallel when a read gets split up into a
> lot of subrequests instead of one per subrequest.
>
> Whilst the parallellism is nice in theory, in practice, the vast
> majority of loads are sequential reads of the whole file, so
> committing a bunch of threads to unlocking folios out of order doesn't
> help in those cases.
>
> (3) It should make it easier to implement content decryption. A folio
> cannot be decrypted until all the requests that contribute to it have
> completed - and, again, most loads are sequential and so, most of the
> time, we want to begin decryption sequentially (though it's great if
> the decryption can happen in parallel).
>
> There is a disadvantage in that we're losing the ability to decrypt and
> unlock things on an as-things-arrive basis which may affect some
> applications.
>
> Signed-off-by: David Howells dhowells@redhat.com
>
> cc: Jeff Layton jlayton@kernel.org
>
> cc: netfs@lists.linux.dev
> cc: linux-fsdevel@vger.kernel.org
> ---
> fs/9p/vfs_addr.c | 3 +-
> fs/afs/dir.c | 8 +-
> fs/ceph/addr.c | 9 +-
> fs/netfs/buffered_read.c | 160 ++++----
> fs/netfs/direct_read.c | 60 +--
> fs/netfs/internal.h | 21 +-
> fs/netfs/main.c | 2 +-
> fs/netfs/objects.c | 34 +-
> fs/netfs/read_collect.c | 716 ++++++++++++++++++++---------------
> fs/netfs/read_pgpriv2.c | 203 ++++------
> fs/netfs/read_retry.c | 207 +++++-----
> fs/netfs/read_single.c | 37 +-
> fs/netfs/write_collect.c | 4 +-
> fs/netfs/write_issue.c | 2 +-
> fs/netfs/write_retry.c | 14 +-
> fs/smb/client/cifssmb.c | 2 +
> fs/smb/client/smb2pdu.c | 5 +-
> include/linux/netfs.h | 16 +-
> include/trace/events/netfs.h | 79 +---
> 19 files changed, 819 insertions(+), 763 deletions(-)

Hello David.

After recent merge from upstream BPF CI started consistently failing
with a task hanging in v9fs_evict_inode. I bisected the failure to
commit e2d46f2ec332, pointing to this patch.

Reverting the patch seems to have helped:
https://github.com/kernel-patches/vmtest/actions/runs/12952856569

Could you please investigate?

Examples of failed jobs:
* https://github.com/kernel-patches/bpf/actions/runs/12941732247
* https://github.com/kernel-patches/bpf/actions/runs/12933849075

A log snippet:

2025-01-24T02:15:03.9009694Z [ 246.932163] INFO: task ip:1055 blocked for more than 122 seconds.
2025-01-24T02:15:03.9013633Z [ 246.932709] Tainted: G OE 6.13.0-g2bcb9cf535b8-dirty #149
2025-01-24T02:15:03.9018791Z [ 246.933249] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
2025-01-24T02:15:03.9025896Z [ 246.933802] task:ip state:D stack:0 pid:1055 tgid:1055 ppid:1054 flags:0x00004002
2025-01-24T02:15:03.9028228Z [ 246.934564] Call Trace:
2025-01-24T02:15:03.9029758Z [ 246.934764] <TASK>
2025-01-24T02:15:03.9032572Z [ 246.934937] __schedule+0xa91/0xe80
2025-01-24T02:15:03.9035126Z [ 246.935224] schedule+0x41/0xb0
2025-01-24T02:15:03.9037992Z [ 246.935459] v9fs_evict_inode+0xfe/0x170
2025-01-24T02:15:03.9041469Z [ 246.935748] ? __pfx_var_wake_function+0x10/0x10
2025-01-24T02:15:03.9043837Z [ 246.936101] evict+0x1ef/0x360
2025-01-24T02:15:03.9046624Z [ 246.936340] __dentry_kill+0xb0/0x220
2025-01-24T02:15:03.9048855Z [ 246.936610] ? dput+0x3a/0x1d0
2025-01-24T02:15:03.9051128Z [ 246.936838] dput+0x114/0x1d0
2025-01-24T02:15:03.9053548Z [ 246.937069] __fput+0x136/0x2b0
2025-01-24T02:15:03.9056154Z [ 246.937305] task_work_run+0x89/0xc0
2025-01-24T02:15:03.9058593Z [ 246.937571] do_exit+0x2c6/0x9c0
2025-01-24T02:15:03.9061349Z [ 246.937816] do_group_exit+0xa4/0xb0
2025-01-24T02:15:03.9064401Z [ 246.938090] __x64_sys_exit_group+0x17/0x20
2025-01-24T02:15:03.9067235Z [ 246.938390] x64_sys_call+0x21a0/0x21a0
2025-01-24T02:15:03.9069924Z [ 246.938672] do_syscall_64+0x79/0x120
2025-01-24T02:15:03.9072746Z [ 246.938941] ? clear_bhb_loop+0x25/0x80
2025-01-24T02:15:03.9075581Z [ 246.939230] ? clear_bhb_loop+0x25/0x80
2025-01-24T02:15:03.9079275Z [ 246.939510] entry_SYSCALL_64_after_hwframe+0x76/0x7e
2025-01-24T02:15:03.9081976Z [ 246.939875] RIP: 0033:0x7fb86f66f21d
2025-01-24T02:15:03.9087533Z [ 246.940153] RSP: 002b:00007ffdb3cf93f8 EFLAGS: 00000202 ORIG_RAX: 00000000000000e7
2025-01-24T02:15:03.9092590Z [ 246.940689] RAX: ffffffffffffffda RBX: 00007fb86f785fa8 RCX: 00007fb86f66f21d
2025-01-24T02:15:03.9097722Z [ 246.941201] RDX: 00000000000000e7 RSI: ffffffffffffff80 RDI: 0000000000000000
2025-01-24T02:15:03.9102762Z [ 246.941705] RBP: 00007ffdb3cf9450 R08: 00007ffdb3cf93a0 R09: 0000000000000000
2025-01-24T02:15:03.9107940Z [ 246.942215] R10: 00007ffdb3cf92ff R11: 0000000000000202 R12: 0000000000000001
2025-01-24T02:15:03.9113002Z [ 246.942723] R13: 0000000000000000 R14: 0000000000000000 R15: 00007fb86f785fc0
2025-01-24T02:15:03.9114614Z [ 246.943244] </TASK>
2025-01-24T02:15:03.9115895Z [ 246.943415]
2025-01-24T02:15:03.9119326Z [ 246.943415] Showing all locks held in the system:
2025-01-24T02:15:03.9122278Z [ 246.943865] 1 lock held by khungtaskd/32:
2025-01-24T02:15:03.9128640Z [ 246.944162] #0: ffffffffa9195d90 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180
2025-01-24T02:15:03.9131426Z [ 246.944792] 2 locks held by kworker/0:2/86:
2025-01-24T02:15:03.9132752Z [ 246.945102]
2025-01-24T02:15:03.9136561Z [ 246.945222] =============================================

It's worth noting that that the hanging does not happen on *every*
test run, but often enough to fail the CI pipeline.

You may try reproducing with a container I used for bisection:

docker pull ghcr.io/theihor/bpf:v9fs_evict_inode-repro
docker run -d --privileged --device=/dev/kvm --cap-add ALL -v /path/to/your/kernel/source:/ci/workspace ghcr.io/theihor/bpf:v9fs_evict_inode-repro
docker exec -it <container_id_or_name> /bin/bash
/ci/run.sh # in the container shell

Note that inside the container it's an "ubuntu" user, and you might
have to run `chown -R ubuntu:ubuntu /ci/workspace` first, or switch to
root.

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