On Fri, Jul 08, 2022 at 09:52:53AM +1000, Dave Chinner wrote: > Hi folks, > > Current work to merge the XFS inode life cycle with the VFS indoe > life cycle is finding some interesting issues. If we have a path > that hits buffer trylocks fairly hard (e.g. a non-blocking > background inode freeing function), we end up hitting massive > contention on the buffer cache hash locks: Hmm. I applied this to a test branch and this fell out of xfs/436 when it runs rmmod xfs. I'll see if I can reproduce it more regularly, but thought I'd put this out there early... XFS (sda3): Unmounting Filesystem ============================================================================= BUG xfs_buf (Not tainted): Objects remaining in xfs_buf on __kmem_cache_shutdown() ----------------------------------------------------------------------------- Slab 0xffffea000443b780 objects=18 used=4 fp=0xffff888110edf340 flags=0x17ff80000010200(slab|head|node=0|zone=2|lastcpupid=0xfff) CPU: 3 PID: 30378 Comm: modprobe Not tainted 5.19.0-rc5-djwx #rc5 bebda13a030d0898279476b6652ddea67c2060cc Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20171121_152543-x86-ol7-builder-01.us.oracle.com-4.el7.1 04/01/2014 Call Trace: <TASK> dump_stack_lvl+0x34/0x44 slab_err+0x95/0xc9 __kmem_cache_shutdown.cold+0x39/0x1e9 kmem_cache_destroy+0x49/0x130 exit_xfs_fs+0x50/0xc57 [xfs 370e1c994a59de083c05cd4df389f629878b8122] __do_sys_delete_module.constprop.0+0x145/0x220 ? exit_to_user_mode_prepare+0x6c/0x100 do_syscall_64+0x35/0x80 entry_SYSCALL_64_after_hwframe+0x46/0xb0 RIP: 0033:0x7fe7d7877c9b Code: 73 01 c3 48 8b 0d 95 21 0f 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 65 21 0f 00 f7 d8 64 89 01 48 RSP: 002b:00007fffb911cab8 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0 RAX: ffffffffffffffda RBX: 0000555a217adcc0 RCX: 00007fe7d7877c9b RDX: 0000000000000000 RSI: 0000000000000800 RDI: 0000555a217add28 RBP: 0000555a217adcc0 R08: 0000000000000000 R09: 0000000000000000 R10: 00007fe7d790fac0 R11: 0000000000000206 R12: 0000555a217add28 R13: 0000000000000000 R14: 0000555a217add28 R15: 00007fffb911ede8 </TASK> Disabling lock debugging due to kernel taint Object 0xffff888110ede000 @offset=0 Object 0xffff888110ede1c0 @offset=448 Object 0xffff888110edefc0 @offset=4032 Object 0xffff888110edf6c0 @offset=5824 --D > - 92.71% 0.05% [kernel] [k] xfs_inodegc_worker > - 92.67% xfs_inodegc_worker > - 92.13% xfs_inode_unlink > - 91.52% xfs_inactive_ifree > - 85.63% xfs_read_agi > - 85.61% xfs_trans_read_buf_map > - 85.59% xfs_buf_read_map > - xfs_buf_get_map > - 85.55% xfs_buf_find > - 72.87% _raw_spin_lock > - do_raw_spin_lock > 71.86% __pv_queued_spin_lock_slowpath > - 8.74% xfs_buf_rele > - 7.88% _raw_spin_lock > - 7.88% do_raw_spin_lock > 7.63% __pv_queued_spin_lock_slowpath > - 1.70% xfs_buf_trylock > - 1.68% down_trylock > - 1.41% _raw_spin_lock_irqsave > - 1.39% do_raw_spin_lock > __pv_queued_spin_lock_slowpath > - 0.76% _raw_spin_unlock > 0.75% do_raw_spin_unlock > > This is basically hammering the pag->pag_buf_lock from lots of CPUs > doing trylocks at the same time. Most of the buffer trylock > operations ultimately fail after we've done the lookup, so we're > really hammering the buf hash lock whilst making no progress. > > We can also see significant spinlock traffic on the same lock just > under normal operation when lots of tasks are accessing metadata > from the same AG, so let's avoid all this by creating a lookup fast > path which leverages the rhashtable's ability to do rcu protected > lookups. > > This is a rework of the initial lockless buffer lookup patch I sent > here: > > https://lore.kernel.org/linux-xfs/20220328213810.1174688-1-david@xxxxxxxxxxxxx/ > > And the alternative cleanup sent by Christoph here: > > https://lore.kernel.org/linux-xfs/20220403120119.235457-1-hch@xxxxxx/ > > This version isn't quite a short as Christophs, but it does roughly > the same thing in killing the two-phase _xfs_buf_find() call > mechanism. It separates the fast and slow paths a little more > cleanly and doesn't have context dependent buffer return state from > the slow path that the caller needs to handle. It also picks up the > rhashtable insert optimisation that Christoph added. > > This series passes fstests under several different configs and does > not cause any obvious regressions in scalability testing that has > been performed. Hence I'm proposing this as potential 5.20 cycle > material. > > Thoughts, comments? > > Version 3: > - rebased onto linux-xfs/for-next > - rearranged some of the changes to avoid repeated shuffling of code > to different locations > - fixed typos in commits > - s/xfs_buf_find_verify/xfs_buf_map_verify/ > - s/xfs_buf_find_fast/xfs_buf_lookup/ > > Version 2: > - https://lore.kernel.org/linux-xfs/20220627060841.244226-1-david@xxxxxxxxxxxxx/ > - based on 5.19-rc2 > - high speed collision of original proposals. > > Initial versions: > - https://lore.kernel.org/linux-xfs/20220403120119.235457-1-hch@xxxxxx/ > - https://lore.kernel.org/linux-xfs/20220328213810.1174688-1-david@xxxxxxxxxxxxx/ > >
