[LU-2948] Client read hang in osc_page_init()

• Type: Bug
• Resolution: Fixed
• Priority: Major
• Fix Version/s: Lustre 2.4.0
• Affects Version/s: Lustre 2.4.0
• Labels: LB sequoia topsequoia
• Environment: Lustre 2.3.58-14chaos (github.com/chaos/lustre)
• Severity: 3
• Assignee: Bruno Faccini (Inactive)
• Reporter: Christopher Morrone (Inactive)

Description

We are seeing threads that get indefinitely stuck (I've personally seen it stuck there for over an hour) sleeping under osc_page_init() while doing a read(). The backtrace looks like this:

2013-03-11 18:11:27.964356 {DefaultControlEventListener} [mmcs]{360}.0.0: sysiod        S 00000fff92016460     0 41292   3080 0x00000000
2013-03-11 18:11:27.964386 {DefaultControlEventListener} [mmcs]{360}.0.0: Call Trace:
2013-03-11 18:11:27.964416 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878e7c0] [c0000003c878e850] 0xc0000003c878e850 (unreliable)
2013-03-11 18:11:27.964446 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878e990] [c000000000009b2c] .__switch_to+0xc4/0x100
2013-03-11 18:11:27.964476 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878ea20] [c00000000044c660] .schedule+0x7d4/0x944
2013-03-11 18:11:27.964506 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878ecd0] [8000000000af2090] .cfs_waitq_wait+0x10/0x30 [libcfs]
2013-03-11 18:11:27.964536 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878ed40] [80000000046f8c58] .osc_page_init+0xb18/0x1130 [osc]
2013-03-11 18:11:27.964567 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878eea0] [800000000252c078] .cl_page_find0+0x378/0xb70 [obdclass]
2013-03-11 18:11:27.964597 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878efc0] [800000000519bf80] .lov_page_init_raid0+0x230/0xa20 [lov]
2013-03-11 18:11:27.964627 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f0e0] [8000000005196160] .lov_page_init+0x50/0xa0 [lov]
2013-03-11 18:11:27.964657 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f170] [800000000252c078] .cl_page_find0+0x378/0xb70 [obdclass]
2013-03-11 18:11:27.964686 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f290] [800000000699e098] .ll_readahead+0xdb8/0x1670 [lustre]
2013-03-11 18:11:27.964717 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f400] [80000000069dd38c] .vvp_io_read_page+0x3bc/0x4d0 [lustre]
2013-03-11 18:11:27.964747 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f510] [800000000253d8a4] .cl_io_read_page+0xf4/0x280 [obdclass]
2013-03-11 18:11:27.964777 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f5d0] [800000000699c5dc] .ll_readpage+0xdc/0x2c0 [lustre]
2013-03-11 18:11:27.964807 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f680] [c000000000099144] .generic_file_aio_read+0x500/0x728
2013-03-11 18:11:27.964837 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f7c0] [80000000069dfe24] .vvp_io_read_start+0x274/0x640 [lustre]
2013-03-11 18:11:27.964867 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f8e0] [80000000025398cc] .cl_io_start+0xcc/0x220 [obdclass]
2013-03-11 18:11:27.964897 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878f980] [8000000002541724] .cl_io_loop+0x194/0x2c0 [obdclass]
2013-03-11 18:11:27.964928 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878fa30] [800000000695a160] .ll_file_io_generic+0x410/0x670 [lustre]
2013-03-11 18:11:27.964958 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878fb30] [800000000695af04] .ll_file_aio_read+0x1d4/0x3a0 [lustre]
2013-03-11 18:11:27.964988 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878fc00] [800000000695b220] .ll_file_read+0x150/0x320 [lustre]
2013-03-11 18:11:27.965018 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878fce0] [c0000000000d429c] .vfs_read+0xd0/0x1c4
2013-03-11 18:11:27.965048 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878fd80] [c0000000000d448c] .SyS_read+0x54/0x98
2013-03-11 18:11:27.965078 {DefaultControlEventListener} [mmcs]{360}.0.0: [c0000003c878fe30] [c000000000000580] syscall_exit+0x0/0x2c

Prakash pointed out that used_mb is maxed out when we found the hung process:

seqio345-ib0:/proc/fs/lustre/llite/ls1-c0000003cc505000$ cat max_cached_mb
users: 384
max_cached_mb: 4096
used_mb: 4096
unused_mb: 0
reclaim_count: 11201

When I tried to cat a file in the same ls1 filesystem, it got stuck in the same call path as the sysiod shown above.

Issuing an "echo 3 > /proc/sys/vm/drop_caches" gets things moving again.

This on a Sequoia ION, ppc64 Lustre client. Lustre version 2.3.58-14chaos.

Comments

[Prakash Surya (Inactive)]

Jinshan, unfortunately I'm not able to easily reproduce LU-2576.

[Jinshan Xiong (Inactive)]

Sorry for delay response.

Hi Bruno, yes, that sounds pretty reasonable. It doesn't make any sense for the process to try it again if someone is already doing the work. Let's see if the patch works and I will do it if I have a chance to work out a new patch.

Hi Prakash, probably there is less LRU pages than lru_shrink_min in that cli. The pages can be in use for rewriting or readahead pages are being read, or write and then read. It's quite common for the caching pages to be reused otherwise we don't need to cache them at all . BTW can you easily reproduce LU-2576 before? We recently found that the patch for LU-2576 causing another problem we want to come up with another fix for that issue.

[Prakash Surya (Inactive)]

Hmm.. I'm not fully convinced this patch will help in that case. Even if osc_lru_reclaim returns > 0, there is still a chance another thread comes in and "steals" those slots before we can decrement cl_lru_left. That hasn't changed as far as I can tell.

This patch does appear to help the case where an OSC has cl_lru_in_list > 0 but returns zero from osc_lru_reclaim. With the patch, it will move on to the next OSC, where it would just return zero previously. It's worth a try, but without evidence pointing to exactly why it failed in the first place it's hard to say whether this will work or not.

One thing I noticed when I was reading the code, is we skip any pages that are "in use":

osc_lru_shrink:
697                 if (cl_page_in_use_noref(page)) {                               
698                         cfs_list_move_tail(&opg->ops_lru, &cli->cl_lru_list);   
699                         continue;                                               
700                 }

What are the chances that the first lru_shrink_min number of pages will be in use? What circumstances marks a page in use?

[Bruno Faccini (Inactive)]

Prakash, thank's for the full stacks log. But it does not show any other issue/info than the threads waiting in osc_page_init().

Jinshan, I agree with you and your patch, parsing all OSCs LRUs in osc_lru_relaim() will do a much better garbage collect job, that will ensure a maximum of pages to become available. But then I think we need to find a way to only do this full scan one time for a bunch of concurrent threads/requesters. It should be simply achieved in osc_lru_reclaim() by testing/spin_trylock() if cache->ccc_lru_lock is already acquired by somebody and return 0 if yes (to wait for LRU frees in osc_lru_reserve()), but/and not release cache->ccc_lru_lock inside the loop.

What do you think ?

[Jinshan Xiong (Inactive)]

In the original implementation, it only tried to shrink other client_obd once if there is no available LRU budget. This is race because the just-freed slots may be immediately used by others this will put the current thread into sleep. And nobody will like to wake it up since we only do it when a page is being released.

[Prakash Surya (Inactive)]

Out of curiosity, what makes you think that will fix the issue? What do you think the underlying issue is?

[Jinshan Xiong (Inactive)]

Please try this patch: http://review.whamcloud.com/5760

This patch has not been verified yet so take your own risk

[Prakash Surya (Inactive)]

I attached `console-seqio345.txt` which contains sysrq-t information from a system stuck in this state.

[Bruno Faccini (Inactive)]

Yes, I agree the "cat" should have caused one more osc_lru_reclaim() call, so it is unlikely the blocking condition will change ...

So we definitely need D_CACHE enabled at least during the "cat", and from beginning if possible. And the /proc counters aleady listed.

I was also thinking that we may learn from a full stack-trace (Alt+SysRq+T) in case some threads are hung in the process of freeing pages.

[Prakash Surya (Inactive)]

I did look at the osc_cached_mb files when this hit, but unfortunately I didn't save them anywhere. IIRC, there were two OSCs with pages in the LRU and none of them were labelled as "busy". Also, then sum of the pages in the two OSC LRU pages didn't add up to 4096 (the max reported by max_cached_mb). Which seems like there might have been a leak of some kind, but I still need to verify that the osc_cached_mb values should sum up to max_cached_mb. If I can reproduce the issue, I'll gather D_CACHE logs and post them.

I'm not convinced a time out would help. Running a "cat" hung in the same location, which ran osc_lru_reclaim prior to getting stuck. So I'd imagine using a timeout would just cause the treads to repeatedly fail to reclaim as well.

[Bruno Faccini (Inactive)]

Yes, I agree that osc_lru_reclaim() should have been able to free some ...

So yes, would be nice to enable D_CACHE when this situation re-occurs, and take a trace after some time, but also to get snapshots of /proc/fs/lustre/llite/<FS>/max_cached_mb and /proc/fs/lustre/osc/<OSC>/osc_cached_mb.

Also I am asking me why we don't wait with a time-out, just in case nothing happen for a single OSC and give osc_lru_reclaim() a new chance to steal LRU-pages from others ??... Humm, but I need to double-check that this is not useless.