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Subject: Re: [REGRESSION] 6.7.1: md: raid5 hang and unresponsive system;
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To: junxiao.bi@oracle.com, Yu Kuai <yukuai1@huaweicloud.com>,
 Song Liu <song@kernel.org>,
 Linux regressions mailing list <regressions@lists.linux.dev>
Cc: gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
 linux-raid@vger.kernel.org, stable@vger.kernel.org,
 Dan Moulding <dan@danm.net>, "yukuai (C)" <yukuai3@huawei.com>
References: <20240123005700.9302-1-dan@danm.net>
 <20240220230658.11069-1-dan@danm.net>
 <7efac6e0-32df-457e-9d21-4945c69328f8@leemhuis.info>
 <CAPhsuW5QYTjBvjAjf8SdcKmPGO20e5-p57n6af5FaXudSiOCmg@mail.gmail.com>
 <739634c3-3e21-44dd-abb1-356cf54e54fd@oracle.com>
 <d3cdebfe-17c0-4f61-9ad9-71d9de2339b2@oracle.com>
 <ecfce4d7-bcf7-c09a-7f01-5c7de88df107@huaweicloud.com>
 <ba26ac4f-160a-451e-a08b-27f577d8d2ba@oracle.com>
 <fa4beda8-986c-1112-f3f2-159f20674d47@huaweicloud.com>
 <e58155bf-71c1-48c4-94c7-b0d9ecbbef99@oracle.com>
From: Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <a447f0f3-a584-a91b-e879-f2a4de774e34@huaweicloud.com>
Date: Fri, 15 Mar 2024 09:30:11 +0800
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Hi,

在 2024/03/15 2:20, junxiao.bi@oracle.com 写道:
> On 3/12/24 6:20 PM, Yu Kuai wrote:
> 
>> Hi,
>>
>> 在 2024/03/13 6:56, junxiao.bi@oracle.com 写道:
>>> On 3/10/24 6:50 PM, Yu Kuai wrote:
>>>
>>>> Hi,
>>>>
>>>> 在 2024/03/09 7:49, junxiao.bi@oracle.com 写道:
>>>>> Here is the root cause for this issue:
>>>>>
>>>>> Commit 5e2cf333b7bd ("md/raid5: Wait for MD_SB_CHANGE_PENDING in 
>>>>> raid5d") introduced a regression, it got reverted through commit 
>>>>> bed9e27baf52 ("Revert "md/raid5: Wait for MD_SB_CHANGE_PENDING in 
>>>>> raid5d"). To fix the original issue commit 5e2cf333b7bd was fixing, 
>>>>> commit d6e035aad6c0 ("md: bypass block throttle for superblock 
>>>>> update") was created, it avoids md superblock write getting 
>>>>> throttled by block layer which is good, but md superblock write 
>>>>> could be stuck in block layer due to block flush as well, and that 
>>>>> is what was happening in this regression report.
>>>>>
>>>>> Process "md0_reclaim" got stuck while waiting IO for md superblock 
>>>>> write done, that IO was marked with REQ_PREFLUSH | REQ_FUA flags, 
>>>>> these 3 steps ( PREFLUSH, DATA and POSTFLUSH ) will be executed 
>>>>> before done, the hung of this process is because the last step 
>>>>> "POSTFLUSH" never done. And that was because of  process 
>>>>> "md0_raid5" submitted another IO with REQ_FUA flag marked just 
>>>>> before that step started. To handle that IO, blk_insert_flush() 
>>>>> will be invoked and hit "REQ_FSEQ_DATA | REQ_FSEQ_POSTFLUSH" case 
>>>>> where "fq->flush_data_in_flight" will be increased. When the IO for 
>>>>> md superblock write was to issue "POSTFLUSH" step through 
>>>>> blk_kick_flush(), it found that "fq->flush_data_in_flight" was not 
>>>>> zero, so it will skip that step, that is expected, because flush 
>>>>> will be triggered when "fq->flush_data_in_flight" dropped to zero.
>>>>>
>>>>> Unfortunately here that inflight data IO from "md0_raid5" will 
>>>>> never done, because it was added into the blk_plug list of that 
>>>>> process, but "md0_raid5" run into infinite loop due to 
>>>>> "MD_SB_CHANGE_PENDING" which made it never had a chance to finish 
>>>>> the blk plug until "MD_SB_CHANGE_PENDING" was cleared. Process 
>>>>> "md0_reclaim" was supposed to clear that flag but it was stuck by 
>>>>> "md0_raid5", so this is a deadlock.
>>>>>
>>>>> Looks like the approach in the RFC patch trying to resolve the 
>>>>> regression of commit 5e2cf333b7bd can help this issue. Once 
>>>>> "md0_raid5" starts looping due to "MD_SB_CHANGE_PENDING", it should 
>>>>> release all its staging IO requests to avoid blocking others. Also 
>>>>> a cond_reschedule() will avoid it run into lockup.
>>>>
>>>> The analysis sounds good, however, it seems to me that the behaviour
>>>> raid5d() pings the cpu to wait for 'MD_SB_CHANGE_PENDING' to be cleared
>>>> is not reasonable, because md_check_recovery() must hold
>>>> 'reconfig_mutex' to clear the flag.
>>>
>>> That's the behavior before commit 5e2cf333b7bd which was added into 
>>> Sep 2022, so this behavior has been with raid5 for many years.
>>>
>>
>> Yes, it exists for a long time doesn't mean it's good. It is really
>> weird to hold spinlock to wait for a mutex.
> I am confused about this, where is the code that waiting mutex while 
> holding spinlock, wouldn't that cause a deadlock?

For example, assume that other contex already holding the
'reconfig_mutex', and this can be slow, then in raid5d:

md_check_recovery
  try lock 'reconfig_mutex' failed

while (1)
  hold spin_lock
  try to issue IO, failed
  release spin_lock
  blk_flush_plug
  hold spin_lock

So, untill other contex release the 'reconfig_mutex', and then
md_check_recovery() update super_block, raid5d() will not make progress, 
meanwhile it will waste one cpu.

Thanks,
Kuai

>>>
>>>>
>>>> Look at raid1/raid10, there are two different behaviour that seems can
>>>> avoid this problem as well:
>>>>
>>>> 1) blk_start_plug() is delayed until all failed IO is handled. This 
>>>> look
>>>> reasonable because in order to get better performance, IO should be
>>>> handled by submitted thread as much as possible, and meanwhile, the
>>>> deadlock can be triggered here.
>>>> 2) if 'MD_SB_CHANGE_PENDING' is not cleared by md_check_recovery(), 
>>>> skip
>>>> the handling of failed IO, and when mddev_unlock() is called, daemon
>>>> thread will be woken up again to handle failed IO.
>>>>
>>>> How about the following patch?
>>>>
>>>> Thanks,
>>>> Kuai
>>>>
>>>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>>>> index 3ad5f3c7f91e..0b2e6060f2c9 100644
>>>> --- a/drivers/md/raid5.c
>>>> +++ b/drivers/md/raid5.c
>>>> @@ -6720,7 +6720,6 @@ static void raid5d(struct md_thread *thread)
>>>>
>>>>         md_check_recovery(mddev);
>>>>
>>>> -       blk_start_plug(&plug);
>>>>         handled = 0;
>>>>         spin_lock_irq(&conf->device_lock);
>>>>         while (1) {
>>>> @@ -6728,6 +6727,14 @@ static void raid5d(struct md_thread *thread)
>>>>                 int batch_size, released;
>>>>                 unsigned int offset;
>>>>
>>>> +               /*
>>>> +                * md_check_recovery() can't clear sb_flags, usually 
>>>> because of
>>>> +                * 'reconfig_mutex' can't be grabbed, wait for 
>>>> mddev_unlock() to
>>>> +                * wake up raid5d().
>>>> +                */
>>>> +               if (test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
>>>> +                       goto skip;
>>>> +
>>>>                 released = release_stripe_list(conf, 
>>>> conf->temp_inactive_list);
>>>>                 if (released)
>>>>                         clear_bit(R5_DID_ALLOC, &conf->cache_state);
>>>> @@ -6766,8 +6773,8 @@ static void raid5d(struct md_thread *thread)
>>>> spin_lock_irq(&conf->device_lock);
>>>>                 }
>>>>         }
>>>> +skip:
>>>>         pr_debug("%d stripes handled\n", handled);
>>>> -
>>>>         spin_unlock_irq(&conf->device_lock);
>>>>         if (test_and_clear_bit(R5_ALLOC_MORE, &conf->cache_state) &&
>>>>             mutex_trylock(&conf->cache_size_mutex)) {
>>>> @@ -6779,6 +6786,7 @@ static void raid5d(struct md_thread *thread)
>>>>                 mutex_unlock(&conf->cache_size_mutex);
>>>>         }
>>>>
>>>> +       blk_start_plug(&plug);
>>>>         flush_deferred_bios(conf);
>>>>
>>>>         r5l_flush_stripe_to_raid(conf->log);
>>>
>>> This patch eliminated the benefit of blk_plug, i think it will not be 
>>> good for IO performance perspective?
>>
>> There is only one daemon thread, so IO should not be handled here as
>> much as possible. The IO should be handled by the thread that is
>> submitting the IO, and let daemon to hanldle the case that IO failed or
>> can't be submitted at that time.
> 
> I am not sure how much it will impact regarding drop the blk_plug.
> 
> Song, what's your take on this?
> 
> Thanks,
> 
> Junxiao.
> 
>>
>> Thanks,
>> Kuai
>>
>>>
>>>
>>> Thanks,
>>>
>>> Junxiao.
>>>
>>>>
>>>>>
>>>>> https://www.spinics.net/lists/raid/msg75338.html
>>>>>
>>>>> Dan, can you try the following patch?
>>>>>
>>>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>>>> index de771093b526..474462abfbdc 100644
>>>>> --- a/block/blk-core.c
>>>>> +++ b/block/blk-core.c
>>>>> @@ -1183,6 +1183,7 @@ void __blk_flush_plug(struct blk_plug *plug, 
>>>>> bool from_schedule)
>>>>>          if (unlikely(!rq_list_empty(plug->cached_rq)))
>>>>>                  blk_mq_free_plug_rqs(plug);
>>>>>   }
>>>>> +EXPORT_SYMBOL(__blk_flush_plug);
>>>>>
>>>>>   /**
>>>>>    * blk_finish_plug - mark the end of a batch of submitted I/O
>>>>> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
>>>>> index 8497880135ee..26e09cdf46a3 100644
>>>>> --- a/drivers/md/raid5.c
>>>>> +++ b/drivers/md/raid5.c
>>>>> @@ -6773,6 +6773,11 @@ static void raid5d(struct md_thread *thread)
>>>>> spin_unlock_irq(&conf->device_lock);
>>>>>                          md_check_recovery(mddev);
>>>>> spin_lock_irq(&conf->device_lock);
>>>>> +               } else {
>>>>> + spin_unlock_irq(&conf->device_lock);
>>>>> +                       blk_flush_plug(&plug, false);
>>>>> +                       cond_resched();
>>>>> + spin_lock_irq(&conf->device_lock);
>>>>>                  }
>>>>>          }
>>>>>          pr_debug("%d stripes handled\n", handled);
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Junxiao.
>>>>>
>>>>> On 3/1/24 12:26 PM, junxiao.bi@oracle.com wrote:
>>>>>> Hi Dan & Song,
>>>>>>
>>>>>> I have not root cause this yet, but would like share some findings 
>>>>>> from the vmcore Dan shared. From what i can see, this doesn't look 
>>>>>> like a md issue, but something wrong with block layer or below.
>>>>>>
>>>>>> 1. There were multiple process hung by IO over 15mins.
>>>>>>
>>>>>> crash> ps -m | grep UN
>>>>>> [0 00:15:50.424] [UN]  PID: 957      TASK: ffff88810baa0ec0 CPU: 1 
>>>>>> COMMAND: "jbd2/dm-3-8"
>>>>>> [0 00:15:56.151] [UN]  PID: 1835     TASK: ffff888108a28ec0 CPU: 2 
>>>>>> COMMAND: "dd"
>>>>>> [0 00:15:56.187] [UN]  PID: 876      TASK: ffff888108bebb00 CPU: 3 
>>>>>> COMMAND: "md0_reclaim"
>>>>>> [0 00:15:56.185] [UN]  PID: 1914     TASK: ffff8881015e6740 CPU: 1 
>>>>>> COMMAND: "kworker/1:2"
>>>>>> [0 00:15:56.255] [UN]  PID: 403      TASK: ffff888101351d80 CPU: 7 
>>>>>> COMMAND: "kworker/u21:1"
>>>>>>
>>>>>> 2. Let pick md0_reclaim to take a look, it is waiting done 
>>>>>> super_block update. We can see there were two pending superblock 
>>>>>> write and other pending io for the underling physical disk, which 
>>>>>> caused these process hung.
>>>>>>
>>>>>> crash> bt 876
>>>>>> PID: 876      TASK: ffff888108bebb00  CPU: 3    COMMAND: 
>>>>>> "md0_reclaim"
>>>>>>  #0 [ffffc900008c3d10] __schedule at ffffffff81ac18ac
>>>>>>  #1 [ffffc900008c3d70] schedule at ffffffff81ac1d82
>>>>>>  #2 [ffffc900008c3d88] md_super_wait at ffffffff817df27a
>>>>>>  #3 [ffffc900008c3dd0] md_update_sb at ffffffff817df609
>>>>>>  #4 [ffffc900008c3e20] r5l_do_reclaim at ffffffff817d1cf4
>>>>>>  #5 [ffffc900008c3e98] md_thread at ffffffff817db1ef
>>>>>>  #6 [ffffc900008c3ef8] kthread at ffffffff8114f8ee
>>>>>>  #7 [ffffc900008c3f30] ret_from_fork at ffffffff8108bb98
>>>>>>  #8 [ffffc900008c3f50] ret_from_fork_asm at ffffffff81000da1
>>>>>>
>>>>>> crash> mddev.pending_writes,disks 0xffff888108335800
>>>>>>   pending_writes = {
>>>>>>     counter = 2  <<<<<<<<<< 2 active super block write
>>>>>>   },
>>>>>>   disks = {
>>>>>>     next = 0xffff88810ce85a00,
>>>>>>     prev = 0xffff88810ce84c00
>>>>>>   },
>>>>>> crash> list -l md_rdev.same_set -s md_rdev.kobj.name,nr_pending 
>>>>>> 0xffff88810ce85a00
>>>>>> ffff88810ce85a00
>>>>>>   kobj.name = 0xffff8881067c1a00 "dev-dm-1",
>>>>>>   nr_pending = {
>>>>>>     counter = 0
>>>>>>   },
>>>>>> ffff8881083ace00
>>>>>>   kobj.name = 0xffff888100a93280 "dev-sde",
>>>>>>   nr_pending = {
>>>>>>     counter = 10 <<<<
>>>>>>   },
>>>>>> ffff8881010ad200
>>>>>>   kobj.name = 0xffff8881012721c8 "dev-sdc",
>>>>>>   nr_pending = {
>>>>>>     counter = 8 <<<<<
>>>>>>   },
>>>>>> ffff88810ce84c00
>>>>>>   kobj.name = 0xffff888100325f08 "dev-sdd",
>>>>>>   nr_pending = {
>>>>>>     counter = 2 <<<<<
>>>>>>   },
>>>>>>
>>>>>> 3. From block layer, i can find the inflight IO for md superblock 
>>>>>> write which has been pending 955s which matches with the hung time 
>>>>>> of "md0_reclaim"
>>>>>>
>>>>>> crash> 
>>>>>> request.q,mq_hctx,cmd_flags,rq_flags,start_time_ns,bio,biotail,state,__data_len,flush,end_io 
>>>>>> ffff888103b4c300
>>>>>>   q = 0xffff888103a00d80,
>>>>>>   mq_hctx = 0xffff888103c5d200,
>>>>>>   cmd_flags = 38913,
>>>>>>   rq_flags = 139408,
>>>>>>   start_time_ns = 1504179024146,
>>>>>>   bio = 0x0,
>>>>>>   biotail = 0xffff888120758e40,
>>>>>>   state = MQ_RQ_COMPLETE,
>>>>>>   __data_len = 0,
>>>>>>   flush = {
>>>>>>     seq = 3, <<<< REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA
>>>>>>     saved_end_io = 0x0
>>>>>>   },
>>>>>>   end_io = 0xffffffff815186e0 <mq_flush_data_end_io>,
>>>>>>
>>>>>> crash> p tk_core.timekeeper.tkr_mono.base
>>>>>> $1 = 2459916243002
>>>>>> crash> eval 2459916243002-1504179024146
>>>>>> hexadecimal: de86609f28
>>>>>>     decimal: 955737218856  <<<<<<< IO pending time is 955s
>>>>>>       octal: 15720630117450
>>>>>>      binary: 
>>>>>> 0000000000000000000000001101111010000110011000001001111100101000
>>>>>>
>>>>>> crash> bio.bi_iter,bi_end_io 0xffff888120758e40
>>>>>>   bi_iter = {
>>>>>>     bi_sector = 8, <<<< super block offset
>>>>>>     bi_size = 0,
>>>>>>     bi_idx = 0,
>>>>>>     bi_bvec_done = 0
>>>>>>   },
>>>>>>   bi_end_io = 0xffffffff817dca50 <super_written>,
>>>>>> crash> dev -d | grep ffff888103a00d80
>>>>>>     8 ffff8881003ab000   sdd        ffff888103a00d80 0 0 0
>>>>>>
>>>>>> 4. Check above request, even its state is "MQ_RQ_COMPLETE", but it 
>>>>>> is still pending. That's because each md superblock write was 
>>>>>> marked with REQ_PREFLUSH | REQ_FUA, so it will be handled in 3 
>>>>>> steps: pre_flush, data, and post_flush. Once each step complete, 
>>>>>> it will be marked in "request.flush.seq", here the value is 3, 
>>>>>> which is REQ_FSEQ_PREFLUSH |  REQ_FSEQ_DATA, so the last step 
>>>>>> "post_flush" has not be done. Another wired thing is that 
>>>>>> blk_flush_queue.flush_data_in_flight is still 1 even "data" step 
>>>>>> already done.
>>>>>>
>>>>>> crash> blk_mq_hw_ctx.fq 0xffff888103c5d200
>>>>>>   fq = 0xffff88810332e240,
>>>>>> crash> blk_flush_queue 0xffff88810332e240
>>>>>> struct blk_flush_queue {
>>>>>>   mq_flush_lock = {
>>>>>>     {
>>>>>>       rlock = {
>>>>>>         raw_lock = {
>>>>>>           {
>>>>>>             val = {
>>>>>>               counter = 0
>>>>>>             },
>>>>>>             {
>>>>>>               locked = 0 '\000',
>>>>>>               pending = 0 '\000'
>>>>>>             },
>>>>>>             {
>>>>>>               locked_pending = 0,
>>>>>>               tail = 0
>>>>>>             }
>>>>>>           }
>>>>>>         }
>>>>>>       }
>>>>>>     }
>>>>>>   },
>>>>>>   flush_pending_idx = 1,
>>>>>>   flush_running_idx = 1,
>>>>>>   rq_status = 0 '\000',
>>>>>>   flush_pending_since = 4296171408,
>>>>>>   flush_queue = {{
>>>>>>       next = 0xffff88810332e250,
>>>>>>       prev = 0xffff88810332e250
>>>>>>     }, {
>>>>>>       next = 0xffff888103b4c348, <<<< the request is in this list
>>>>>>       prev = 0xffff888103b4c348
>>>>>>     }},
>>>>>>   flush_data_in_flight = 1,  >>>>>> still 1
>>>>>>   flush_rq = 0xffff888103c2e000
>>>>>> }
>>>>>>
>>>>>> crash> list 0xffff888103b4c348
>>>>>> ffff888103b4c348
>>>>>> ffff88810332e260
>>>>>>
>>>>>> crash> request.tag,state,ref 0xffff888103c2e000 >>>> flush_rq of 
>>>>>> hw queue
>>>>>>   tag = -1,
>>>>>>   state = MQ_RQ_IDLE,
>>>>>>   ref = {
>>>>>>     counter = 0
>>>>>>   },
>>>>>>
>>>>>> 5. Looks like the block layer or underlying(scsi/virtio-scsi) may 
>>>>>> have some issue which leading to the io request from md layer 
>>>>>> stayed in a partial complete statue. I can't see how this can be 
>>>>>> related with the commit bed9e27baf52 ("Revert "md/raid5: Wait for 
>>>>>> MD_SB_CHANGE_PENDING in raid5d"")
>>>>>>
>>>>>>
>>>>>> Dan,
>>>>>>
>>>>>> Are you able to reproduce using some regular scsi disk, would like 
>>>>>> to rule out whether this is related with virtio-scsi?
>>>>>>
>>>>>> And I see the kernel version is 6.8.0-rc5 from vmcore, is this the 
>>>>>> official mainline v6.8-rc5 without any other patches?
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> Junxiao.
>>>>>>
>>>>>> On 2/23/24 6:13 PM, Song Liu wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> On Fri, Feb 23, 2024 at 12:07 AM Linux regression tracking (Thorsten
>>>>>>> Leemhuis) <regressions@leemhuis.info> wrote:
>>>>>>>> On 21.02.24 00:06, Dan Moulding wrote:
>>>>>>>>> Just a friendly reminder that this regression still exists on the
>>>>>>>>> mainline. It has been reverted in 6.7 stable. But I upgraded a
>>>>>>>>> development system to 6.8-rc5 today and immediately hit this issue
>>>>>>>>> again. Then I saw that it hasn't yet been reverted in Linus' tree.
>>>>>>>> Song Liu, what's the status here? I aware that you fixed with 
>>>>>>>> quite a
>>>>>>>> few regressions recently, but it seems like resolving this one is
>>>>>>>> stalled. Or were you able to reproduce the issue or make some 
>>>>>>>> progress
>>>>>>>> and I just missed it?
>>>>>>> Sorry for the delay with this issue. I have been occupied with some
>>>>>>> other stuff this week.
>>>>>>>
>>>>>>> I haven't got luck to reproduce this issue. I will spend more 
>>>>>>> time looking
>>>>>>> into it next week.
>>>>>>>
>>>>>>>> And if not, what's the way forward here wrt to the release of 6.8?
>>>>>>>> Revert the culprit and try again later? Or is that not an option 
>>>>>>>> for one
>>>>>>>> reason or another?
>>>>>>> If we don't make progress with it in the next week, we will do 
>>>>>>> the revert,
>>>>>>> same as we did with stable kernels.
>>>>>>>
>>>>>>>> Or do we assume that this is not a real issue? That it's caused 
>>>>>>>> by some
>>>>>>>> oddity (bit-flip in the metadata or something like that?) only 
>>>>>>>> to be
>>>>>>>> found in Dan's setup?
>>>>>>> I don't think this is because of oddities. Hopefully we can get more
>>>>>>> information about this soon.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Song
>>>>>>>
>>>>>>>> Ciao, Thorsten (wearing his 'the Linux kernel's regression 
>>>>>>>> tracker' hat)
>>>>>>>> -- 
>>>>>>>> Everything you wanna know about Linux kernel regression tracking:
>>>>>>>> https://linux-regtracking.leemhuis.info/about/#tldr
>>>>>>>> If I did something stupid, please tell me, as explained on that 
>>>>>>>> page.
>>>>>>>>
>>>>>>>> #regzbot poke
>>>>>>>>
>>>>>
>>>>> .
>>>>>
>>>>
>>> .
>>>
>>
> .
>