linux-kernel - Re: md_raid: mdX_raid6 looping after sync

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Date:   Tue, 26 Jan 2021 17:05:40 +0100
From:   Donald Buczek <buczek@...gen.mpg.de>
To:     Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
        Song Liu <song@...nel.org>, linux-raid@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        it+raid@...gen.mpg.de
Subject: Re: md_raid: mdX_raid6 looping after sync_action "check" to "idle"
 transition

Dear Guoqing,

On 26.01.21 15:06, Guoqing Jiang wrote:
> 
> 
> On 1/26/21 13:58, Donald Buczek wrote:
>>
>>
>>> Hmm, how about wake the waiter up in the while loop of raid5d?
>>>
>>> @@ -6520,6 +6532,11 @@ static void raid5d(struct md_thread *thread)
>>>                          md_check_recovery(mddev);
>>>                          spin_lock_irq(&conf->device_lock);
>>>                  }
>>> +
>>> +               if ((atomic_read(&conf->active_stripes)
>>> +                    < (conf->max_nr_stripes * 3 / 4) ||
>>> +                    (test_bit(MD_RECOVERY_INTR, &mddev->recovery))))
>>> +                       wake_up(&conf->wait_for_stripe);
>>>          }
>>>          pr_debug("%d stripes handled\n", handled);
>>
>> Hmm... With this patch on top of your other one, we still have the basic symptoms (md3_raid6 busy looping), but the sync thread is now hanging at
>>
>>      root@...th:~# cat /proc/$(pgrep md3_resync)/stack
>>      [<0>] md_do_sync.cold+0x8ec/0x97c
>>      [<0>] md_thread+0xab/0x160
>>      [<0>] kthread+0x11b/0x140
>>      [<0>] ret_from_fork+0x22/0x30
>>
>> instead, which is https://elixir.bootlin.com/linux/latest/source/drivers/md/md.c#L8963
> 
> Not sure why recovery_active is not zero, because it is set 0 before blk_start_plug, and raid5_sync_request returns 0 and skipped is also set to 1. Perhaps handle_stripe calls md_done_sync.
> 
> Could you double check the value of recovery_active? Or just don't wait if resync thread is interrupted.
> 
> wait_event(mddev->recovery_wait,
>         test_bit(MD_RECOVERY_INTR,&mddev->recovery) ||
>         !atomic_read(&mddev->recovery_active));

With that added, md3_resync goes into a loop, too. Not 100% busy, though.

root@...th:~# cat /proc/$(pgrep md3_resync)/stack

[<0>] raid5_get_active_stripe+0x1e7/0x6b0  # https://elixir.bootlin.com/linux/v5.11-rc5/source/drivers/md/raid5.c#L735
[<0>] raid5_sync_request+0x2a7/0x3d0       # https://elixir.bootlin.com/linux/v5.11-rc5/source/drivers/md/raid5.c#L6274
[<0>] md_do_sync.cold+0x3ee/0x97c          # https://elixir.bootlin.com/linux/v5.11-rc5/source/drivers/md/md.c#L8883
[<0>] md_thread+0xab/0x160
[<0>] kthread+0x11b/0x140
[<0>] ret_from_fork+0x22/0x30

Sometimes top of stack is raid5_get_active_stripe+0x1ef/0x6b0 instead of raid5_get_active_stripe+0x1e7/0x6b0, so I guess it sleeps, its woken, but the conditions don't match so its sleeps again.

Best
   Donald

> 
>> And, unlike before, "md: md3: data-check interrupted." from the pr_info two lines above appears in dmesg.
> 
> Yes, that is intentional since MD_RECOVERY_INTR is set by write idle.
> 
> Anyway, will try the script and investigate more about the issue.
> 
> Thanks,
> Guoqing

-- 
Donald Buczek
buczek@...gen.mpg.de
Tel: +49 30 8413 1433