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Date: Tue, 2 Feb 2021 16:42:04 +0100
From: Guoqing Jiang <guoqing.jiang@...ud.ionos.com>
To: Donald Buczek <buczek@...gen.mpg.de>, 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
Hi Donald,
On 1/26/21 17:05, Donald Buczek wrote:
> 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.
I don't know why the condition was not true after the change since the
RECOVERY_INTR is set and the caller is raid5_sync_request.
wait_event_lock_irq(conf->wait_for_stripe,
(test_bit(MD_RECOVERY_INTR, &mddev->recovery) && sync_req) ||
/* the previous condition */,
*(conf->hash_locks + hash));
BTW, I think there some some possible ways:
1. let "echo idle" give up the reconfig_mutex if there are limited
number of active stripe, but I feel it is ugly to check sh number from
action_store (kind of layer violation).
2. make raid5_sync_request -> raid5_get_active_stripe can quit from the
current situation (this was we tried, though it doesn't work so far).
3. set MD_ALLOW_SB_UPDATE as you said though I am not sure the safety
(but maybe I am wrong).
4. given the write IO keeps coming from upper layer which decrease the
available stripes. Maybe we need to call grow_one_stripe at the
beginning of raid5_make_request for this case, then call drop_one_stripe
at the end of make_request.
5. maybe don't hold reconfig_mutex when try to unregister sync_thread,
like this.
/* resync has finished, collect result */
mddev_unlock(mddev);
md_unregister_thread(&mddev->sync_thread);
mddev_lock(mddev);
My suggestion would be try 2 + 4 together since the reproducer triggers
both sync io and write io. Or try 5. Perhaps there is better way which I
just can't find.
Thanks,
Guoqing
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