linux-kernel - Re: bug in tag handling in blk-mq?

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Date:   Tue, 8 May 2018 19:09:04 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Mike Galbraith <efault@....de>,
        Paolo Valente <paolo.valente@...aro.org>
Cc:     Christoph Hellwig <hch@....de>,
        linux-block <linux-block@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>
Subject: Re: bug in tag handling in blk-mq?

On 5/8/18 3:19 PM, Jens Axboe wrote:
> On 5/8/18 2:37 PM, Jens Axboe wrote:
>> On 5/8/18 10:42 AM, Mike Galbraith wrote:
>>> On Tue, 2018-05-08 at 08:55 -0600, Jens Axboe wrote:
>>>>
>>>> All the block debug files are empty...
>>>
>>> Sigh.  Take 2, this time cat debug files, having turned block tracing
>>> off before doing anything else (so trace bits in dmesg.txt should end
>>> AT the stall).
>>
>> OK, that's better. What I see from the traces:
>>
>> - You have regular IO and some non-fs IO (from scsi_execute()). This mix
>>   may be key.
>>
>> - sdd has nothing pending, yet has 6 active waitqueues.
>>
>> I'm going to see if I can reproduce this. Paolo, what kind of attempts
>> to reproduce this have you done?
> 
> No luck so far. Out of the patches you referenced, I can only find the
> shallow depth change, since that's in the parent of this email. Can
> you send those as well?
> 
> Perhaps also expand a bit on exactly what you are running. File system,
> mount options, etc.

Alright, I managed to reproduce it. What I think is happening is that
BFQ is limiting the inflight case to something less than the wake
batch for sbitmap, which can lead to stalls. I don't have time to test
this tonight, but perhaps you can give it a go when you are back at it.
If not, I'll try tomorrow morning.

If this is the issue, I can turn it into a real patch. This is just to
confirm that the issue goes away with the below.

diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index e6a9c06ec70c..94ced15b6428 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -272,6 +272,7 @@ EXPORT_SYMBOL_GPL(sbitmap_bitmap_show);
 
 static unsigned int sbq_calc_wake_batch(unsigned int depth)
 {
+#if 0
 	unsigned int wake_batch;
 
 	/*
@@ -284,6 +285,9 @@ static unsigned int sbq_calc_wake_batch(unsigned int depth)
 		wake_batch = max(1U, depth / SBQ_WAIT_QUEUES);
 
 	return wake_batch;
+#else
+	return 1;
+#endif
 }
 
 int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,

-- 
Jens Axboe