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

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Date:   Wed, 9 May 2018 07:06:31 +0200
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Mike Galbraith <efault@....de>
Cc:     Jens Axboe <axboe@...nel.dk>, 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?



> Il giorno 09 mag 2018, alle ore 06:11, Mike Galbraith <efault@....de> ha scritto:
> 
> On Tue, 2018-05-08 at 19:09 -0600, Jens Axboe wrote:
>> 
>> 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.
> 
> Confirmed.  Impressive high speed bug stomping.
> 

Great! It's a real relief that this ghost is gone.

Thank you both,
Paolo

>> 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,
>>