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

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Message-ID: <1525839089.15732.1.camel@gmx.de>
Date:   Wed, 09 May 2018 06:11:29 +0200
From:   Mike Galbraith <efault@....de>
To:     Jens Axboe <axboe@...nel.dk>,
        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 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.

> 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,
>