linux-kernel - Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze

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Date:   Wed, 14 Aug 2019 13:38:25 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>, kernel-team@...roid.com,
        Narayan Kamath <narayan@...gle.com>,
        Dario Freni <dariofreni@...gle.com>,
        Nikita Ioffe <ioffe@...gle.com>,
        Jiyong Park <jiyong@...gle.com>,
        Martijn Coenen <maco@...gle.com>
Subject: Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze_queue() when possible.

On Wed, Aug 14, 2019 at 1:34 PM Ming Lei <ming.lei@...hat.com> wrote:
> Another candidate is to not switch to q_usage_counter's percpu mode
> until loop becomes Lo_bound, and this way may be more clean.

Thanks! I had considered this too, but thought it a bit risky to mess
with the init flag from the loop driver. Maybe we could delay
switching q_usage_counter to per-cpu mode in the block layer in
general, until the first request comes in?

This would also address our issues, and potentially be an even smaller change.

Martijn
>
> Something like the following patch:
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index a7461f482467..8791f9242583 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -1015,6 +1015,9 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
>          */
>         bdgrab(bdev);
>         mutex_unlock(&loop_ctl_mutex);
> +
> +       percpu_ref_switch_to_percpu(&lo->lo_queue->q_usage_counter);
> +
>         if (partscan)
>                 loop_reread_partitions(lo, bdev);
>         if (claimed_bdev)
> @@ -1171,6 +1174,8 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
>         lo->lo_state = Lo_unbound;
>         mutex_unlock(&loop_ctl_mutex);
>
> +       percpu_ref_switch_to_atomic(&lo->lo_queue->q_usage_counter, NULL);
> +
>         /*
>          * Need not hold loop_ctl_mutex to fput backing file.
>          * Calling fput holding loop_ctl_mutex triggers a circular
> @@ -2003,6 +2008,12 @@ static int loop_add(struct loop_device **l, int i)
>         }
>         lo->lo_queue->queuedata = lo;
>
> +       /*
> +        * cheat block layer for not switching to q_usage_counter's
> +        * percpu mode before loop becomes Lo_bound
> +        */
> +       blk_queue_flag_set(QUEUE_FLAG_INIT_DONE, lo->lo_queue);
> +
>         blk_queue_max_hw_sectors(lo->lo_queue, BLK_DEF_MAX_SECTORS);
>
>         /*
>
>
> thanks,
> Ming