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

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Date:   Thu, 15 Aug 2019 15:57:27 +0100
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 12:47 PM Ming Lei <ming.lei@...hat.com> wrote:
> blk_queue_init_done() is only called in blk_queue_init_done() for
> this purpose, so this approach should be fine, IMO.

I was thinking somebody might add more stuff to "init" in the future,
and then that new stuff would now no longer be executed for the loop
driver. The name "init" is pretty generic...but if that's not a
concern I'm happy with your proposal as well. There's one more
"freeze" I'd like to get rid of - we also call LOOP_SET_STATUS(64),
and there's a freeze in there because lo->transfer is modified. That
makes sense, but I was hoping we can make that freeze conditional on
whether lo->transfer would actually change value; if it stays the
same, I think freezing is not necessary.

>
> > switching q_usage_counter to per-cpu mode in the block layer in
> > general, until the first request comes in?
>
> This approach may not help your issue on loop, IO request comes
> just after disk is added, such as by systemd, or reading partition table.

That's a good point, I thought we could avoid the partition scan, but
on some Android devices we'll have max_part set, and there will be a
partition scan.

Thanks,
Martijn

>
> However, loop only starts to handle IO really after it becomes 'Lo_bound'.
>
> thanks,
> Ming