Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze_queue() when possible.

From: Ming Lei <tom.leiming@gmail.com>
To: Martijn Coenen <maco@android.com>
Cc: Ming Lei <ming.lei@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	linux-block <linux-block@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	kernel-team@android.com, Narayan Kamath <narayan@google.com>,
	Dario Freni <dariofreni@google.com>,
	Nikita Ioffe <ioffe@google.com>, Jiyong Park <jiyong@google.com>,
	Martijn Coenen <maco@google.com>
Subject: Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze_queue() when possible.
Date: Fri, 16 Aug 2019 00:34:44 +0800	[thread overview]
Message-ID: <CACVXFVP0JrpUgterqHs5bvCQn7L9a-XrjDCD3BmQOLe+rgC1KQ@mail.gmail.com> (raw)
In-Reply-To: <CAB0TPYGc8H1pJZrDX1r5wO1gyYV9rzgi3acT9mp-vxxrdA-pyA@mail.gmail.com>

On Thu, Aug 15, 2019 at 11:38 PM Martijn Coenen <maco@android.com> wrote:
>
> On Wed, Aug 14, 2019 at 12:47 PM Ming Lei <ming.lei@redhat.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.

The queue freeze in SET_STATUS may not be avoided, not only
.transfer, there are also .lo_offset, .size, filename, dio and others.

If nothing will change, why does the userspace bother to send
SET_STATUS?

Thanks,
Ming Lei