Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sun, 14 Jul 2019 01:01:18 -0300
From:   Marcos Paulo de Souza <marcos.souza.org@gmail.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     linux-kernel@vger.kernel.org,
        "open list:BLOCK LAYER" <linux-block@vger.kernel.org>
Subject: Re: [PATCH] block: elevator.c: Check elevator kernel argument again
Message-ID: <20190714040118.GA19237@continental>
References: <20190713035221.31508-1-marcos.souza.org@gmail.com>
 <75c5681a-389f-cfaf-7f3f-31a2daec9da4@kernel.dk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <75c5681a-389f-cfaf-7f3f-31a2daec9da4@kernel.dk>
User-Agent: Mutt/1.11.3 (2019-02-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Jul 13, 2019 at 03:53:28PM -0600, Jens Axboe wrote:
> On 7/12/19 9:52 PM, Marcos Paulo de Souza wrote:
> > Since the inclusion of blk-mq, elevator= kernel argument was not being
> > considered anymore, making it impossible to specify a specific elevator
> > at boot time as it was used before.
> > 
> > This is done by checking chosen_elevator global variable, which is
> > populated once elevator= kernel argument is passed. Without this patch,
> > mq-deadline is the only elevator that is can be used at boot time.
> > 
> > Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
> > ---
> > 
> >   I found this issue while inspecting why noop scheduler was gone, and
> >   so I found that was now impossible to use a scheduler different from
> >   mq-deadeline.
> > 
> >   Am I missing something? Is this a desirable behavior?
> 
> Just google, I'm sure 2-3 discussions on this topic will come up.
> 
> tldr is that the original parameter was a mistake and doesn't work at
> all for multiple devices. Today it's even worse, as we have device
> types that won't even work properly with any scheduler, liked the
> zoned devices. The parameter was never enabled for blk-mq because of
> that, and hence died when the legacy IO path was scrapped. It's not
> coming back.

Thanks Jens. So it makes sense to remove all leftover code of elevator argument with
some dead documentation about it, avoiding confusion about it in the future
again. I will send some patches tomorrow.

> 
> -- 
> Jens Axboe
>