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[66.29.164.166]) by smtp.gmail.com with ESMTPSA id d14sm17326936pfo.154.2019.07.13.14.53.29 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 13 Jul 2019 14:53:30 -0700 (PDT) Subject: Re: [PATCH] block: elevator.c: Check elevator kernel argument again To: Marcos Paulo de Souza , linux-kernel@vger.kernel.org Cc: "open list:BLOCK LAYER" References: <20190713035221.31508-1-marcos.souza.org@gmail.com> From: Jens Axboe Message-ID: <75c5681a-389f-cfaf-7f3f-31a2daec9da4@kernel.dk> Date: Sat, 13 Jul 2019 15:53:28 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20190713035221.31508-1-marcos.souza.org@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > --- > > 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. -- Jens Axboe