linux-kernel - Why do we use cpu nice priority for ioprio?

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Message-ID: <BANLkTimiL5VZki4oUaw5hQYgB=E2jYwKrw@mail.gmail.com>
Date:	Thu, 24 Mar 2011 19:38:50 -0700
From:	Justin TerAvest <teravest@...gle.com>
To:	jaxboe@...ionio.com, Vivek Goyal <vgoyal@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Chad Talbott <ctalbott@...gle.com>
Subject: Why do we use cpu nice priority for ioprio?

It's not clear why the cpu nice value should be mapped to the ioprio
for a task when none is picked.

Internally, at cfq_init_prio_data(), we just set:
		ioprio = IOPRIO_NORM;
		ioprio_class = IOPRIO_CLASS_BE;
if ioprio_class is IOPRIO_CLASS_NONE.

The problem is that  today, SCHED_RR and SCHED_FIFO threads
automatically get bumped up to RT class. This all happens behind the
curtains and the io_class of the thread is still shown as NONE with
sys_ioprio_get(). What's the motivation behind this promotion of
ioprio class?

I'd be happy to send a patch to change the behavior, but I'm curious
why the code promotes based on task nice values today.

Thanks!
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