We noticed that the user interface of Android devices becomes very slow
under memory pressure. This is because Android uses the zram driver on top
of the loop driver for swapping, because under memory pressure the swap
code alternates reads and writes quickly, because mq-deadline is the
default scheduler for loop devices and because mq-deadline delays writes by
five seconds for such a workload with default settings. Fix this by making
the kernel select I/O scheduler 'none' from inside add_disk() for loop
devices. This default can be overridden at any time from user space,
e.g. via a udev rule. This approach has an advantage compared to changing
the I/O scheduler from userspace from 'mq-deadline' into 'none', namely
that synchronize_rcu() does not get called.
Additionally, this patch reduces the Android boot time on my test setup
with 0.5 seconds compared to configuring the loop I/O scheduler from user
space.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Bug: 194450129
(cherry picked from commit 2112f5c133 git://git.kernel.dk/linux-block/ for-5.15/block)
Change-Id: I6f9579b4cd2cb22fcb5c858d4f292f1870336fdd
Signed-off-by: Bart Van Assche <bvanassche@google.com>
bda49ad060