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commit 2112f5c1330a671fa852051d85cb9eadc05d7eb7 upstream. 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. This patch changes the default I/O scheduler for loop devices from 'mq-deadline' into 'none'. 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. Cc: Christoph Hellwig <hch@lst.de> Cc: Ming Lei <ming.lei@redhat.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Martijn Coenen <maco@android.com> Cc: Jaegeuk Kim <jaegeuk@kernel.org> Signed-off-by: Bart Van Assche <bvanassche@acm.org> Link: https://lore.kernel.org/r/20210805174200.3250718-3-bvanassche@acm.org Signed-off-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.