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a1a2d8f016
The kworker routine update_writeback_rate() is schedued to update the writeback rate in every 5 seconds by default. Before calling __update_writeback_rate() to do real job, semaphore dc->writeback_lock should be held by the kworker routine. At the same time, bcache writeback thread routine bch_writeback_thread() also needs to hold dc->writeback_lock before flushing dirty data back into the backing device. If the dirty data set is large, it might be very long time for bch_writeback_thread() to scan all dirty buckets and releases dc->writeback_lock. In such case update_writeback_rate() can be starved for long enough time so that kernel reports a soft lockup warn- ing started like: watchdog: BUG: soft lockup - CPU#246 stuck for 23s! [kworker/246:31:179713] Such soft lockup condition is unnecessary, because after the writeback thread finishes its job and releases dc->writeback_lock, the kworker update_writeback_rate() may continue to work and everything is fine indeed. This patch avoids the unnecessary soft lockup by the following method, - Add new member to struct cached_dev - dc->rate_update_retry (0 by default) - In update_writeback_rate() call down_read_trylock(&dc->writeback_lock) firstly, if it fails then lock contention happens. - If dc->rate_update_retry <= BCH_WBRATE_UPDATE_MAX_SKIPS (15), doesn't acquire the lock and reschedules the kworker for next try. - If dc->rate_update_retry > BCH_WBRATE_UPDATE_MAX_SKIPS, no retry anymore and call down_read(&dc->writeback_lock) to wait for the lock. By the above method, at worst case update_writeback_rate() may retry for 1+ minutes before blocking on dc->writeback_lock by calling down_read(). For a 4TB cache device with 1TB dirty data, 90%+ of the unnecessary soft lockup warning message can be avoided. When retrying to acquire dc->writeback_lock in update_writeback_rate(), of course the writeback rate cannot be updated. It is fair, because when the kworker is blocked on the lock contention of dc->writeback_lock, the writeback rate cannot be updated neither. This change follows Jens Axboe's suggestion to a more clear and simple version. Signed-off-by: Coly Li <colyli@suse.de> Link: https://lore.kernel.org/r/20220528124550.32834-2-colyli@suse.de Signed-off-by: Jens Axboe <axboe@kernel.dk> |
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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.