Messages in this thread | | | Date | Fri, 12 Feb 2021 10:47:05 +0100 | From | Petr Mladek <> | Subject | Re: [PATCH v2] printk: avoid prb_first_valid_seq() where possible |
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On Thu 2021-02-11 18:37:52, John Ogness wrote: > If message sizes average larger than expected (more than 32 > characters), the data_ring will wrap before the desc_ring. Once the > data_ring wraps, it will start invalidating descriptors. These > invalid descriptors hang around until they are eventually recycled > when the desc_ring wraps. Readers do not care about invalid > descriptors, but they still need to iterate past them. If the > average message size is much larger than 32 characters, then there > will be many invalid descriptors preceding the valid descriptors. > > The function prb_first_valid_seq() always begins at the oldest > descriptor and searches for the first valid descriptor. This can > be rather expensive for the above scenario. And, in fact, because > of its heavy usage in /dev/kmsg, there have been reports of long > delays and even RCU stalls. > > For code that does not need to search from the oldest record, > replace prb_first_valid_seq() usage with prb_read_valid_*() > functions, which provide a start sequence number to search from. > > Fixes: 896fbe20b4e2333fb55 ("printk: use the lockless ringbuffer") > Reported-by: kernel test robot <oliver.sang@intel.com> > Reported-by: J. Avila <elavila@google.com> > Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
I am going to push the patch later today. I would prefer to do it later and give a chance others to react. But the merge window will likely start the following week and Sergey was fine with this approach.
Best Regard, Petr
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