On (21/02/05 15:23), 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.s...@intel.com> > Reported-by: J. Avila <elav...@google.com> > Signed-off-by: John Ogness <john.ogn...@linutronix.de>
Acked-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 5a95c688621f..035aae771ea1 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -735,9 +735,9 @@ static ssize_t devkmsg_read(struct file *file, char > __user *buf, > logbuf_lock_irq(); > } > > - if (user->seq < prb_first_valid_seq(prb)) { > + if (r->info->seq != user->seq) { > /* our last seen message is gone, return error and reset */ > - user->seq = prb_first_valid_seq(prb); Yeah, I can see how this pattern can be expensive, it would have been less obvious and harder to spot had it been something like this valid_seq = prb_first_valid_seq(prb); if (user->seq < valid_seq) { user->seq = valid_seq; ... } Great analysis, John. I wonder if Intel test robot measures all test execution times; I do recall "we saw N% performance improvement after patch P" emails, but not sure if all of the tests are being tracked. -ss