On (04/16/18 10:47), Sergey Senozhatsky wrote: > On (04/14/18 11:35), Sergey Senozhatsky wrote: > > On (04/13/18 10:12), Steven Rostedt wrote: > > > > > > > The interval is set to one hour. It is rather arbitrary selected time. > > > > It is supposed to be a compromise between never print these messages, > > > > do not lockup the machine, do not fill the entire buffer too quickly, > > > > and get information if something changes over time. > > > > > > > > > I think an hour is incredibly long. We only allow 100 lines per hour for > > > printks happening inside another printk? > > > > > > I think 5 minutes (at most) would probably be plenty. One minute may be > > > good enough. > > > > Besides 100 lines is absolutely not enough for any real lockdep splat. > > My call would be - up to 1000 lines in a 1 minute interval. > > Well, if we want to basically turn printk_safe() into > printk_safe_ratelimited(). > I'm not so sure about it. > > Besides the patch also rate limits printk_nmi->logbuf - the logbuf > PRINTK_NMI_DEFERRED_CONTEXT_MASK bypass, which is way too important > to rate limit it - for no reason. > > Dunno, can we keep printk_safe() the way it is and introduce a new > printk_safe_ratelimited() specifically for call_console_drivers()? > > Lockdep splat is a one time event, if we lose half of it - we, most > like, lose the entire report. And call_console_drivers() is not the > one and only source of warnings/errors/etc. So if we turn printk_safe > into printk_safe_ratelimited() [not sure we want to do it] for all > then I want restrictions to be as low as possible, IOW to log_store() > as many lines as possible.
One more thing, I'd really prefer to rate limit the function which flushes per-CPU printk_safe buffers; not the function that appends new messages to the per-CPU printk_safe buffers. There is a significant difference. printk_safe does not help us when we are dealing with any external locks - and call_console_drivers() is precisely that type of case. The very next thing to happen after lockdep splat, or spin_lock debugging report, etc. can be an actual deadlock->panic(). Thus I want to have the entire report in per-CPU buffer [if possible], so we can flush_on_panic() per-CPU buffers, or at least move the data to the logbuf and make it accessible in vmcore. If we rate limit the function that appends data to the per-CPU buffer then we may simply suppress [rate limit] the report, so there will be nothing to flush_on_panic(). -ss