Hi Steven, Am Donnerstag, den 03.12.2020, 21:18 -0500 schrieb Steven Rostedt: > Sorry for the really late reply, but I received this while I was on > vacation, and my backlog was so big when I got back that I left most of > it unread. :-/ And to make matters worse, my out-of-office script > wasn't working, to let people know I was on vacation.
No problem, I already figured that this might have fallen through the cracks. It's also not really a high prio issue for us. > On Mon, 07 Sep 2020 18:16:52 +0200 > Lucas Stach <l.st...@pengutronix.de> wrote: > > > Hi all, > > > > one of my colleagues has taken a look at device boot times and stumbled > > across a pretty big amount of kernel boot time being spent in > > tracer_init_tracefs(). On this particular i.MX6Q based device the > > kernel spends more than 1 second in this function, which is a > > significant amount of the overall kernel inititalization time. While > > this machine is no rocket with its Cortex A9 @ 800MHz, the amount of > > CPU time being used there is pretty irritating. > > > > Specifically the issue lies within trace_event_eval_update where ~1100 > > trace_event_calls get updated with ~500 trace_eval_maps. I haven't had > > a chance yet to dig any deeper or try to understand more of what's > > going on there, but I wanted to get the issue out there in case anyone > > has some cycles to spare to help us along. > > OK, that makes sense. The macro TRACE_DEFINE_ENUM() will make a mapping > of enums into their values. This is needed because if an enum is used > in tp_printk() of a TRACE_EVENT(), the name of the ENUM is passed to > user space. The enum name is useless to user space, so this function > will scan the strings that are exported to user space and convert the > enum name to the enum values. > > > > > The obvious questions for now are: > > 1. Why is this function so damn expensive (at least on this whimpy ARM > > machine)? and > > Well, it's doing a string substitution for thousands of events. > > > > 2. Could any of this be done asynchronously, to not block the kernel in > > early init? > > Yes :-) > > We could make a thread that does this, that the init wakes up and runs, > letting the kernel to move forward. Would you like to make that patch > or shall I? I guess you are much more likely to come up with a correct patch, as I'm not really clear yet on when we would need to synchronize this thread, to make sure things are available before they get used by something. I likely won't have time in the near future to read enough code in this particular spot of the kernel. I would be happy to test a patch on our whimpy machines, though. :) Regards, Lucas