Messages in this thread | | | Date | Wed, 13 Jul 2022 07:05:50 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -printk] printk, tracing: fix console tracepoint |
| |
On Wed, Jul 13, 2022 at 01:25:41PM +0200, Petr Mladek wrote: > On Tue 2022-07-12 08:16:55, Paul E. McKenney wrote: > > On Tue, Jul 12, 2022 at 10:53:53AM -0400, Steven Rostedt wrote: > > > On Tue, 12 Jul 2022 06:49:16 -0700 > > > "Paul E. McKenney" <paulmck@kernel.org> wrote: > > > > > > > > I guess the question is, can we have printk() in such a place? Because this > > > > > tracepoint is attached to printk and where ever printk is done so is this > > > > > tracepoint. > > > > > > > > As I understand it, code in such a place should be labeled noinstr. > > > > Then the call to printk() would be complained about as an illegal > > > > noinstr-to-non-noinstr call. > > > > > > > > But where exactly is that printk()? > > > > > > Perhaps the fix is to remove the _rcuidle() from trace_console_rcuidle(). > > > If printk() can never be called from noinstr (aka RCU not watching). > > > > Maybe printk() is supposed to be invoked from noinstr. It might be a > > special case in the tooling. I have no idea. ;-) > > I think that it is ok to do _not_ support printk() in noinstr parts. > > > However, the current SRCU read-side algorithm will tolerate being invoked > > from noinstr as long as it is not also an NMI handler. Much though > > debugging tools might (or might not) complain. > > > > Don't get me wrong, I can make SRCU tolerate being called while RCU is > > not watching. It is not even all that complicated. The cost is that > > architectures that have NMIs but do not have NMI-safe this_cpu*() > > operations have an SRCU reader switch from explicit smp_mb() and > > interrupt disabling to a cmpxchg() loop relying on the implicit barriers > > in cmpxchg(). > > > > For arm64, this was reportedly a win. > > IMHO, the tracepoint in printk() is not worth slowing down other > important fast paths. > > The tracepoint was moved into vprintk_store() in 5.19-rc1. It used > to be in console_unlock() before. The previous location was not > reliable by definition. Old messages might be overridden by new > ones before they reach console. Also messages in NMI context > used to be stored in per-CPU buffers. There was even bigger > risk that they would not reach the console.
Fair enough, works for me!
Thanx, Paul
| |