Re: [PATCH 24/36] printk: Remove trace_.*_rcuidle() usage

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Jun 09, 2022 at 11:16:46AM +0200, Petr Mladek wrote:
> On Wed 2022-06-08 16:27:47, Peter Zijlstra wrote:
> > The problem, per commit fc98c3c8c9dc ("printk: use rcuidle console
> > tracepoint"), was printk usage from the cpuidle path where RCU was
> > already disabled.
> > 
> > Per the patches earlier in this series, this is no longer the case.
> 
> My understanding is that this series reduces a lot the amount
> of code called with RCU disabled. As a result the particular printk()
> call mentioned by commit fc98c3c8c9dc ("printk: use rcuidle console
> tracepoint") is called with RCU enabled now. Hence this particular
> problem is fixed better way now.
> 
> But is this true in general?
> Does this "prevent" calling printk() a safe way in code with
> RCU disabled?

On x86_64, yes. Other architectures, less so.

Specifically, the objtool noinstr validation pass will warn at build
time (DEBUG_ENTRY=y) if any noinstr/cpuidle code does a call to
non-vetted code like printk().

At the same time; there's a few hacks that allow WARN to work, but
mostly if you hit WARN in entry/noinstr you get to keep the pieces in
any case.

On other architecture we'll need to rely on runtime coverage with
PROVE_RCU. That is, if a splat like in the above mentioned commit
happens again, we'll need to fix it by adjusting the callchain, not by
mucking about with RCU state.

> I am not sure if anyone cares. printk() is the best effort
> functionality because of the consoles code anyway. Also I wonder
> if anyone uses this trace_console().

This is the tracepoint used to spool all of printk into ftrace, I
suspect there's users, but I haven't used it myself.

> Therefore if this patch allows to remove some tricky tracing
> code then it might be worth it. But if trace_console_rcuidle()
> variant is still going to be available then I would keep using it.

My ultimate goal is to delete trace_.*_rcuidle() and RCU_NONIDLE()
entirely. We're close, but not quite there yet.