Re: [RFC PATCH 00/11] printk: safe printing in NMI context

[Jiri Kosina]

On Wed, 18 Jun 2014, Paul E. McKenney wrote:

> > >  	/* Complain about tasks blocking the grace period. */
> > > @@ -1044,8 +1041,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
> > >  	pr_cont(" (t=%lu jiffies g=%ld c=%ld q=%lu)\n",
> > >  		jiffies - rsp->gp_start,
> > >  		(long)rsp->gpnum, (long)rsp->completed, totqlen);
> > > -	if (!trigger_all_cpu_backtrace())
> > > -		dump_stack();
> > > +	rcu_dump_cpu_stacks(rsp);
> > 
> > This is prone to producing not really consistent stacktraces though, 
> > right? As the target task is still running at the time the stack is being 
> > walked, it might produce stacktraces that are potentially nonsensial.
> 
> If a CPU is stuck, the stack trace down to where it is stuck is
> likely to be static.  But yes, there is some potential for confusion.
> My (admittedly limited) rcutorture testing produced sensible stack traces,
> but things might be a bit uglier in other situations.

I agree that it might work nicely for RCU stall detector indeed. I was 
looking for solution that'd work nicely both for RCU and for sysrq-l 
(where we can't rely on processess being stuck in any way).

> > How about sending NMI to the target CPU, so that the task is actually 
> > stopped, but printing its stacktrace from the CPU that detected the stall 
> > while it's stopped?
> > 
> > That way, there is no printk()-from-NMI, but also the stacktrace is 
> > guaranteed to be self-consistent.
> 
> I believe that this was what Steven was suggesting, though by using
> tracing.  

My understanding was that Steven is suggesting using trace_printk() from 
NMI.

> Of course, if my current approach isn't up to the job, then something 
> like this general approach would look quite good.

Thanks,

-- 
Jiri Kosina
SUSE Labs
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