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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