On Tue, Nov 29, 2016 at 06:07:34AM -0800, Paul E. McKenney wrote:
> On Tue, Nov 29, 2016 at 10:16:50AM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 28, 2016 at 11:52:41PM -0600, Josh Poimboeuf wrote:
> > > > We used to do that, but the resulting NMIs were problematic on some
> > > > platforms. Perhaps things have gotten better?
> > >
> > > Did a little digging on git blame and found the following commit (which
> > > seems to be the cause of the KASAN warning and missing stack dump):
> > >
> > > bc1dce514e9b ("rcu: Don't use NMIs to dump other CPUs' stacks")
> > >
> > > I presume this commit is still needed because of the NMI printk deadlock
> > > issues which were discussed at Kernel Summit. I guess those issues need
> > > to be sorted out before the above commit can be reverted.
> >
> > so printk should more or less work from NMI, esp. after:
> >
> > 42a0bb3f7138 ("printk/nmi: generic solution for safe printk in NMI")
>
> And of course bc1dce514e9b doesn't revert cleanly, but see hand reversion
> below. Also, 42a0bb3f7138's commit log calls out MN10300 and Xtensa as
> needing more work. Has that happened?
Petr M, any idea?
> But I really like the fact that RCU CPU stall warnings dump only those
> stacks that are likely to be involved, and the patch below goes back
> to dumping everyone. Shouldn't be that hard to fix, though...
There's a new trigger_single_cpu_backtrace() function which can be used
for that.
> ------------------------------------------------------------------------
>
> commit e7c9d76ed508fe978c6657e33f4de1b160ee4efe
> Author: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Date: Tue Nov 29 05:49:06 2016 -0800
>
> rcu: Once again use NMI-based stack traces in stall warnings
>
> This commit is for all intents and purposes a revert of bc1dce514e9b
> ("rcu: Don't use NMIs to dump other CPUs' stacks"). The reason to
> suppose that this can now safely be reverted is the presence of
> 42a0bb3f7138 ("printk/nmi: generic solution for safe printk in NMI"),
> which is said to have made NMI-based stack dumps safe.
>
> Not-yet-signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> Cc: Petr Mladek <pmla...@suse.com>
> Cc: Josh Poimboeuf <jpoim...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 91a68e4e6671..d73ccd4bed86 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1396,7 +1396,10 @@ static void rcu_check_gp_kthread_starvation(struct
> rcu_state *rsp)
> }
>
> /*
> - * Dump stacks of all tasks running on stalled CPUs.
> + * Dump stacks of all tasks running on stalled CPUs. First try using
> + * NMIs, but fall back to manual remote stack tracing on architectures
> + * that don't support NMI-based stack dumps. The NMI-triggered stack
> + * traces are more accurate because they are printed by the target CPU.
> */
> static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
> {
> @@ -1404,6 +1407,8 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
> unsigned long flags;
> struct rcu_node *rnp;
>
> + if (trigger_all_cpu_backtrace())
> + return;
> rcu_for_each_leaf_node(rsp, rnp) {
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> if (rnp->qsmask != 0) {
>
--
Josh
