Re: Endless soft-lockups for compiling workload since next-20200519

[Peter Zijlstra]

On Thu, May 21, 2020 at 02:40:36AM +0200, Frederic Weisbecker wrote:
> On Wed, May 20, 2020 at 02:50:56PM +0200, Peter Zijlstra wrote:
> > On Tue, May 19, 2020 at 11:58:17PM -0400, Qian Cai wrote:
> > > Just a head up. Repeatedly compiling kernels for a while would trigger
> > > endless soft-lockups since next-20200519 on both x86_64 and powerpc.
> > > .config are in,
> > 
> > Could be 90b5363acd47 ("sched: Clean up scheduler_ipi()"), although I've
> > not seen anything like that myself. Let me go have a look.
> > 
> > 
> > In as far as the logs are readable (they're a wrapped mess, please don't
> > do that!), they contain very little useful, as is typical with IPIs :/
> > 
> > > [ 1167.993773][    C1] WARNING: CPU: 1 PID: 0 at kernel/smp.c:127
> > > flush_smp_call_function_queue+0x1fa/0x2e0
> 
> So I've tried to think of a race that could produce that and here is
> the only thing I could come up with. It's a bit complicated unfortunately:

This:

>         smp_call_function_single_async() {             smp_call_function_single_async() {
>             // verified csd->flags != CSD_LOCK             // verified csd->flags != CSD_LOCK
>             csd->flags = CSD_LOCK                          csd->flags = CSD_LOCK

concurrent smp_call_function_single_async() using the same csd is what
I'm looking at as well. Now in the ILB case there is an easy cure:

(because there is only a single ilb target)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 01f94cf52783..b6d8a7b991f0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10033,7 +10033,7 @@ static void kick_ilb(unsigned int flags)
 	 * is idle. And the softirq performing nohz idle load balance
 	 * will be run before returning from the IPI.
 	 */
-	smp_call_function_single_async(ilb_cpu, &cpu_rq(ilb_cpu)->nohz_csd);
+	smp_call_function_single_async(ilb_cpu, &this_rq()->nohz_csd);
 }
 
 /*

Qian, can you give that a spin?

But I'm still not convinced of your scenario:

>                                                    kick_ilb() {
>                                                        atomic_fetch_or(...., nohz_flags(0))

>         atomic_fetch_or(...., nohz_flags(0))           #VMENTER
>         smp_call_function_single_async() {             smp_call_function_single_async() {
>             // verified csd->flags != CSD_LOCK             // verified csd->flags != CSD_LOCK
>             csd->flags = CSD_LOCK                          csd->flags = CSD_LOCK

Note that we check the return value of atomic_fetch_or() and bail if
someone else set a flag in KICK_MASK before us.

Aah, I suppose you're saying this can happen when:

  !(flags & NOHZ_KICK_MASK)

? That's not supposed to happen though.


Anyway, let me go stare at the remove wake-up case, because i'm afraid
that might have the same problem too...