Endless soft-lockups for compiling workload since next-20200519

[Frederic Weisbecker]

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:

CPU 0                                              CPU 1
-----                                              -----

tick {
    trigger_load_balance() {
        raise_softirq(SCHED_SOFTIRQ);
        //but nohz_flags(0) = 0
    }
                                                   kick_ilb() {
                                                       atomic_fetch_or(...., nohz_flags(0))
    softirq() {                                        #VMEXIT or anything that could stop a CPU for a while
        run_rebalance_domain() {
            nohz_idle_balance() {
                atomic_andnot(NOHZ_KICK_MASK, nohz_flag(0))
            }
         }
     }
}

// schedule
nohz_newidle_balance() {
    kick_ilb() { // pick current CPU
        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
            //execute in place                             //queue and send IPI
            csd->flags = 0
            nohz_csd_func()
	}
    }
}


IPI�{
    flush_smp_call_function_queue() {
        csd_unlock() {
            WARN_ON(csd->flags != CSD_LOCK) <---------!!!!!



The root cause here would be that trigger_load_balance() unconditionally raise
the softirq. And I have to confess I'm not clear why since the softirq is
essentially a no-op when nohz_flags() is 0.

Thanks.