Re: [PATCH RFC x86/mce] Make mce_timed_out() identify holdout CPUs

[Paul E. McKenney]

On Thu, Jan 07, 2021 at 12:26:19AM +0000, Luck, Tony wrote:
> > Please see below for an updated patch.
> 
> Yes. That worked:
> 
> [   78.946069] mce: mce_timed_out: MCE holdout CPUs (may include false positives): 24-47,120-143
> [   78.946151] mce: mce_timed_out: MCE holdout CPUs (may include false positives): 24-47,120-143
> [   78.946153] Kernel panic - not syncing: Timeout: Not all CPUs entered broadcast exception handler
> 
> I guess that more than one CPU hit the timeout and so your new message was printed twice
> before the panic code took over?

Could well be.

It would be easy to add a flag that allowed only one CPU to print the
message.  Does that make sense?  (See off-the-cuff probably-broken
delta patch below for one approach.)

> Once again, the whole of socket 1 is MIA rather than just the pair of threads on one of the cores there.
> But that's a useful improvement (eliminating the other three sockets on this system).
> 
> Tested-by: Tony Luck <tony.luck@xxxxxxxxx>

Thank you very much!  I will apply this.

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
index 7a6e1f3..b46ac56 100644
--- a/arch/x86/kernel/cpu/mce/core.c
+++ b/arch/x86/kernel/cpu/mce/core.c
@@ -882,6 +882,7 @@ static atomic_t mce_callin;
  */
 static cpumask_t mce_present_cpus;
 static cpumask_t mce_missing_cpus;
+static atomic_t mce_missing_cpus_gate;
 
 /*
  * Check if a timeout waiting for other CPUs happened.
@@ -900,7 +901,7 @@ static int mce_timed_out(u64 *t, const char *msg)
 	if (!mca_cfg.monarch_timeout)
 		goto out;
 	if ((s64)*t < SPINUNIT) {
-		if (mca_cfg.tolerant <= 1) {
+		if (mca_cfg.tolerant <= 1 && !atomic_xchg(&mce_missing_cpus_gate, 1)) {
 			if (cpumask_andnot(&mce_missing_cpus, cpu_online_mask, &mce_present_cpus))
 				pr_info("%s: MCE holdout CPUs (may include false positives): %*pbl\n",
 					__func__, cpumask_pr_args(&mce_missing_cpus));
@@ -1017,6 +1018,7 @@ static int mce_start(int *no_way_out)
 	 */
 	order = atomic_inc_return(&mce_callin);
 	cpumask_set_cpu(smp_processor_id(), &mce_present_cpus);
+	atomic_set(&mce_missing_cpus_gate, 0);
 
 	/*
 	 * Wait for everyone.
@@ -1126,6 +1128,7 @@ static int mce_end(int order)
 	atomic_set(&global_nwo, 0);
 	atomic_set(&mce_callin, 0);
 	cpumask_clear(&mce_present_cpus);
+	atomic_set(&mce_missing_cpus_gate, 0);
 	barrier();
 
 	/*
@@ -2725,6 +2728,7 @@ static void mce_reset(void)
 	atomic_set(&mce_callin, 0);
 	atomic_set(&global_nwo, 0);
 	cpumask_clear(&mce_present_cpus);
+	atomic_set(&mce_missing_cpus_gate, 0);
 }
 
 static int fake_panic_get(void *data, u64 *val)