On Wed, Jan 06, 2021 at 11:17:08AM -0800, Paul E. McKenney wrote:

On Wed, Jan 06, 2021 at 06:39:30PM +0000, Luck, Tony wrote:

quoted

quoted

The "Timeout: Not all CPUs entered broadcast exception handler" message
will appear from time to time given enough systems, but this message does
not identify which CPUs failed to enter the broadcast exception handler.
This information would be valuable if available, for example, in order to
correlated with other hardware-oriented error messages.  This commit
therefore maintains a cpumask_t of CPUs that have entered this handler,
and prints out which ones failed to enter in the event of a timeout.

I tried doing this a while back, but found that in my test case where I forced
an error that would cause both threads from one core to be "missing", the
output was highly unpredictable. Some random number of extra CPUs were
reported as missing. After I added some extra breadcrumbs it became clear
that pretty much all the CPUs (except the missing pair) entered do_machine_check(),
but some got hung up at various points beyond the entry point. My only theory
was that they were trying to snoop caches from the dead core (or access some
other resource held by the dead core) and so they hung too.

Your code is much neater than mine ... and perhaps works in other cases, but
maybe the message needs to allow for the fact that some of the cores that
are reported missing may just be collateral damage from the initial problem.

Understood.  The system is probably not in the best shape if this code
is ever executed, after all.  ;-)

So how about like this?

	pr_info("%s: MCE holdout CPUs (may include false positives): %*pbl\n",

That looks fine.

Easy enough if so!

quoted

If I get time in the next day or two, I'll run my old test against your code to
see what happens.

I got time today (plenty of meetings in which to run experiments in background).

This code:

-               if (mca_cfg.tolerant <= 1)
+               if (mca_cfg.tolerant <= 1) {
+                       if (!cpumask_andnot(&mce_missing_cpus, cpu_online_mask, &mce_present_cpus))
+                               pr_info("%s: MCE holdout CPUs: %*pbl\n",
+                                       __func__, cpumask_pr_args(&mce_missing_cpus));
                        mce_panic(msg, NULL, NULL);

didn't trigger ... so maybe that cpumask_andnot() didn't return the value you expected?

I added a:

+                       pr_info("%s: MCE present CPUs: %*pbl\n", __func__, cpumask_pr_args(&mce_present_cpus));

to check that the mask was being set correctly, and saw:

[  219.329767] mce: mce_timed_out: MCE present CPUs: 0-23,48-119,144-191

So the every core of socket 1 failed to show up for this test.

For my own testing, is this still the right thing to use?

	https://github.com/andikleen/mce-inject

That fakes up errors (by hooking into the mce_rdmsr() code to return arbitrary
user supplied values).  The plus side of this is that you can fake any error
signature without needing special h/w or f/w. The downside is that it is all fake
and you can't create situations where some CPUs don't show up in the machine
check handler.

-Tony