Re: [PATCH v5 10/18] watchdog/hardlockup: Add a "cpu" param to watchdog_hardlockup_check()

Hi,

On Tue, May 23, 2023 at 9:02 AM Petr Mladek [off-list ref] wrote:

On Fri 2023-05-19 10:18:34, Douglas Anderson wrote:

quoted

In preparation for the buddy hardlockup detector where the CPU
checking for lockup might not be the currently running CPU, add a
"cpu" parameter to watchdog_hardlockup_check().

As part of this change, make hrtimer_interrupts an atomic_t since now
the CPU incrementing the value and the CPU reading the value might be
different. Technially this could also be done with just READ_ONCE and
WRITE_ONCE, but atomic_t feels a little cleaner in this case.

While hrtimer_interrupts is made atomic_t, we change
hrtimer_interrupts_saved from "unsigned long" to "int". The "int" is
needed to match the data type backing atomic_t for hrtimer_interrupts.
Even if this changes us from 64-bits to 32-bits (which I don't think
is true for most compilers), it doesn't really matter. All we ever do
is increment it every few seconds and compare it to an old value so
32-bits is fine (even 16-bits would be). The "signed" vs "unsigned"
also doesn't matter for simple equality comparisons.

hrtimer_interrupts_saved is _not_ switched to atomic_t nor even
accessed with READ_ONCE / WRITE_ONCE. The hrtimer_interrupts_saved is
always consistently accessed with the same CPU. NOTE: with the
upcoming "buddy" detector there is one special case. When a CPU goes
offline/online then we can change which CPU is the one to consistently
access a given instance of hrtimer_interrupts_saved. We still can't
end up with a partially updated hrtimer_interrupts_saved, however,
because we end up petting all affected CPUs to make sure the new and
old CPU can't end up somehow read/write hrtimer_interrupts_saved at
the same time.

--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c

@@ -87,29 +87,34 @@ __setup("nmi_watchdog=", hardlockup_panic_setup);

 #if defined(CONFIG_HARDLOCKUP_DETECTOR_PERF)

-static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
-static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved);
+static DEFINE_PER_CPU(atomic_t, hrtimer_interrupts);
+static DEFINE_PER_CPU(int, hrtimer_interrupts_saved);
 static DEFINE_PER_CPU(bool, watchdog_hardlockup_warned);
 static unsigned long watchdog_hardlockup_all_cpu_dumped;

-static bool is_hardlockup(void)
+static bool is_hardlockup(unsigned int cpu)
 {
-     unsigned long hrint = __this_cpu_read(hrtimer_interrupts);
+     int hrint = atomic_read(&per_cpu(hrtimer_interrupts, cpu));

-     if (__this_cpu_read(hrtimer_interrupts_saved) == hrint)
+     if (per_cpu(hrtimer_interrupts_saved, cpu) == hrint)
              return true;

-     __this_cpu_write(hrtimer_interrupts_saved, hrint);
+     /*
+      * NOTE: we don't need any fancy atomic_t or READ_ONCE/WRITE_ONCE
+      * for hrtimer_interrupts_saved. hrtimer_interrupts_saved is
+      * written/read by a single CPU.
+      */
+     per_cpu(hrtimer_interrupts_saved, cpu) = hrint;

      return false;
 }

 static void watchdog_hardlockup_kick(void)
 {
-     __this_cpu_inc(hrtimer_interrupts);
+     atomic_inc(raw_cpu_ptr(&hrtimer_interrupts));

Is there any particular reason why raw_*() is needed, please?

My expectation is that the raw_ API should be used only when
there is a good reason for it. Where a good reason might be
when the checks might fail but the consistency is guaranteed
another way.

IMHO, we should use:

        atomic_inc(this_cpu_ptr(&hrtimer_interrupts));

To be honest, I am a bit lost in the per_cpu API definitions.

But this_cpu_ptr() seems to be implemented the same way as
per_cpu_ptr() when CONFIG_DEBUG_PREEMPT is enabled.
And we use per_cpu_ptr() in is_hardlockup().

Also this_cpu_ptr() is used more commonly:

$> git grep this_cpu_ptr | wc -l
1385
$> git grep raw_cpu_ptr | wc -l
114

Hmmm, I think maybe I confused myself. The old code purposely used the
double-underscore prefixed version of this_cpu_inc(). I couldn't find
a double-underscore version of this_cpu_ptr() and I somehow convinced
myself that the raw() version was the right equivalent version.

You're right that this_cpu_ptr() is a better choice here and I don't
see any reason why we'd need the raw version.

quoted

 }

-void watchdog_hardlockup_check(struct pt_regs *regs)
+void watchdog_hardlockup_check(unsigned int cpu, struct pt_regs *regs)
 {
      /*
       * Check for a hardlockup by making sure the CPU's timer

@@ -117,35 +122,42 @@ void watchdog_hardlockup_check(struct pt_regs *regs)
       * fired multiple times before we overflow'd. If it hasn't
       * then this is a good indication the cpu is stuck
       */
-     if (is_hardlockup()) {
+     if (is_hardlockup(cpu)) {
              unsigned int this_cpu = smp_processor_id();
+             struct cpumask backtrace_mask = *cpu_online_mask;

Does this work, please?

IMHO, we should use cpumask_copy().

Ah, good call, thanks!

quoted

              /* Only print hardlockups once. */
-             if (__this_cpu_read(watchdog_hardlockup_warned))
+             if (per_cpu(watchdog_hardlockup_warned, cpu))
                      return;

Otherwise, it looks good to me.

Neither change seems urgent though both are important to fix, I'll
wait a day or two to see if you have feedback on any of the other
patches and I'll send a fixup series.

-Doug

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