linux-kernel - Re: [PATCH 2/4] watchdog/softlockup: Replace "watchdog/%u" threads with cpu_stop

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Date:   Fri, 8 Jun 2018 15:57:04 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, gkohli@...eaurora.org, tglx@...utronix.de,
        mpe@...erman.id.au, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, will.deacon@....com
Subject: Re: [PATCH 2/4] watchdog/softlockup: Replace "watchdog/%u" threads
 with cpu_stop_work

On 06/07, Peter Zijlstra wrote:
>
> On Thu, Jun 07, 2018 at 02:33:12PM +0200, Peter Zijlstra wrote:
>
> > +static int softlockup_stop_fn(void *data)
> >  {
> > +	watchdog_disable(smp_processor_id());
> > +	return 0;
> >  }
> >
> > +static void softlockup_stop_all(void)
> >  {
> > +	int cpu;
> > +
> > +	for_each_cpu(cpu, &watchdog_allowed_mask)
> > +		stop_one_cpu(cpu, softlockup_stop_fn, NULL);
> > +
> > +	cpumask_clear(&watchdog_allowed_mask);
> >  }
>
> Bugger, that one doesn't quite work.. watchdog_disable() ends up calling
> a sleeping function. I forgot to enable all the debug cruft when
> testing..

And probably there is another problem. Both watchdog_disable(cpu) and
watchdog_nmi_disable(cpu) assume that cpu == smp_processor_id(), this arg
is simply ignored.

but lockup_detector_offline_cpu(cpu) is called by cpuhp_invoke_callback(),
so in this case watchdog_disable(dying_cpu) is simply wrong.

May be we can do something like below? Then softlockup_stop_all() can simply do

	for_each_cpu(cpu, &watchdog_allowed_mask)
		watchdog_disable(cpu);

watchdog_nmi_disable() is __weak, but at first glance arch/sparc/kernel/nmi.c
does everything correctly.

Oleg.

--- x/kernel/watchdog.c
+++ x/kernel/watchdog.c
@@ -108,13 +108,13 @@ __setup("hardlockup_all_cpu_backtrace=",
  */
 int __weak watchdog_nmi_enable(unsigned int cpu)
 {
-	hardlockup_detector_perf_enable();
+	hardlockup_detector_perf_enable(cpu);
 	return 0;
 }
 
 void __weak watchdog_nmi_disable(unsigned int cpu)
 {
-	hardlockup_detector_perf_disable();
+	hardlockup_detector_perf_disable(cpu);
 }
 
 /* Return 0, if a NMI watchdog is available. Error code otherwise */
@@ -479,7 +479,7 @@ static void watchdog_enable(unsigned int
 
 static void watchdog_disable(unsigned int cpu)
 {
-	struct hrtimer *hrtimer = this_cpu_ptr(&watchdog_hrtimer);
+	struct hrtimer *hrtimer = per_cpu_ptr(&watchdog_hrtimer, cpu);
 
 	watchdog_set_prio(SCHED_NORMAL, 0);
 	/*
--- x/kernel/watchdog_hld.c
+++ x/kernel/watchdog_hld.c
@@ -162,9 +162,8 @@ static void watchdog_overflow_callback(s
 	return;
 }
 
-static int hardlockup_detector_event_create(void)
+static int hardlockup_detector_event_create(int cpu)
 {
-	unsigned int cpu = smp_processor_id();
 	struct perf_event_attr *wd_attr;
 	struct perf_event *evt;
 
@@ -179,37 +178,37 @@ static int hardlockup_detector_event_cre
 			PTR_ERR(evt));
 		return PTR_ERR(evt);
 	}
-	this_cpu_write(watchdog_ev, evt);
+	raw_cpu_write(watchdog_ev, evt);
 	return 0;
 }
 
 /**
  * hardlockup_detector_perf_enable - Enable the local event
  */
-void hardlockup_detector_perf_enable(void)
+void hardlockup_detector_perf_enable(int cpu)
 {
-	if (hardlockup_detector_event_create())
+	if (hardlockup_detector_event_create(cpu))
 		return;
 
 	/* use original value for check */
 	if (!atomic_fetch_inc(&watchdog_cpus))
 		pr_info("Enabled. Permanently consumes one hw-PMU counter.\n");
 
-	perf_event_enable(this_cpu_read(watchdog_ev));
+	perf_event_enable(raw_cpu_read(watchdog_ev));
 }
 
 /**
  * hardlockup_detector_perf_disable - Disable the local event
  */
-void hardlockup_detector_perf_disable(void)
+void hardlockup_detector_perf_disable(int cpu)
 {
-	struct perf_event *event = this_cpu_read(watchdog_ev);
+	struct perf_event *event = per_cpu_read(watchdog_ev, cpu);
 
 	if (event) {
 		perf_event_disable(event);
-		this_cpu_write(watchdog_ev, NULL);
-		this_cpu_write(dead_event, event);
-		cpumask_set_cpu(smp_processor_id(), &dead_events_mask);
+		raw_cpu_write(watchdog_ev, cpu, NULL);
+		raw_cpu_write(dead_event, cpu, event);
+		cpumask_set_cpu(cpu, &dead_events_mask);
 		atomic_dec(&watchdog_cpus);
 	}
 }