linux-kernel - Re: [patch 14/41] x86/smp: Use static key for reschedule interrupt tracing

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Date:   Fri, 25 Aug 2017 22:07:33 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [patch 14/41] x86/smp: Use static key for reschedule interrupt
 tracing

On Fri, 25 Aug 2017, Steven Rostedt wrote:
> On Fri, 25 Aug 2017 12:31:17 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> 
> > -__visible void __irq_entry smp_trace_reschedule_interrupt(struct pt_regs *regs)
> > -{
> > -	/*
> > -	 * Need to call irq_enter() before calling the trace point.
> > -	 * __smp_reschedule_interrupt() calls irq_enter/exit() too (in
> > -	 * scheduler_ipi(). This is OK, since those functions are allowed
> > -	 * to nest.
> > -	 */
> > -	ipi_entering_ack_irq();
> > -	trace_reschedule_entry(RESCHEDULE_VECTOR);
> > +	if (trace_irqvectors_enabled()) {
> > +		/*
> > +		 * scheduler_ipi() might call irq_enter() as well, but
> > +		 * nested calls are fine.
> > +		 */
> > +		irq_enter();
> > +		trace_reschedule_entry(RESCHEDULE_VECTOR);
> > +	}
> > +
> >  	inc_irq_stat(irq_resched_count);
> >  	scheduler_ipi();
> > -	trace_reschedule_exit(RESCHEDULE_VECTOR);
> > -	exiting_irq();
> > +
> > +	if (trace_irqvectors_enabled()) {
> > +		trace_reschedule_exit(RESCHEDULE_VECTOR);
> > +		irq_exit();
> 
> This is racy. If the static key gets changed on one and not the other,
> we lose the irq_enter/irq_exit match.

Indeed. Good point. Fixed up version below.

Thanks,

	tglx
8<---------------------
--- a/arch/x86/include/asm/hw_irq.h
+++ b/arch/x86/include/asm/hw_irq.h
@@ -48,7 +48,7 @@ extern asmlinkage void call_function_sin
 
 #ifdef CONFIG_TRACING
 /* Interrupt handlers registered during init_IRQ */
-extern void trace_reschedule_interrupt(void);
+#define trace_reschedule_interrupt	reschedule_interrupt
 #define trace_call_function_interrupt	call_function_interrupt
 #define trace_call_function_single_interrupt	call_function_single_interrupt
 #define trace_thermal_interrupt	thermal_interrupt
--- a/arch/x86/kernel/smp.c
+++ b/arch/x86/kernel/smp.c
@@ -254,37 +254,27 @@ static void native_stop_other_cpus(int w
 }
 
 /*
- * Reschedule call back.
+ * Reschedule call back. KVM uses this interrupt to force a cpu out of
+ * guest mode
  */
-static inline void __smp_reschedule_interrupt(void)
-{
-	inc_irq_stat(irq_resched_count);
-	scheduler_ipi();
-}
-
 __visible void __irq_entry smp_reschedule_interrupt(struct pt_regs *regs)
 {
 	ack_APIC_irq();
-	__smp_reschedule_interrupt();
-	/*
-	 * KVM uses this interrupt to force a cpu out of guest mode
-	 */
-}
-
-__visible void __irq_entry smp_trace_reschedule_interrupt(struct pt_regs *regs)
-{
-	/*
-	 * Need to call irq_enter() before calling the trace point.
-	 * __smp_reschedule_interrupt() calls irq_enter/exit() too (in
-	 * scheduler_ipi(). This is OK, since those functions are allowed
-	 * to nest.
-	 */
-	ipi_entering_ack_irq();
-	trace_reschedule_entry(RESCHEDULE_VECTOR);
 	inc_irq_stat(irq_resched_count);
+
+	if (trace_irqvectors_enabled()) {
+		/*
+		 * scheduler_ipi() might call irq_enter() as well, but
+		 * nested calls are fine.
+		 */
+		irq_enter();
+		trace_reschedule_entry(RESCHEDULE_VECTOR);
+		scheduler_ipi();
+		trace_reschedule_exit(RESCHEDULE_VECTOR);
+		irq_exit();
+		return;
+	}
 	scheduler_ipi();
-	trace_reschedule_exit(RESCHEDULE_VECTOR);
-	exiting_irq();
 }
 
 __visible void __irq_entry smp_call_function_interrupt(struct pt_regs *regs)