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Date:   Thu, 23 Mar 2023 00:14:28 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Valentin Schneider <vschneid@redhat.com>
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        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI
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On Wed, Mar 22, 2023 at 06:22:28PM +0000, Valentin Schneider wrote:
> On 22/03/23 18:22, Peter Zijlstra wrote:

> >>        hackbench-157   [001]    10.894320: ipi_send_cpu:         cpu=3 callsite=check_preempt_curr+0x37 callback=0x0
> >
> > Arguably we should be setting callback to scheduler_ipi(), except
> > ofcourse, that's not an actual function...
> >
> > Maybe we can do "extern inline" for the actual users and provide a dummy
> > function for the symbol when tracing.
> >
> 
> Huh, I wasn't aware that was an option, I'll look into that. I did scribble
> down a comment next to smp_send_reschedule(), but having a decodable
> function name would be better!

So clang-15 builds the below (and generates the expected code), but
gcc-12 vomits nonsense about a non-static inline calling a static inline
or somesuch bollocks :-/

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1991,7 +1991,7 @@ extern char *__get_task_comm(char *to, s
 })
 
 #ifdef CONFIG_SMP
-static __always_inline void scheduler_ipi(void)
+extern __always_inline void scheduler_ipi(void)
 {
 	/*
 	 * Fold TIF_NEED_RESCHED into the preempt_count; anybody setting
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -130,9 +130,9 @@ extern void arch_smp_send_reschedule(int
  * scheduler_ipi() is inline so can't be passed as callback reason, but the
  * callsite IP should be sufficient for root-causing IPIs sent from here.
  */
-#define smp_send_reschedule(cpu) ({		  \
-	trace_ipi_send_cpu(cpu, _RET_IP_, NULL);  \
-	arch_smp_send_reschedule(cpu);		  \
+#define smp_send_reschedule(cpu) ({				\
+	trace_ipi_send_cpu(cpu, _RET_IP_, &scheduler_ipi);	\
+	arch_smp_send_reschedule(cpu);				\
 })
 
 /*
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3790,6 +3790,15 @@ static int ttwu_runnable(struct task_str
 }
 
 #ifdef CONFIG_SMP
+void scheduler_ipi(void)
+{
+	/*
+	 * Actual users should end up using the extern inline, this is only
+	 * here for the symbol.
+	 */
+	BUG();
+}
+
 void sched_ttwu_pending(void *arg)
 {
 	struct llist_node *llist = arg;