Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI

From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <vschneid@redhat.com>
Cc: linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
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	Frederic Weisbecker <frederic@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Marc Zyngier <maz@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	Nicholas Piggin <npiggin@gmail.com>, Guo Ren <guoren@kernel.org>,
	"David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH v5 7/7] sched, smp: Trace smp callback causing an IPI
Date: Thu, 23 Mar 2023 00:14:28 +0100	[thread overview]
Message-ID: <20230322231428.GV2017917@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <xhsmhh6ucd4t7.mognet@vschneid.remote.csb>

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;

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