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

[Peter Zijlstra]

On Wed, Mar 22, 2023 at 12:20:28PM +0000, Valentin Schneider wrote:
> On 22/03/23 10:53, Peter Zijlstra wrote:

> > Hurmph... so we only really consume @func when we IPI. Would it not be
> > more useful to trace this thing for *every* csd enqeued?
> 
> It's true that any CSD enqueued on that CPU's call_single_queue in the
> [first CSD llist_add()'ed, IPI IRQ hits] timeframe is a potential source of
> interference.
> 
> However, can we be sure that first CSD isn't an indirect cause for the
> following ones? say the target CPU exits RCU EQS due to the IPI, there's a
> bit of time before it gets to flush_smp_call_function_queue() where some other CSD
> could be enqueued *because* of that change in state.
> 
> I couldn't find a easy example of that, I might be biased as this is where
> I'd like to go wrt IPI'ing isolated CPUs in usermode. But regardless, when
> correlating an IPI IRQ with its source, we'd always have to look at the
> first CSD in that CSD stack.

So I was thinking something like this:

---
Subject: trace,smp: Trace all smp_function_call*() invocations
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Wed Mar 22 14:58:36 CET 2023

(Ab)use the trace_ipi_send_cpu*() family to trace all
smp_function_call*() invocations, not only those that result in an
actual IPI.

The queued entries log their callback function while the actual IPIs
are traced on generic_smp_call_function_single_interrupt().

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/smp.c |   58 ++++++++++++++++++++++++++++++----------------------------
 1 file changed, 30 insertions(+), 28 deletions(-)

--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -106,18 +106,20 @@ void __init call_function_init(void)
 }
 
 static __always_inline void
-send_call_function_single_ipi(int cpu, smp_call_func_t func)
+send_call_function_single_ipi(int cpu)
 {
 	if (call_function_single_prep_ipi(cpu)) {
-		trace_ipi_send_cpu(cpu, _RET_IP_, func);
+		trace_ipi_send_cpu(cpu, _RET_IP_,
+				   generic_smp_call_function_single_interrupt);
 		arch_send_call_function_single_ipi(cpu);
 	}
 }
 
 static __always_inline void
-send_call_function_ipi_mask(const struct cpumask *mask, smp_call_func_t func)
+send_call_function_ipi_mask(const struct cpumask *mask)
 {
-	trace_ipi_send_cpumask(mask, _RET_IP_, func);
+	trace_ipi_send_cpumask(mask, _RET_IP_,
+			       generic_smp_call_function_single_interrupt);
 	arch_send_call_function_ipi_mask(mask);
 }
 
@@ -318,25 +320,6 @@ static __always_inline void csd_unlock(s
 	smp_store_release(&csd->node.u_flags, 0);
 }
 
-static __always_inline void
-raw_smp_call_single_queue(int cpu, struct llist_node *node, smp_call_func_t func)
-{
-	/*
-	 * The list addition should be visible to the target CPU when it pops
-	 * the head of the list to pull the entry off it in the IPI handler
-	 * because of normal cache coherency rules implied by the underlying
-	 * llist ops.
-	 *
-	 * If IPIs can go out of order to the cache coherency protocol
-	 * in an architecture, sufficient synchronisation should be added
-	 * to arch code to make it appear to obey cache coherency WRT
-	 * locking and barrier primitives. Generic code isn't really
-	 * equipped to do the right thing...
-	 */
-	if (llist_add(node, &per_cpu(call_single_queue, cpu)))
-		send_call_function_single_ipi(cpu, func);
-}
-
 static DEFINE_PER_CPU_SHARED_ALIGNED(call_single_data_t, csd_data);
 
 void __smp_call_single_queue(int cpu, struct llist_node *node)
@@ -356,10 +339,23 @@ void __smp_call_single_queue(int cpu, st
 		func = CSD_TYPE(csd) == CSD_TYPE_TTWU ?
 			sched_ttwu_pending : csd->func;
 
-		raw_smp_call_single_queue(cpu, node, func);
-	} else {
-		raw_smp_call_single_queue(cpu, node, NULL);
+		trace_ipi_send_cpu(cpu, _RET_IP_, func);
 	}
+
+	/*
+	 * The list addition should be visible to the target CPU when it pops
+	 * the head of the list to pull the entry off it in the IPI handler
+	 * because of normal cache coherency rules implied by the underlying
+	 * llist ops.
+	 *
+	 * If IPIs can go out of order to the cache coherency protocol
+	 * in an architecture, sufficient synchronisation should be added
+	 * to arch code to make it appear to obey cache coherency WRT
+	 * locking and barrier primitives. Generic code isn't really
+	 * equipped to do the right thing...
+	 */
+	if (llist_add(node, &per_cpu(call_single_queue, cpu)))
+		send_call_function_single_ipi(cpu);
 }
 
 /*
@@ -798,14 +794,20 @@ static void smp_call_function_many_cond(
 		}
 
 		/*
+		 * Trace each smp_function_call_*() as an IPI, actual IPIs
+		 * will be traced with func==generic_smp_call_function_single_ipi().
+		 */
+		trace_ipi_send_cpumask(cfd->cpumask_ipi, _RET_IP_, func);
+
+		/*
 		 * Choose the most efficient way to send an IPI. Note that the
 		 * number of CPUs might be zero due to concurrent changes to the
 		 * provided mask.
 		 */
 		if (nr_cpus == 1)
-			send_call_function_single_ipi(last_cpu, func);
+			send_call_function_single_ipi(last_cpu);
 		else if (likely(nr_cpus > 1))
-			send_call_function_ipi_mask(cfd->cpumask_ipi, func);
+			send_call_function_ipi_mask(cfd->cpumask_ipi);
 	}
 
 	if (run_local && (!cond_func || cond_func(this_cpu, info))) {