[RFC PATCH v3 1/1] trace,smp: Add tracepoints around remotelly called functions

[RFC PATCH v3 1/1] trace,smp: Add tracepoints around remotelly called functions

[Leonardo Bras]

When running RT workloads in isolated CPUs, many cases of deadline misses
are caused by remote CPU requests such as smp_call_function*().

For those cases, having the names of those functions running around the
deadline miss moment could help (a lot) finding a target for the next
improvements.

Add tracepoints for acquiring the function name & csd before entry and
after returning from the remote-cpu requested function.

Also, add tracepoints on the remote cpus requesting them.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
---

Changes since RFCv2:
- Fixed some spacing issues and trace calls

Changes since RFCv1:
- Implemented trace_csd_queue_cpu() as suggested by Valentin Schneider
- Using EVENT_CLASS in order to avoid duplication
- Introduced new helper: csd_do_func()
- Name change from smp_call_function_* to csd_function_*
- Rebased on top of torvalds/master

 include/trace/events/smp.h | 72 ++++++++++++++++++++++++++++++++++++++
 kernel/smp.c               | 41 +++++++++++++---------
 2 files changed, 96 insertions(+), 17 deletions(-)
 create mode 100644 include/trace/events/smp.h

diff --git a/include/trace/events/smp.h b/include/trace/events/smp.h
new file mode 100644
index 000000000000..c304318a0203
--- /dev/null
+++ b/include/trace/events/smp.h
@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM smp
+
+#if !defined(_TRACE_SMP_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_SMP_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(csd_queue_cpu,
+
+	TP_PROTO(const unsigned int cpu,
+		 unsigned long callsite,
+		 smp_call_func_t func,
+		 call_single_data_t *csd),
+
+	TP_ARGS(cpu, callsite, func, csd),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, cpu)
+		__field(void *, callsite)
+		__field(void *, func)
+		__field(void *, csd)
+	),
+
+	TP_fast_assign(
+		__entry->cpu = cpu;
+		__entry->callsite = (void *)callsite;
+		__entry->func = func;
+		__entry->csd  = csd;
+	),
+
+	TP_printk("cpu=%u callsite=%pS func=%pS csd=%p",
+		  __entry->cpu, __entry->callsite, __entry->func, __entry->csd)
+);
+
+/*
+ * Tracepoints for a function which is called as an effect of smp_call_function.*
+ */
+DECLARE_EVENT_CLASS(csd_function,
+
+	TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
+
+	TP_ARGS(func, csd),
+
+	TP_STRUCT__entry(
+		__field(void *,	func)
+		__field(void *,	csd)
+	),
+
+	TP_fast_assign(
+		__entry->func	= func;
+		__entry->csd	= csd;
+	),
+
+	TP_printk("function %ps, csd = %p", __entry->func, __entry->csd)
+);
+
+DEFINE_EVENT(csd_function, csd_function_entry,
+	TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
+	TP_ARGS(func, csd)
+);
+
+DEFINE_EVENT(csd_function, csd_function_exit,
+	TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
+	TP_ARGS(func, csd)
+);
+
+#endif /* _TRACE_SMP_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>
diff --git a/kernel/smp.c b/kernel/smp.c
index ab3e5dad6cfe..cada433c5c1f 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -27,6 +27,9 @@
 #include <linux/jump_label.h>
 
 #include <trace/events/ipi.h>
+#define CREATE_TRACE_POINTS
+#include <trace/events/smp.h>
+#undef CREATE_TRACE_POINTS
 
 #include "smpboot.h"
 #include "sched/smp.h"
@@ -121,6 +124,14 @@ send_call_function_ipi_mask(struct cpumask *mask)
 	arch_send_call_function_ipi_mask(mask);
 }
 
+static __always_inline void
+csd_do_func(smp_call_func_t func, void *info, call_single_data_t *csd)
+{
+	trace_csd_function_entry(func, csd);
+	func(info);
+	trace_csd_function_exit(func, csd);
+}
+
 #ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
 
 static DEFINE_STATIC_KEY_MAYBE(CONFIG_CSD_LOCK_WAIT_DEBUG_DEFAULT, csdlock_debug_enabled);
@@ -329,7 +340,7 @@ void __smp_call_single_queue(int cpu, struct llist_node *node)
 	 * even if we haven't sent the smp_call IPI yet (e.g. the stopper
 	 * executes migration_cpu_stop() on the remote CPU).
 	 */
-	if (trace_ipi_send_cpu_enabled()) {
+	if (trace_csd_queue_cpu_enabled()) {
 		call_single_data_t *csd;
 		smp_call_func_t func;
 
@@ -337,7 +348,7 @@ void __smp_call_single_queue(int cpu, struct llist_node *node)
 		func = CSD_TYPE(csd) == CSD_TYPE_TTWU ?
 			sched_ttwu_pending : csd->func;
 
-		trace_ipi_send_cpu(cpu, _RET_IP_, func);
+		trace_csd_queue_cpu(cpu, _RET_IP_, func, csd);
 	}
 
 	/*
@@ -375,7 +386,7 @@ static int generic_exec_single(int cpu, struct __call_single_data *csd)
 		csd_lock_record(csd);
 		csd_unlock(csd);
 		local_irq_save(flags);
-		func(info);
+		csd_do_func(func, info, csd);
 		csd_lock_record(NULL);
 		local_irq_restore(flags);
 		return 0;
@@ -477,7 +488,7 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
 			}
 
 			csd_lock_record(csd);
-			func(info);
+			csd_do_func(func, info, csd);
 			csd_unlock(csd);
 			csd_lock_record(NULL);
 		} else {
@@ -508,7 +519,7 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
 
 				csd_lock_record(csd);
 				csd_unlock(csd);
-				func(info);
+				csd_do_func(func, info, csd);
 				csd_lock_record(NULL);
 			} else if (type == CSD_TYPE_IRQ_WORK) {
 				irq_work_single(csd);
@@ -522,8 +533,10 @@ static void __flush_smp_call_function_queue(bool warn_cpu_offline)
 	/*
 	 * Third; only CSD_TYPE_TTWU is left, issue those.
 	 */
-	if (entry)
-		sched_ttwu_pending(entry);
+	if (entry) {
+		csd = llist_entry(entry, typeof(*csd), node.llist);
+		csd_do_func(sched_ttwu_pending, entry, csd);
+	}
 }
 
 
@@ -728,7 +741,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 	int cpu, last_cpu, this_cpu = smp_processor_id();
 	struct call_function_data *cfd;
 	bool wait = scf_flags & SCF_WAIT;
-	int nr_cpus = 0, nr_queued = 0;
+	int nr_cpus = 0;
 	bool run_remote = false;
 	bool run_local = false;
 
@@ -786,21 +799,15 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 			csd->node.src = smp_processor_id();
 			csd->node.dst = cpu;
 #endif
+			trace_csd_queue_cpu(cpu, _RET_IP_, func, csd);
+
 			if (llist_add(&csd->node.llist, &per_cpu(call_single_queue, cpu))) {
 				__cpumask_set_cpu(cpu, cfd->cpumask_ipi);
 				nr_cpus++;
 				last_cpu = cpu;
 			}
-			nr_queued++;
 		}
 
-		/*
-		 * Trace each smp_function_call_*() as an IPI, actual IPIs
-		 * will be traced with func==generic_smp_call_function_single_ipi().
-		 */
-		if (nr_queued)
-			trace_ipi_send_cpumask(cfd->cpumask, _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
@@ -816,7 +823,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
 		unsigned long flags;
 
 		local_irq_save(flags);
-		func(info);
+		csd_do_func(func, info, NULL);
 		local_irq_restore(flags);
 	}
 
-- 
2.40.1

Re: [RFC PATCH v3 1/1] trace,smp: Add tracepoints around remotelly called functions

[Valentin Schneider]

On 10/05/23 20:01, Leonardo Bras wrote:
> When running RT workloads in isolated CPUs, many cases of deadline misses
> are caused by remote CPU requests such as smp_call_function*().
>
> For those cases, having the names of those functions running around the
> deadline miss moment could help (a lot) finding a target for the next
> improvements.
>
> Add tracepoints for acquiring the function name & csd before entry and
> after returning from the remote-cpu requested function.
>
> Also, add tracepoints on the remote cpus requesting them.

Per the v1 thread, I got some context as to why we want this, but this
changelog is quite sparse on the subject. How about:

"""
The recently added ipi_send_{cpu,cpumask} tracepoints allow finding sources
of IPIs targeting CPUs running latency-sensitive applications.

For NOHZ_FULL CPUs, all IPIs are interference, and those tracepoints are
sufficient to find them and work on getting rid of them. In some setups
however, not *all* IPIs are to be suppressed, but long-running IPI
callbacks can still be problematic.

Add a pair of tracepoints to mark the start and end of processing a CSD IPI
callback, similar to what exists for softirq, workqueue or timer callbacks.
"""

And you can probably split the csd_queue_cpu TP into a separate commit,
with something along the lines of:

"""
Add a tracepoint for when a CSD is queued to a remote CPU's
call_single_queue. This allows finding exactly which CPU queued a given CSD
when looking at a csd_function_{entry,exit} event, and also enables us to
accurately measure IPI delivery time with e.g. a synthetic event:

  $ echo 'hist:keys=cpu,csd.hex:ts=common_timestamp.usecs' >\
      /sys/kernel/tracing/events/smp/csd_queue_cpu/trigger
  $ echo 'csd_latency unsigned int dst_cpu; unsigned long csd; u64 time' >\
      /sys/kernel/tracing/synthetic_events
  $ echo \
  'hist:keys=common_cpu,csd.hex:'\
  'time=common_timestamp.usecs-$ts:'\
  'onmatch(smp.csd_queue_cpu).trace(csd_latency,common_cpu,csd,$time)' >\
      /sys/kernel/tracing/events/smp/csd_function_entry/trigger

  $ trace-cmd record -e 'synthetic:csd_latency' hackbench
  $ trace-cmd report
  <...>-467   [001]    21.824263: csd_queue_cpu:        cpu=0 callsite=try_to_wake_up+0x2ea func=sched_ttwu_pending csd=0xffff8880076148b8
  <...>-467   [001]    21.824280: ipi_send_cpu:         cpu=0 callsite=try_to_wake_up+0x2ea callback=generic_smp_call_function_single_interrupt+0x0
  <...>-489   [000]    21.824299: csd_function_entry:   func=sched_ttwu_pending csd=0xffff8880076148b8
  <...>-489   [000]    21.824320: csd_latency:          dst_cpu=0, csd=18446612682193848504, time=36
"""

>
> Signed-off-by: Leonardo Bras <leobras@redhat.com>

Overall I like the CSD vs IPI split, it's a saner approach than logging it
all as IPIs, even if it does generate more events as we have to emit an
event for every csd queued (i.e. no _cpumask() variant is possible).

Some nitpicks below, and one other thing: are we happy with the smp event
namespace, and with the new TP names? Should the namespace be csd instead,
to match the trace_<namespace>_* nomenclature?

> ---
>
> Changes since RFCv2:
> - Fixed some spacing issues and trace calls
>
> Changes since RFCv1:
> - Implemented trace_csd_queue_cpu() as suggested by Valentin Schneider
> - Using EVENT_CLASS in order to avoid duplication
> - Introduced new helper: csd_do_func()
> - Name change from smp_call_function_* to csd_function_*
> - Rebased on top of torvalds/master
>
>  include/trace/events/smp.h | 72 ++++++++++++++++++++++++++++++++++++++
>  kernel/smp.c               | 41 +++++++++++++---------
>  2 files changed, 96 insertions(+), 17 deletions(-)
>  create mode 100644 include/trace/events/smp.h
>
> diff --git a/include/trace/events/smp.h b/include/trace/events/smp.h
> new file mode 100644
> index 000000000000..c304318a0203
> --- /dev/null
> +++ b/include/trace/events/smp.h
> @@ -0,0 +1,72 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#undef TRACE_SYSTEM
> +#define TRACE_SYSTEM smp
> +
> +#if !defined(_TRACE_SMP_H) || defined(TRACE_HEADER_MULTI_READ)
> +#define _TRACE_SMP_H
> +
> +#include <linux/tracepoint.h>
> +
> +TRACE_EVENT(csd_queue_cpu,
> +
> +	TP_PROTO(const unsigned int cpu,
> +		 unsigned long callsite,
> +		 smp_call_func_t func,
> +		 call_single_data_t *csd),
> +
> +	TP_ARGS(cpu, callsite, func, csd),
> +
> +	TP_STRUCT__entry(
> +		__field(unsigned int, cpu)
> +		__field(void *, callsite)
> +		__field(void *, func)
> +		__field(void *, csd)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->cpu = cpu;
> +		__entry->callsite = (void *)callsite;
> +		__entry->func = func;
> +		__entry->csd  = csd;
> +	),
> +
> +	TP_printk("cpu=%u callsite=%pS func=%pS csd=%p",
                                       ^^^^^^^^
This can be func=%ps, offsets should always be 0 so not very useful (I know
I put %pS for the IPI TPs, that's a force of habit...)

> +		  __entry->cpu, __entry->callsite, __entry->func, __entry->csd)
> +);

> +
> +/*
> + * Tracepoints for a function which is called as an effect of smp_call_function.*
> + */
> +DECLARE_EVENT_CLASS(csd_function,
> +
> +	TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
> +
> +	TP_ARGS(func, csd),
> +
> +	TP_STRUCT__entry(
> +		__field(void *,	func)
> +		__field(void *,	csd)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->func	= func;
> +		__entry->csd	= csd;
> +	),
> +
> +	TP_printk("function %ps, csd = %p", __entry->func, __entry->csd)

To match the style of the other TPs, that should be:

        TP_printk("func=%ps csd=%p", __entry->func, __entry->csd)

Re: [RFC PATCH v3 1/1] trace,smp: Add tracepoints around remotelly called functions

[Leonardo Brás]

On Thu, 2023-05-11 at 11:56 +0100, Valentin Schneider wrote:
> On 10/05/23 20:01, Leonardo Bras wrote:
> > When running RT workloads in isolated CPUs, many cases of deadline misses
> > are caused by remote CPU requests such as smp_call_function*().
> > 
> > For those cases, having the names of those functions running around the
> > deadline miss moment could help (a lot) finding a target for the next
> > improvements.
> > 
> > Add tracepoints for acquiring the function name & csd before entry and
> > after returning from the remote-cpu requested function.
> > 
> > Also, add tracepoints on the remote cpus requesting them.
> 
> Per the v1 thread, I got some context as to why we want this, but this
> changelog is quite sparse on the subject. How about:
> 
> """
> The recently added ipi_send_{cpu,cpumask} tracepoints allow finding sources
> of IPIs targeting CPUs running latency-sensitive applications.
> 
> For NOHZ_FULL CPUs, all IPIs are interference, and those tracepoints are
> sufficient to find them and work on getting rid of them. In some setups
> however, not *all* IPIs are to be suppressed, but long-running IPI
> callbacks can still be problematic.
> 
> Add a pair of tracepoints to mark the start and end of processing a CSD IPI
> callback, similar to what exists for softirq, workqueue or timer callbacks.
> """
> 
> And you can probably split the csd_queue_cpu TP into a separate commit,
> with something along the lines of:
> 
> """
> Add a tracepoint for when a CSD is queued to a remote CPU's
> call_single_queue. This allows finding exactly which CPU queued a given CSD
> when looking at a csd_function_{entry,exit} event, and also enables us to
> accurately measure IPI delivery time with e.g. a synthetic event:
> 
>   $ echo 'hist:keys=cpu,csd.hex:ts=common_timestamp.usecs' >\
>       /sys/kernel/tracing/events/smp/csd_queue_cpu/trigger
>   $ echo 'csd_latency unsigned int dst_cpu; unsigned long csd; u64 time' >\
>       /sys/kernel/tracing/synthetic_events
>   $ echo \
>   'hist:keys=common_cpu,csd.hex:'\
>   'time=common_timestamp.usecs-$ts:'\
>   'onmatch(smp.csd_queue_cpu).trace(csd_latency,common_cpu,csd,$time)' >\
>       /sys/kernel/tracing/events/smp/csd_function_entry/trigger
> 
>   $ trace-cmd record -e 'synthetic:csd_latency' hackbench
>   $ trace-cmd report
>   <...>-467   [001]    21.824263: csd_queue_cpu:        cpu=0 callsite=try_to_wake_up+0x2ea func=sched_ttwu_pending csd=0xffff8880076148b8
>   <...>-467   [001]    21.824280: ipi_send_cpu:         cpu=0 callsite=try_to_wake_up+0x2ea callback=generic_smp_call_function_single_interrupt+0x0
>   <...>-489   [000]    21.824299: csd_function_entry:   func=sched_ttwu_pending csd=0xffff8880076148b8
>   <...>-489   [000]    21.824320: csd_latency:          dst_cpu=0, csd=18446612682193848504, time=36
> """

Sure, I will split the patch and add the commit msg as suggested.


> 
> > 
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> 
> Overall I like the CSD vs IPI split, it's a saner approach than logging it
> all as IPIs, even if it does generate more events as we have to emit an
> event for every csd queued (i.e. no _cpumask() variant is possible).
> 
> Some nitpicks below, and one other thing: are we happy with the smp event
> namespace, and with the new TP names? Should the namespace be csd instead,
> to match the trace_<namespace>_* nomenclature?

I selected "smp.h" filename because those tracepoints are getting used on smp.c.

I think keeping a generic name is good for any future tracepoints people end up
designing for smp.c, but I am open to discussion.

> 
> > ---
> > 
> > Changes since RFCv2:
> > - Fixed some spacing issues and trace calls
> > 
> > Changes since RFCv1:
> > - Implemented trace_csd_queue_cpu() as suggested by Valentin Schneider
> > - Using EVENT_CLASS in order to avoid duplication
> > - Introduced new helper: csd_do_func()
> > - Name change from smp_call_function_* to csd_function_*
> > - Rebased on top of torvalds/master
> > 
> >  include/trace/events/smp.h | 72 ++++++++++++++++++++++++++++++++++++++
> >  kernel/smp.c               | 41 +++++++++++++---------
> >  2 files changed, 96 insertions(+), 17 deletions(-)
> >  create mode 100644 include/trace/events/smp.h
> > 
> > diff --git a/include/trace/events/smp.h b/include/trace/events/smp.h
> > new file mode 100644
> > index 000000000000..c304318a0203
> > --- /dev/null
> > +++ b/include/trace/events/smp.h
> > @@ -0,0 +1,72 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#undef TRACE_SYSTEM
> > +#define TRACE_SYSTEM smp
> > +
> > +#if !defined(_TRACE_SMP_H) || defined(TRACE_HEADER_MULTI_READ)
> > +#define _TRACE_SMP_H
> > +
> > +#include <linux/tracepoint.h>
> > +
> > +TRACE_EVENT(csd_queue_cpu,
> > +
> > +	TP_PROTO(const unsigned int cpu,
> > +		 unsigned long callsite,
> > +		 smp_call_func_t func,
> > +		 call_single_data_t *csd),
> > +
> > +	TP_ARGS(cpu, callsite, func, csd),
> > +
> > +	TP_STRUCT__entry(
> > +		__field(unsigned int, cpu)
> > +		__field(void *, callsite)
> > +		__field(void *, func)
> > +		__field(void *, csd)
> > +	),
> > +
> > +	TP_fast_assign(
> > +		__entry->cpu = cpu;
> > +		__entry->callsite = (void *)callsite;
> > +		__entry->func = func;
> > +		__entry->csd  = csd;
> > +	),
> > +
> > +	TP_printk("cpu=%u callsite=%pS func=%pS csd=%p",
>                                        ^^^^^^^^
> This can be func=%ps, offsets should always be 0 so not very useful (I know
> I put %pS for the IPI TPs, that's a force of habit...)
> 

I agree, let's do as suggested.

> > +		  __entry->cpu, __entry->callsite, __entry->func, __entry->csd)
> > +);
> 
> > +
> > +/*
> > + * Tracepoints for a function which is called as an effect of smp_call_function.*
> > + */
> > +DECLARE_EVENT_CLASS(csd_function,
> > +
> > +	TP_PROTO(smp_call_func_t func, call_single_data_t *csd),
> > +
> > +	TP_ARGS(func, csd),
> > +
> > +	TP_STRUCT__entry(
> > +		__field(void *,	func)
> > +		__field(void *,	csd)
> > +	),
> > +
> > +	TP_fast_assign(
> > +		__entry->func	= func;
> > +		__entry->csd	= csd;
> > +	),
> > +
> > +	TP_printk("function %ps, csd = %p", __entry->func, __entry->csd)
> 
> To match the style of the other TPs, that should be:
> 
>         TP_printk("func=%ps csd=%p", __entry->func, __entry->csd)
> 

Done!


Thanks for reviewing!
Leo