Re: [RFC PATCH 1/1] smp: Add tracepoints for functions called with smp_call_function*()

[Valentin Schneider <vschneid@redhat.com>]

On 10/05/23 17:27, Leonardo Brás wrote:
> On Thu, 2023-05-04 at 12:59 +0100, Valentin Schneider wrote:
>> +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)
>> +);
>
> This is for the caller side, right?
>

Yep, see usage lower down.

>> +
>> +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("func=%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)
>> +);
>
> Oh, this is what event_class is for. Thanks for the example :)
>
>> +
>> +#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 ab3e5dad6cfe9..7d28db303e9bc 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);
>> +}
>> +
>
> Good one, a helper to avoid calling those traces everywhere.
>
> Honest question:
> Since info == csd->info and func == csd->func, we could just pass csd, right?
> I suppose the suggestion on the 3-argument version is to use the values already
> fetched from memory instead of fetching them again. Is that correct?
>

There's also the special case of CSD_TYPE_TTWU where there is no csd->func,
instead we have an implicit func mapping to sched_ttwu_pending). I think
it's preferable to directly feed the right things to the TP than to
duplicate the "decoding" logic against the *csd passed as TP argument.