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From:   Valentin Schneider <vschneid@redhat.com>
To:     Leonardo =?utf-8?Q?Br=C3=A1s?= <leobras@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yury Norov <yury.norov@gmail.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Nadav Amit <namit@vmware.com>,
        Zhen Lei <thunder.leizhen@huawei.com>,
        Chen Zhongjin <chenzhongjin@huawei.com>,
        linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Daniel Bristot de Oliveira <bristot@kernel.org>
Subject: Re: [RFC PATCH 1/1] smp: Add tracepoints for functions called with
 smp_call_function*()
In-Reply-To: <f96442e9166e4f73be772656a93c89093eddbb8b.camel@redhat.com>
References: <20230406075718.68672-1-leobras@redhat.com>
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 <f294a74cd17bf932621a6a746d592f49770d25ec.camel@redhat.com>
 <20230503145900.GC1689514@hirez.programming.kicks-ass.net>
 <xhsmh7ctojoia.mognet@vschneid.remote.csb>
 <f96442e9166e4f73be772656a93c89093eddbb8b.camel@redhat.com>
Date:   Thu, 11 May 2023 09:13:27 +0100
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On 10/05/23 17:27, Leonardo Br=C3=A1s 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 =3D cpu;
>> +		__entry->callsite =3D (void *)callsite;
>> +		__entry->func =3D func;
>> +		__entry->csd  =3D csd;
>> +	),
>> +
>> +	TP_printk("cpu=3D%u callsite=3D%pS func=3D%pS csd=3D%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	=3D func;
>> +		__entry->csd	=3D csd;
>> +	),
>> +
>> +	TP_printk("func=3D%pS csd=3D%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 =3D=3D csd->info and func =3D=3D csd->func, we could just pass=
 csd, right?
> I suppose the suggestion on the 3-argument version is to use the values a=
lready
> 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.