Re: [PATCH tracing/kprobes 4/7] tracing/kprobes: Add event profiling support

[Masami Hiramatsu <mhiramat@redhat.com>]

Frederic Weisbecker wrote:
> On Thu, Sep 10, 2009 at 07:53:30PM -0400, Masami Hiramatsu wrote:
>> +#ifdef CONFIG_EVENT_PROFILE
>> +
>> +/* Kprobe profile handler */
>> +static __kprobes int kprobe_profile_func(struct kprobe *kp,
>> +					 struct pt_regs *regs)
>> +{
>> +	struct trace_probe *tp = container_of(kp, struct trace_probe, rp.kp);
>> +	struct ftrace_event_call *call =&tp->call;
>> +	struct kprobe_trace_entry *entry;
>> +	int size, i, pc;
>> +	unsigned long irq_flags;
>> +
>> +	local_save_flags(irq_flags);
>> +	pc = preempt_count();
>> +
>> +	size = SIZEOF_KPROBE_TRACE_ENTRY(tp->nr_args);
>
>
>
> Note that the end-result must be u64 aligned for perf ring buffer.
> And this is a bit tricky.
> What is inserted in the perf ring buffer is:
>
> raw_trace + (u32)raw_trace_size
>
> So we must ensure that sizeof(raw_trace) + sizeof(u32)
> is well u64 aligned.
>
> We don't insert the trace_size ourself though, this is done
> from kernel/perf_counter.c
>
> But we need to handle the size of the size (sorry) in the final
> alignment.
> To sum-up: sizeof(raw_trace) doesn't need (shouldn't) to be u64
> aligned but sizeof(raw_trace) + sizeof(u32) must be.
>
> Given this aligned size, we then substract it by sizeof(u32)
> to have the needed size of the raw entry.
>
> This result gives you the size of char raw_data[], which
> is also the same size passed in perf_tpcounter_event().
>
> See?

Ah, I see. So the size to write to perf_tpcounter_event must be
'(a multiple number of sizeof(u64)) - sizeof(u32)', right?

(Hmm, why would not perf_counter align data by itself? :)

>
> That's why we have this in trace/ftrace.h:
>
> __data_size = "the real entry data size"
> __entry_size = ALIGN(__data_size + sizeof(*entry) + sizeof(u32), sizeof(u64));
> __entry_size -= sizeof(u32);
>
> do {
> 		char raw_data[__entry_size];
> 		...
> 		perf_tpcounter_event(event_call->id, __addr, __count, entry,
> 			     __entry_size);
> 		...
> } while (0);

Ok, I'll do that.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com