Subject: Re: [PATCH v3 net-next 4/4] net: switchdev: Add tracepoints

From: Tobias Waldekranz <[email-masked]>
Date: Mon, 04 Mar 2024 23:40:49 +0100

On ons, feb 28, 2024 at 09:56, Steven Rostedt <[email-masked]> wrote:
> On Wed, 28 Feb 2024 11:47:24 +0100
> Tobias Waldekranz <[email-masked]> wrote:
>
>> >> > +	TP_fast_assign(
>> >> > +		__entry->val = val;
>> >> > +		__assign_str(dev, info->dev ? netdev_name(info->dev) : "(null)");
>> >> > +		__entry->info = info;
>> >> > +		__entry->err = err;
>> >> > +		switchdev_notifier_str(val, info, __entry->msg, SWITCHDEV_TRACE_MSG_MAX);  
>> >> 
>> >> Is it possible to just store the information in the trace event and then
>> >> call the above function in the read stage?  
>> >
>> > I agree with Steven: it looks like that with the above code the
>> > tracepoint itself will become measurably costily in terms of CPU
>> > cycles: we want to avoid that.
>> >
>> > Perhaps using different tracepoints with different notifier_block type
>> > would help? so that each trace point could just copy a few specific
>> > fields.  
>> 
>> This can be done, but you will end up having to duplicate the decoding
>> and formatting logic from switchdev-str.c, with the additional hurdle of
>> having to figure out the sizes of all referenced objects in order to
>> create flattened versions of every notification type.
>
> Would it help if you could pass a trace_seq to it? The TP_printk() has a
> "magical" trace_seq variable that trace events can use in the TP_printk()
> called "p".
>
> Look at:
>
>   include/trace/events/libata.h:
>
> const char *libata_trace_parse_status(struct trace_seq*, unsigned char);
> #define __parse_status(s) libata_trace_parse_status(p, s)
>
> Where we have:
>
> const char *
> libata_trace_parse_status(struct trace_seq *p, unsigned char status)
> {
> 	const char *ret = trace_seq_buffer_ptr(p);
>
> 	trace_seq_printf(p, "{ ");
> 	if (status & ATA_BUSY)
> 		trace_seq_printf(p, "BUSY ");
> 	if (status & ATA_DRDY)
> 		trace_seq_printf(p, "DRDY ");
> 	if (status & ATA_DF)
> 		trace_seq_printf(p, "DF ");
> 	if (status & ATA_DSC)
> 		trace_seq_printf(p, "DSC ");
> 	if (status & ATA_DRQ)
> 		trace_seq_printf(p, "DRQ ");
> 	if (status & ATA_CORR)
> 		trace_seq_printf(p, "CORR ");
> 	if (status & ATA_SENSE)
> 		trace_seq_printf(p, "SENSE ");
> 	if (status & ATA_ERR)
> 		trace_seq_printf(p, "ERR ");
> 	trace_seq_putc(p, '}');
> 	trace_seq_putc(p, 0);
>
> 	return ret;
> }
>
> The "trace_seq p" is a pointer to trace_seq descriptor that can build
> strings, and then you can use it to print a custom string in the trace
> output.

Yes I managed to decode the hidden variable :) I also found
trace_seq_acquire() (and its macro alter ego __get_buf()), which would
let me keep the generic stringer functions. So far, so good.

I think the foundational problem remains though: TP_printk() is not
executed until a user reads from the trace_pipe; at which point the
object referenced by __entry->info may already be dead and
buried. Right?

>> 
>> What I like about the current approach is that when new notification and
>> object types are added, switchdev_notifier_str will automatically be
>> able to decode them and give you some rough idea of what is going on,
>> even if no new message specific decoding logic is added. It is also
>> reusable by drivers that might want to decode notifications or objects
>> in error messages.
>> 
>> Would some variant of (how I understand) Steven's suggestion to instead
>> store the formatted message in a dynamic array (__assign_str()), rather
>> than in the tracepoint entry, be acceptable?
>
> Matters if you could adapt using a trace_seq for the output. Or at least
> use a seq_buf, as that's what is under the covers of trace_seq. If you
> rather just use seq_buf, the above could pretty much be the same by passing
> in: &p->seq.
>
> -- Steve