linux-kernel - Re: [PATCH 10/17] hyper-v: trace vmbus

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Date:   Mon, 30 Oct 2017 11:07:01 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     kys@...rosoft.com, linux-kernel@...r.kernel.org,
        devel@...uxdriverproject.org, olaf@...fle.de, apw@...onical.com,
        jasowang@...hat.com, leann.ogasawara@...onical.com,
        marcelo.cerri@...onical.com, sthemmin@...rosoft.com,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 10/17] hyper-v: trace vmbus_open()

Greg KH <gregkh@...uxfoundation.org> writes:

> On Mon, Oct 30, 2017 at 09:16:19AM +0100, Vitaly Kuznetsov wrote:
>> Greg KH <gregkh@...uxfoundation.org> writes:
>> 
>> > On Sun, Oct 29, 2017 at 12:21:09PM -0700, kys@...hange.microsoft.com wrote:
>> >> From: Vitaly Kuznetsov <vkuznets@...hat.com>
>> >> 
>> >> Add tracepoint to CHANNELMSG_OPENCHANNEL sender.
>> >> 
>> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> >> Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
>> >> ---
>> >>  drivers/hv/channel.c  |  2 ++
>> >>  drivers/hv/hv_trace.h | 27 +++++++++++++++++++++++++++
>> >>  2 files changed, 29 insertions(+)
>> >> 
>> >> diff --git a/drivers/hv/channel.c b/drivers/hv/channel.c
>> >> index a406beb10dd0..739b3fe1e0fb 100644
>> >> --- a/drivers/hv/channel.c
>> >> +++ b/drivers/hv/channel.c
>> >> @@ -185,6 +185,8 @@ int vmbus_open(struct vmbus_channel *newchannel, u32 send_ringbuffer_size,
>> >>  	ret = vmbus_post_msg(open_msg,
>> >>  			     sizeof(struct vmbus_channel_open_channel), true);
>> >>  
>> >> +	trace_vmbus_open(open_msg, ret);
>> >
>> > Why add tracepoints for things that ftrace can handle automatically?
>> 
>> This series adds pretty prints for structures printing what is needed
>> and in the right format significantly simplifying debugging. And it
>> wouldn't make sense to add tracepoints to *some* messages-related
>> functions and skip others where parsing is more trivial.
>
> Tracepoints add memory usage and take up real space.  If you don't need
> them for something, as there are other ways to already get the
> information needed, why add new ones that you now need to drag around
> for all time?
>

Are you opposed to the series as a whole (AKA 'no tracepoints in
drivers') or only to some tracepoints we add here?

Debugging stuff is always a tradeof between some memory overhead and
convenience (and CONFIG_TRACEPOINTS is the knob). Here I'd like to see
vmbus_* structures parsed in output. When things go wrong I can tell
someone "do echo 1 > /sys/kernel/debug/tracing/events/hyperv/enable and
show me the output" and I will have enough information to figure out
what's going on.

I'm probably missing something about easy 'alternative' ways to get what
I need and why people add tracepoints to drivers in general.

-- 
  Vitaly