linux-kernel - Re: [PATCH 04/15] ftrace: pack event structures.

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Date:	Thu, 10 Mar 2011 22:37:22 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	David Sharp <dhsharp@...gle.com>
Cc:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	mrubin@...gle.com
Subject: Re: [PATCH 04/15] ftrace: pack event structures.

On Thu, 2011-03-10 at 15:21 -0800, David Sharp wrote:

> Yes, I think that should have the same effect. Of course the next time
> we come along and remove other useless fields like 'flags' and
> 'preempt_count', a similar condition could arise.
> 
> (Note I don't necessarily think that 'flags' and 'preempt_count' are
> useless; although I don't know what they're used for, or why they need
> to be in every entry. fwiw, we've taken trace_entry down to 4 bytes of
> {unsigned short type; unsigned short pid;}, and we even have plans to
> completely remove pid.)

The flags and preempt count are legacy from the latency-tracer. But they
do come in quite handy when debugging -rt code. I've gotten trace data
from customers that knowing if preemption is enabled or disable and
whether or not something happened in interrupt context was detrimental.

That said, I would love to figure out a nice way to make these optional.

-- Steve


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