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Subject: Re: Help adding trace events to xHCI
From: Steven Rostedt <rostedt@goodmis.org>
To: Johannes Berg <johannes@sipsolutions.net>
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Date: Fri, 26 Jul 2013 09:17:29 -0400
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On Fri, 2013-07-26 at 15:06 +0200, Johannes Berg wrote:
> On Fri, 2013-07-26 at 08:28 -0400, Steven Rostedt wrote:

> Ah, yes, that'd work. I was considering putting it into the trace event
> handling itself so I don't have to allocate those buffers and put the
> handling into every tracepoint, but I don't know how that'd work with
> interrupts coming in.

If you create helper functions, it shouldn't be too hard.

> 
> If we assume that interrupts coming in in the middle of a tracepoint
> should be rare, we could do something like
> 
>  * allocate max buffer in on the tracing ringbuffer page
>  * write data into it
>  * if no interrupt came in, reduce reservation
> 
> but I'm not sure how to implement step 3 :)
> 

It's possible to reduce the ring buffer, it's just not implemented. I'm
not sure I want to do that either. Interrupts coming in is not so rare
as it can be any interrupt being traced. This means your tracepoints
will likely waste a lot of buffer space if you are tracing interrupts as
well.

That said, I can probably implement a reduce feature of the ring buffer
if needed.

-- Steve