linux-kernel - Re: [RFC PATCH 0/2] tracing: Teach FETCH_MTD

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Date:	Wed, 27 Nov 2013 09:37:21 +0900
From:	Namhyung Kim <namhyung.kim@....com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH 0/2] tracing: Teach FETCH_MTD_symbol to handle per-cpu
 data

Hi Oleg,

(Please use my korg email address, otherwise I might not see - at least 
in time - the messages.)

2013-11-27 AM 2:43, Oleg Nesterov wrote:
> On 11/26, Masami Hiramatsu wrote:
>> If the symbol is
>> percpu, it should be automatically translated to something like
>> FETCH_percpu, instead of such additional expression.
>
> OK. Probably yes, it should be automatically translated, please
> see the patches.
>
> This doesn't allow to read the data from other CPUs, but at least
> the changes are simple and this_cpu_ is better than the reading
> from the obviously wrong address.

Agreed.  I also like the automatic translation.

>
>> For the dynamic allocated per-cpu things, it is also good to give
>> a straight operation, like +10(percpu(%rdi)).
>
> Probably yes, but this needs more changes and I am still not sure
> this is really useful. And if we do this, we probably also need
> to allow to read from other CPUs...

Right.  This should be another story.

Do we also need something similar to user-space TLS?

>
>> Also, please do not be afraid to add fetch operation,
>
> Well, this is what I am trying to avoid ;)

Well, it looks like a trade-off between code-size (or performance) and 
readability (or modularity).  For this specific case, I don't have any 
preference though.

Thanks,
Namhyung
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