linux-kernel - Re: [ANNOUNCE] New utility: 'trace'

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Date:	Wed, 17 Nov 2010 13:36:42 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Tom Zanussi <tzanussi@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Ted Ts'o <tytso@....edu>, Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Jason Baron <jbaron@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Christoph Hellwig <hch@....de>,
	Pekka Enberg <penberg@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [ANNOUNCE] New utility: 'trace'

* Tom Zanussi (tzanussi@...il.com) wrote:
[...]
> IIRC, I think the conclusion we came to was that it could be done
> mechanically if for example the right-hand-side of an assignment in
> TP_fast_assign() only involved a simple variable assignment, but as
> Steve pointed out, some assignments are more complicated than that.

Yep, we came up to the same conclusions in UST.

> For example, in the sched_switch tracepoint assignments:
> 
> 	__entry->prev_prio      = prev->prio;
>         __entry->prev_state     = __trace_sched_switch_state(prev);
> 
> so the prev_prio should be able to be tested 'in-line' but the
> prev_state would require a temporary buffer to write the value into
> before doing the test as mentioned by Steve.  At which point you're no
> further ahead (in that case) than the current situation...

if we change all assignments to, e.g.:

_tp_assign(__entry->prev_prio, prev->prio)
_tp_assign(__entry->prev_state, __trace_sched_switch_state(prev))

then we can redefine the macros for filtering much more easily than with the
" = " assignment operator.

About your comment above, what is the problem with evaluating
"__trace_sched_switch_state(prev)" twice ? It will typically be cache-hot after
the first evaluation, so I wonder if, in practice, we really save a significant
amount of cycles by saving its result between filtering and writing into trace
buffers. As I pointed out earlier, for my customers, having a very, very fast
filter "out" case is more important that trying to squeeze a few cycles out of
the filter passed case.

Also, how many of these "__trace_sched_switch_state(prev)" are static inlines vs
actual function calls ? If it's mostly static inlines to dereference a few
pointers, doing it the second time when the filter passed won't hurt much.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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