Subject: Re: [ANNOUNCE] New utility: 'trace'
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Tom Zanussi <tzanussi@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@elte.hu>, "Ted Ts'o" <tytso@mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arjan van de Ven <arjan@infradead.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Li Zefan <lizf@cn.fujitsu.com>, Jason Baron <jbaron@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Christoph Hellwig <hch@lst.de>, Pekka Enberg <penberg@kernel.org>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Eric Dumazet <eric.dumazet@gmail.com>
In-Reply-To: <20101117183642.GE13717@Krystal>
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Date: Wed, 17 Nov 2010 14:25:06 -0500
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On Wed, 2010-11-17 at 13:36 -0500, Mathieu Desnoyers wrote:
> * Tom Zanussi (tzanussi@gmail.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))

I would just call it __assign().


> 
> 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.

Yes, something like this could work. It would require a bit more CPP
magic to handle it though.

I could write something up to do this. I just have to add it to the 100
other things I'm doing at the same time :-p

-- Steve


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