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Date:	Wed, 17 Nov 2010 13:25:29 -0600
From:	Tom Zanussi <tzanussi@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.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'

On Wed, 2010-11-17 at 14:02 -0500, Mathieu Desnoyers wrote: 
> * Tom Zanussi (tzanussi@...il.com) wrote:
> > On Wed, 2010-11-17 at 13:36 -0500, Mathieu Desnoyers wrote:
> > > * 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.
> > > 
> > 
> > But the idea is to avoid allocating the trace buffer in the first place,
> > until we've decided we want the event.  So how do you check the result
> > of __trace_sched_switch_state(prev) with the filter value if you don't
> > have it temporarily stored somewhere (not in the trace buffer, which
> > doesn't exist yet as far as this event is concerned)?
> 
> It seems I might be missing something important, but what's wrong with using
> registers or the stack to hold the value for comparison ? In this case, it's a

Nothing, it shouldn't matter where the temporary storage is, as long as
it's not in the trace buffer.

Tom

> "long", so a register seems perfectly reasonable. But again, I feel I'm missing
> a key point -- what is it ?
> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Tom
> > 
> > > 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
> > > 
> > 
> > 
> 



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