linux-kernel - Re: [RFC PATCH 1/2] Marker probes in futex.c

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Message-Id: <1208361092.6395.81.camel@twins>
Date:	Wed, 16 Apr 2008 17:51:32 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	prasad@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca
Subject: Re: [RFC PATCH 1/2] Marker probes in futex.c

On Tue, 2008-04-15 at 21:22 +0530, K. Prasad wrote:
> On Tue, Apr 15, 2008 at 02:02:22PM +0200, Peter Zijlstra wrote:
> > On Tue, 2008-04-15 at 17:23 +0530, K. Prasad wrote:
> > 
> > > +	trace_mark(futex_wait_called, "uaddr:%p fshared:%p val:%u "
> > > +			"abs_time:%p bitset:%d",
> > > +			uaddr, fshared, val, abs_time, bitset);
> > 
> > This is some seriuosly ugly looking gunk, why would we want stuff like
> > that scattered across the code?
> > 
> > What is wrong with a few simple hooks like:
> > 
> >   trace_futex_wait(uaddr, fshares, val, abs_time, bitset);
> > 
> > and then deal with that.
> > 
> > Also, you seem to expose way too much futex internals; do you really
> > need that? People will go use this marker crap like ABI and further
> > restrain us from changing the code.
> The idea is to export as much data as possible (considering that the
> cost involved in doing so is less) and use them only when required.
> 
> If we were to log just the futex_ops, just as you had suggested,
> "Just log:
>  
>   futex: <uaddr> wait
>   futex: <uaddr> wakeup"
> it would provide only cursory information about code-flow and not enough
> debug information to help the user solve the issue he's trying to debug.
> 
> Say for e.g. in futex_wait you may want to know if abs_time had a
> non-zero value but the primitive debugging information would end
> up being a handicap from discovering that.
> 
> If you can specifically point me to information you think would be
> absolutely unnecessary, I can get them out of the trace_mark().

I'm thinking everything is superflous; you're basically logging what
strace already gives you; except worse by encoding local variable names
and exposing kernel pointers.

Also get rid of that futex_markers.h thing - its utterly useless and
makes the code less readable.

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