linux-kernel - Re: [PATCH 2/2] Markers Implementation for Preempt RCU Boost Tracing

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Date:	Mon, 14 Jan 2008 14:36:53 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Pavel Machek <pavel@....cz>, Ingo Molnar <mingo@...e.hu>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"K. Prasad" <prasad@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	dipankar@...ibm.com, ego@...ibm.com, paulmck@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] Markers Implementation for Preempt RCU Boost
	Tracing

* Linus Torvalds (torvalds@...ux-foundation.org) wrote:
> 
> 
> On Mon, 14 Jan 2008, Mathieu Desnoyers wrote:
> > 
> > We would have to figure out if enabling -freorder-blocks-and-partition
> > makes sense kernel-wide.
> 
> Last I saw, it generates crappy code, with lots more jumps back and forth, 
> and the image just blows up.
> 
> There's a reason we use -Os, and that's that small footprint I$ is 
> generally more important than fake compiler optimizations that don't 
> actually help except on microbenchmarks where everything fits in the 
> cache.
> 
> Taking a branch instruction from two bytes to five is almost always a 
> mistake, unless you *know* that the code it jumps to will effectively 
> never be done at all (which is not necessarily the case at all). It also 
> makes debugging much nastier, because if now things like backtraces 
> probably look like crap too!
> 
> Don't go there. The *best* we can do is to just use the optimizations that 
> generate good-looking code that humans can read. The rest is just compiler 
> masturbation.
> 

I agree that turning this flag on does not seem like an interesting
solution.

Well, I wonder how important this issue of not sharing L1 instruction
cachelines with scheduler code is. If we care as much about it as Ingo
states, I wonder why we leave about 22 BUG() macros in sched.c
(calculated from the number of ud2 instructions generated on x86), which
adds up to 7 bytes at the end of many scheduler functions (7 bytes
coming from ud2, jmp and .p2align on x86).

And 22 markers in sched.c is already much more than needed. I actually
propose only 5 in my patchset.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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