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

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Date:	Mon, 14 Jan 2008 10:35:05 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Pavel Machek <pavel@....cz>
Cc:	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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/2] Markers Implementation for Preempt RCU Boost
	Tracing

* Pavel Machek (pavel@....cz) wrote:
> On Mon 2008-01-07 13:59:54, Mathieu Desnoyers wrote:
> > * Ingo Molnar (mingo@...e.hu) wrote:
> > > 
> > > * Frank Ch. Eigler <fche@...hat.com> wrote:
> > > 
> > > > > [...] this is a general policy matter. It is _so much easier_ to add 
> > > > > markers if they _can_ have near-zero overhead (as in 1-2 
> > > > > instructions). Otherwise we'll keep arguing about it, especially if 
> > > > > any is added to performance-critical codepath. (where we are 
> > > > > counting instructions)
> > > > 
> > > > The effect of the immediate-values patch, combined with gcc 
> > > > CFLAGS+=-freorder-blocks, *is* to keep the overhead at 1-2 
> > > > dcache-impact-free instructions.  The register saves, parameter 
> > > > evaluation, the function call, can all be moved out of line.
> > > 
> > > well, -freorder-blocks seems to be default-enabled at -O2 on gcc 4.2, so 
> > > we should already be getting that, right?
> > > 
> > > There's one thing that would make out-of-line tracepoints have a lot 
> > > less objectionable to me: right now the 'out of line' area is put to the 
> > > end of functions. That splinters the kernel image with inactive, rarely 
> > > taken areas of code - blowing up its icache footprint considerably. For 
> > > example sched.o has ~100 functions, with the average function size being 
> > > 200 bytes. At 64 bytes L1 cacheline size that's a 10-20% icache waste 
> > > already.
> > 
> > Hrm, I agree this can be a problem on architectures with more standard
> > associative icaches, but aren't most x86_64 machines (and modern x86_32)
> > using an instruction trace cache instead ? This makes the problem
> > irrelevant.
> > 
> > But I agree that, as Frank proposed, -freorder-blocks-and-partition
> > could help us in that matter for the architectures using an associative
> > L1 icache.
> 
> I thought trace cache died with P4?
> -- 

And you are absolutely right.

We would have to figure out if enabling -freorder-blocks-and-partition
makes sense kernel-wide.

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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