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

[Pavel Machek <pavel@ucw.cz>]

On Mon 2008-01-07 13:59:54, Mathieu Desnoyers wrote:
> * Ingo Molnar (mingo@elte.hu) wrote:
> > 
> > * Frank Ch. Eigler <fche@redhat.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?
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