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SubjectRe: [PATCH 2/2] Markers Implementation for Preempt RCU Boost Tracing
* Pavel Machek (pavel@ucw.cz) wrote:
> 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?
> --

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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