* Ingo Molnar (mingo@xxxxxxx) wrote: > > * Frank Ch. Eigler <fche@xxxxxxxxxx> 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. Mathieu > > It's true that keeping the off-site code within the function keeps total > codesize slightly smaller, because the offsets (and hence the > conditional jumps) are thus 8 bit - but that's below 1% and the > cache-blow-up aspect is more severe in practice at 10-20%. > > So it would be nice if we could collect all this offline code and stuff > it away into another portion of the kernel image. (or, into another > portion of the object file - which would still be good enough in > practice) > > Ingo -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
