* 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. 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 - 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
