On Sat, Jan 10, 2015 at 10:27 PM, Linus Torvalds <[email protected]> wrote: > On Sat, Jan 10, 2015 at 1:09 PM, Denys Vlasenko > <[email protected]> wrote: >> >> I think using push/pop is okay. In the very hottest code paths >> you may want to prefer mov's. > > For kernel entrypoints in particular, the code sequence is quite > possibly constrained by the decoder and instruction fetch rather than > the execution engine. Even if the entrypoint were to be in the L1 I$ > (which is not generally the case except in microbenchmarks), I am > pretty sure that even Intel doesn't actually speculatively decode > across system call boundaries, so unlike normal nice code, you don't > have the front end running ahead of the execution engine. > > Looking at the system call hotpath, for example, it looks like we > save/restore 8 registers. So 16 instructions or about 80 bytes of > code. I could easily imagine us avoiding one cacheline access by using > shorter 1- and 2-byte push/pop instructions (depending a bit on how > the cacheline alignment works out, of course). > > Depending on how well it prefetches from L2 and/or exact decoder > details, that kind of issue *can* overshadow the actual execution > costs. Of course, on microbenchmarks (eg some system call benchmark > that does "getppid()" in a loop), even the kernel side stays in the > L1, so those might show possible execution issues more. And > macrobenchmarks probably won't show a cycle or two in the system call > or fault path anyway. > > Linus
Looks like consensus to me. I'm resending patches with patch #3 reworked. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
