> On Nov 20, 2014, at 11:33 PM, Rui Paulo <rpa...@me.com> wrote: > > On Nov 13, 2014, at 14:11, Scott Long <sco...@freebsd.org> wrote: >> >> Author: scottl >> Date: Thu Nov 13 22:11:44 2014 >> New Revision: 274489 >> URL: https://svnweb.freebsd.org/changeset/base/274489 >> >> Log: >> Extend earlier addition of stack frames to most of support.S. This makes >> stack traces in KDB, HWPMC, and DTrace much more reliable and useful. > > No performance differences? The kernel enables/disables the compiler option > to omit the frame pointer based on the kernel config file. If DDB, DTrace, > or HWPMC is enabled, the frame pointer is always saved in C functions. > > Some of these functions are in the hot path, so if you didn't see any > performance problem, I wonder if we should disable -fomit-frame-pointer > always.
That’s a good question to look further into. I didn’t see any measurable differences with this change. I think that the cost of the function call itself masks the cost of a few extra instructions, but I didn’t test with switching it on/off for the entire kernel. That said, I purposely implemented this as macros so it could be easily changed in the future. If someone finds that this measurably impacts a certain workload, I wouldn’t object to making it conditional, though it does complicate any hand-written ASM code that tries to access the stack via %esp offsets. We don’t have anything like that now, but Kip was concerned enough about it in the future that I left it enabled unconditionally. Scott _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
