Date: Fri, 21 Nov 2014 16:26:47 -0700 From: Scott Long <scott4long@yahoo.com> To: Rui Paulo <rpaulo@me.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Scott Long <scottl@FreeBSD.org> Subject: Re: svn commit: r274489 - in head/sys/amd64: amd64 include Message-ID: <13EC3116-6146-42FC-8941-2C7C009224B3@yahoo.com> In-Reply-To: <35E5EAD8-99C1-43C0-8D01-B3B5B86ECA25@me.com> References: <201411132211.sADMBjP3009246@svn.freebsd.org> <35E5EAD8-99C1-43C0-8D01-B3B5B86ECA25@me.com>
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> On Nov 20, 2014, at 11:33 PM, Rui Paulo <rpaulo@me.com> wrote: > > On Nov 13, 2014, at 14:11, Scott Long <scottl@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. Scotthelp
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