* Michal Hocko <mhocko@xxxxxxxxxx> wrote: > From: Michal Hocko <mhocko@xxxxxxxx> > > x86 implementation of __down_write is using inline asm to optimize the > code flow. This however requires that it has go over an additional hop > for the slow path call_rwsem_down_write_failed which has to > save_common_regs/restore_common_regs to preserve the calling convention. > This, however doesn't add much because the fast path only saves one > register push/pop (rdx) when compared to the generic implementation: > > Before: > 0000000000000019 <down_write>: > 19: e8 00 00 00 00 callq 1e <down_write+0x5> > 1e: 55 push %rbp > 1f: 48 ba 01 00 00 00 ff movabs $0xffffffff00000001,%rdx > 26: ff ff ff > 29: 48 89 f8 mov %rdi,%rax > 2c: 48 89 e5 mov %rsp,%rbp > 2f: f0 48 0f c1 10 lock xadd %rdx,(%rax) > 34: 85 d2 test %edx,%edx > 36: 74 05 je 3d <down_write+0x24> > 38: e8 00 00 00 00 callq 3d <down_write+0x24> > 3d: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax > 44: 00 00 > 46: 5d pop %rbp > 47: 48 89 47 38 mov %rax,0x38(%rdi) > 4b: c3 retq > > After: > 0000000000000019 <down_write>: > 19: e8 00 00 00 00 callq 1e <down_write+0x5> > 1e: 55 push %rbp > 1f: 48 b8 01 00 00 00 ff movabs $0xffffffff00000001,%rax > 26: ff ff ff > 29: 48 89 e5 mov %rsp,%rbp > 2c: 53 push %rbx > 2d: 48 89 fb mov %rdi,%rbx > 30: f0 48 0f c1 07 lock xadd %rax,(%rdi) > 35: 48 85 c0 test %rax,%rax > 38: 74 05 je 3f <down_write+0x26> > 3a: e8 00 00 00 00 callq 3f <down_write+0x26> > 3f: 65 48 8b 04 25 00 00 mov %gs:0x0,%rax > 46: 00 00 > 48: 48 89 43 38 mov %rax,0x38(%rbx) > 4c: 5b pop %rbx > 4d: 5d pop %rbp > 4e: c3 retq I'm not convinced about the removal of this optimization at all. > This doesn't seem to justify the code obfuscation and complexity. Use > the generic implementation instead. > > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > --- > arch/x86/include/asm/rwsem.h | 17 +++++------------ > arch/x86/lib/rwsem.S | 9 --------- > 2 files changed, 5 insertions(+), 21 deletions(-) Turn the argument around, would we be willing to save two instructions off the fast path of a commonly used locking construct, with such a simple optimization: > arch/x86/include/asm/rwsem.h | 17 ++++++++++++----- > arch/x86/lib/rwsem.S | 9 +++++++++ > 2 files changed, 21 insertions(+), 5 deletions(-) ? Yes! So, if you want to remove the assembly code - can we achieve that without hurting the generated fast path, using the compiler? Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
