On Wed 03-02-16 09:10:16, Ingo Molnar wrote: > > * 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. OK, fair enough. As I've mentioned in the cover letter I do not really insist on this patch. I just found the current code too ugly to live without a good reason because down_write is a call so saving one push/pop seems like really negligible to the call itself. Moreover this is a write lock which is expected to be heavier. It is the read path which is expected to be light and contention (slow path) is expected on the write lock. That being said, if you really believe that the current code is easier to maintain then I will not pursue this patch. The rest doesn't really depend on it. I will just respin the follow up x86 specifi __down_write_killable to follow the same code convention. [...] > So, if you want to remove the assembly code - can we achieve that without hurting > the generated fast path, using the compiler? One way would be to do the same thing as mutex does and do the fast path as an inline. This could bloat the kernel and require some additional changes to allow arch specific reimplementations though so I didn't want to go that path. -- Michal Hocko SUSE Labs -- 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
