On 10/05/2015 08:58 AM, Linus Torvalds wrote:
...
> Dave, mind sharing the micro-benchmark or perhaps even just a kernel
> profile of it? How is that "iov_iter_fault_in_readable()" so
> noticeable? It really shouldn't be a big deal.
The micro was just plugging this test:
https://www.sr71.net/~dave/intel/write1byte.c
In to will-it-scale:
https://github.com/antonblanchard/will-it-scale
iov_iter_fault_in_readable() shows up as the third-most expensive kernel
function in a profile:
> 7.45% write1byte_proc [kernel.kallsyms] [k]
> copy_user_enhanced_fast_string
> 6.51% write1byte_proc [kernel.kallsyms] [k] unlock_page
>
> 6.04% write1byte_proc [kernel.kallsyms] [k]
> iov_iter_fault_in_readable
> 5.23% write1byte_proc libc-2.20.so [.] __GI___libc_write
>
> 4.86% write1byte_proc [kernel.kallsyms] [k] entry_SYSCALL_64
>
> 4.48% write1byte_proc [kernel.kallsyms] [k]
> iov_iter_copy_from_user_atomic
> 3.94% write1byte_proc [kernel.kallsyms] [k] generic_perform_write
>
> 3.74% write1byte_proc [kernel.kallsyms] [k] mutex_lock
>
> 3.59% write1byte_proc [kernel.kallsyms] [k]
> entry_SYSCALL_64_after_swapgs
> 3.55% write1byte_proc [kernel.kallsyms] [k] find_get_entry
>
> 3.53% write1byte_proc [kernel.kallsyms] [k] vfs_write
>
> 3.17% write1byte_proc [kernel.kallsyms] [k] find_lock_entry
>
> 3.17% write1byte_proc [kernel.kallsyms] [k] put_page
>
The disassembly points at the stac/clac pair being the culprits inside
the function (copy/paste from 'perf top' disassebly here):
...
> │ stac
> 24.57 │ mov (%rcx),%sil
> 15.70 │ clac
> 28.77 │ test %eax,%eax
> 2.15 │ mov %sil,-0x1(%rbp)
> 8.93 │ ↓ jne 66
> 2.31 │ movslq %edx,%rdx
One thing I've been noticing on Skylake is that barriers (implicit and
explicit) are showing up more in profiles. What we're seeing here
probably isn't actually stac/clac overhead, but the cost of finishing
some other operations that are outstanding before we can proceed through
here.
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