[RFC] change non-atomic bitops method

[Rasmus Villemoes]

On Mon, Feb 09 2015, "Wang, Yalin" <Yalin.Wang at sonymobile.com> wrote:

> I te-test the patch on 3.10 kernel.
> The result like this:
>
> VmallocChunk:   251498164 kB
> __set_bit_miss_count:11730 __set_bit_success_count:1036316
> __clear_bit_miss_count:209640 __clear_bit_success_count:4806556
> __test_and_set_bit_miss_count:0 __test_and_set_bit_success_count:121
> __test_and_clear_bit_miss_count:0 __test_and_clear_bit_success_count:445
>
> __clear_bit miss rate is a little high,
> I check the log, and most miss coming from this code:
>
> <6>[  442.701798] [<ffffffc00021d084>] warn_slowpath_fmt+0x4c/0x58
> <6>[  442.701805] [<ffffffc0002461a8>] __clear_bit+0x98/0xa4
> <6>[  442.701813] [<ffffffc0003126ac>] __alloc_fd+0xc8/0x124
> <6>[  442.701821] [<ffffffc000312768>] get_unused_fd_flags+0x28/0x34
> <6>[  442.701828] [<ffffffc0002f9370>] do_sys_open+0x10c/0x1c0
> <6>[  442.701835] [<ffffffc0002f9458>] SyS_openat+0xc/0x18
> In __clear_close_on_exec(fd, fdt);
>
>
>
> <6>[  442.695354] [<ffffffc00021d084>] warn_slowpath_fmt+0x4c/0x58
> <6>[  442.695359] [<ffffffc0002461a8>] __clear_bit+0x98/0xa4
> <6>[  442.695367] [<ffffffc000312340>] dup_fd+0x1d4/0x280
> <6>[  442.695375] [<ffffffc00021b07c>] copy_process.part.56+0x42c/0xe38
> <6>[  442.695382] [<ffffffc00021bb9c>] do_fork+0xe0/0x360
> <6>[  442.695389] [<ffffffc00021beb4>] SyS_clone+0x10/0x1c
> In __clear_open_fd(open_files - i, new_fdt);
>
> Do we need test_bit() before clear_bit()at these 2 place?
>

In the second case, new_fdt->open_fds has just been filled by a
memcpy, and no-one can possibly have written to that cache line in the
meantime. 

In the first case, testing is also likely wasteful if fdt->max_fds is
less than half the number of bits in a cacheline (fdt->close_on_exec and
fdt->open_fds are always contiguous, and the latter is unconditionally
written to).

Rasmus