On Mon, Feb 09 2015, "Wang, Yalin" <Yalin.Wang@xxxxxxxxxxxxxx> 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 -- 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
