> -----Original Message----- > From: Andrew Morton [mailto:akpm@xxxxxxxxxxxxxxxxxxxx] > Sent: Tuesday, February 10, 2015 4:34 AM > To: Wang, Yalin > Cc: 'Kirill A. Shutemov'; 'arnd@xxxxxxxx'; 'linux-arch@xxxxxxxxxxxxxxx'; > 'linux-kernel@xxxxxxxxxxxxxxx'; 'linux@xxxxxxxxxxxxxxxx'; 'linux-arm- > kernel@xxxxxxxxxxxxxxxxxxx' > Subject: Re: [RFC] change non-atomic bitops method > > On Mon, 9 Feb 2015 16:18:10 +0800 "Wang, Yalin" <Yalin.Wang@xxxxxxxxxxxxxx> > wrote: > > > > That we're running clear_bit against a cleared bit 10% of the time is a > > > bit alarming. I wonder where that's coming from. > > > > > > The enormous miss count in test_and_clear_bit() might indicate an > > > inefficiency somewhere. > > 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? > > I don't know. I was happily typing in this: > > diff -puN include/linux/bitops.h~a include/linux/bitops.h > --- a/include/linux/bitops.h~a > +++ a/include/linux/bitops.h > @@ -226,5 +226,37 @@ extern unsigned long find_last_bit(const > unsigned long size); > #endif > > +/** > + * __set_clear_bit - non-atomically set a bit if it is presently clear > + * @nr: The bit number > + * @addr: The base address of the operation > + * > + * __set_clear_bit() and similar functions avoid unnecessarily dirtying a > + * cacheline when the operation will have no effect. > + */ > +static inline void __set_clear_bit(unsigned nr, volatile unsigned long > *addr) > +{ > + if (!test_bit(nr, addr)) > + __set_bit(nr, addr); > +} > + > +static inline void __clear_set_bit(unsigned nr, volatile unsigned long > *addr) > +{ > + if (test_bit(nr, addr)) > + __clear_bit(nr, addr); > +} > + > +static inline void set_clear_bit(unsigned nr, volatile unsigned long > *addr) > +{ > + if (!test_bit(nr, addr)) > + set_bit(nr, addr); > +} > + > +static inline void clear_set_bit(unsigned nr, volatile unsigned long > *addr) > +{ > + if (test_bit(nr, addr)) > + clear_bit(nr, addr); > +} > + > #endif /* __KERNEL__ */ > #endif > > (maybe __set_bit_if_clear would be a better name) > > But I don't know if it will do anything useful. The CPU *should* be > able to avoid dirtying the cacheline on its own: it has all the info it > needs to know that no writeback will be needed. But I don't know which > (if any) CPUs perform this optimisation. I will send a new patch for your review . Thanks -- 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
