RE: [RFC] change non-atomic bitops method

[Wang, Yalin]

> -----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

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