[PATCH] lib: Make _find_next_bit helper function inline
Yury
yury.norov at gmail.com
Tue Jul 28 14:38:45 PDT 2015
On 29.07.2015 00:23, Yury wrote:
> On 28.07.2015 22:09, Cassidy Burden wrote:
>> I've tested Yury Norov's find_bit reimplementation with the
>> test_find_bit
>> module (https://lkml.org/lkml/2015/3/8/141) and measured about 35-40%
>> performance degradation on arm64 3.18 run with fixed CPU frequency.
>>
>> The performance degradation appears to be caused by the
>> helper function _find_next_bit. After inlining this function into
>> find_next_bit and find_next_zero_bit I get slightly better performance
>> than the old implementation:
>>
>> find_next_zero_bit find_next_bit
>> old new inline old new inline
>> 26 36 24 24 33 23
>> 25 36 24 24 33 23
>> 26 36 24 24 33 23
>> 25 36 24 24 33 23
>> 25 36 24 24 33 23
>> 25 37 24 24 33 23
>> 25 37 24 24 33 23
>> 25 37 24 24 33 23
>> 25 36 24 24 33 23
>> 25 37 24 24 33 23
>>
>> Signed-off-by: Cassidy Burden <cburden at codeaurora.org>
>> Cc: Alexey Klimov <klimov.linux at gmail.com>
>> Cc: David S. Miller <davem at davemloft.net>
>> Cc: Daniel Borkmann <dborkman at redhat.com>
>> Cc: Hannes Frederic Sowa <hannes at stressinduktion.org>
>> Cc: Lai Jiangshan <laijs at cn.fujitsu.com>
>> Cc: Mark Salter <msalter at redhat.com>
>> Cc: AKASHI Takahiro <takahiro.akashi at linaro.org>
>> Cc: Thomas Graf <tgraf at suug.ch>
>> Cc: Valentin Rothberg <valentinrothberg at gmail.com>
>> Cc: Chris Wilson <chris at chris-wilson.co.uk>
>> ---
>> lib/find_bit.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/find_bit.c b/lib/find_bit.c
>> index 18072ea..d0e04f9 100644
>> --- a/lib/find_bit.c
>> +++ b/lib/find_bit.c
>> @@ -28,7 +28,7 @@
>> * find_next_zero_bit. The difference is the "invert" argument, which
>> * is XORed with each fetched word before searching it for one bits.
>> */
>> -static unsigned long _find_next_bit(const unsigned long *addr,
>> +static inline unsigned long _find_next_bit(const unsigned long *addr,
>> unsigned long nbits, unsigned long start, unsigned long
>> invert)
>> {
>> unsigned long tmp;
>
> Hi Cassidi,
>
> At first, I'm really surprised that there's no assembler implementation
> of find_bit routines for aarch64. Aarch32 has ones...
>
> I was thinking on inlining the helper, but decided not to do this....
>
> 1. Test is not too realistic. https://lkml.org/lkml/2015/2/1/224
> The typical usage pattern is to look for a single bit or range of bits.
> So in practice nobody calls find_next_bit thousand times.
>
> 2. Way more important to fit functions into as less cache lines as
> possible. https://lkml.org/lkml/2015/2/12/114
> In this case, inlining increases cache lines consumption almost twice...
>
> 3. Inlining prevents compiler from some other possible optimizations.
> It's
> probable that in real module compiler will inline callers of
> _find_next_bit,
> and final output will be better. I don't like to point out the
> compiler how
> it should do its work.
>
> Nevertheless, if this is your real case, and inlining helps, I'm OK
> with it.
>
> But I think, before/after for x86 is needed as well.
> And why don't you consider '__always_inline__'? Simple inline is only
> a hint and
> guarantees nothing.
(Sorry for typo in your name. Call me Yuri next time.)
Adding Rasmus and George to CC
More information about the linux-arm-kernel
mailing list