On Mon, May 15, 2017 at 6:17 PM, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, May 15, 2017 at 06:06:18PM +0200, Arnd Bergmann wrote:
>> On Mon, May 15, 2017 at 5:47 PM, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote:
>> > On Sun, May 14, 2017 at 08:09:17PM +0200, Ingo Molnar wrote:
>> >
>> > I also think that sched_find_first_bit() may be faster that find_first_bit()
>> > because it's inlined in the caller. We can do so for find_first_bit() if
>> > it takes small sizes at compile time, and so all parts of kernel will
>> > use fast find_first_bit, not only sched.
>>
>> I suspect the first step would be to 'select GENERIC_FIND_FIRST_BIT'
>> on ARM64, which should already improve the performance for those
>> files that never call the 'next' variants.
>>
>> Adding an inline version of find_first_{,zero_}bit could also help, but
>> is harder to quantify.
>>
>
> I checked again, and in fact I measured on top of this patch:
> https://lkml.org/lkml/2017/5/13/137
> So find_first_bit is already enabled.
Ok. I've played around with this for a bit more and came to a generic
version that is almost as good as the current sched_find_first_bit()
on x86 (one extra comparison):
+#define sched_find_first_bit(b) find_first_bit(b, 128)
-extern unsigned long find_first_bit(const unsigned long *addr,
+extern unsigned long __find_first_bit(const unsigned long *addr,
unsigned long size);
+static inline unsigned long find_first_bit(const unsigned long *addr,
+ unsigned long size)
+{
+ unsigned long idx;
+
+ if (!__builtin_constant_p(size))
+ return __find_first_bit(addr, size);
+
+ idx = 0;
+ switch (size) {
+ case BITS_PER_LONG * 4:
+ if (addr[0])
+ return __ffs(addr[0]) + idx;
+ addr++;
+ idx += BITS_PER_LONG;
+ case BITS_PER_LONG * 3:
+ if (addr[0])
+ return __ffs(addr[0]) + idx;
+ addr++;
+ idx += BITS_PER_LONG;
+ case BITS_PER_LONG * 2:
+ if (addr[0])
+ return __ffs(addr[0]) + idx;
+ addr++;
+ idx += BITS_PER_LONG;
+ case BITS_PER_LONG * 1:
+ if (addr[0])
+ return __ffs(addr[0]) + idx;
+ addr++;
+ idx += BITS_PER_LONG;
+ return idx;
+ }
+
+ return __find_first_bit(addr, size);
+}
However, on architectures that rely on
include/asm-generic/bitops/__ffs.h or something
similarly verbose, this would just add needless bloat
to the size rather than actually making a difference
in performance.
Arnd
