| Message ID | 20120905193001.99d06c26.yoshikawa.takuya@oss.ntt.co.jp (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, Sep 05, 2012 at 07:30:01PM +0900, Takuya Yoshikawa wrote: > find_highest_vector() and count_vectors(): > - Instead of using magic values, define and use proper macros. > > find_highest_vector(): > - Remove likely() which is there only for historical reasons and not > doing correct branch predictions anymore. Using such heuristics > to optimize this function is not worth it now. Let CPUs predict > things instead. > > - Stop checking word[0] separately. This was only needed for doing > likely() optimization. > > - Use for loop, not while, to iterate over the register array to make > the code clearer. > > Note that we actually confirmed that the likely() did wrong predictions > by inserting debug code. > > Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> > Cc: Michael S. Tsirkin <mst@redhat.com> Looks like a nice cleanup. Acked-by: Michael S. Tsirkin <mst@redhat.com> > --- > arch/x86/kvm/lapic.c | 30 ++++++++++++++++++------------ > 1 files changed, 18 insertions(+), 12 deletions(-) > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c > index 18d149d..8ace252 100644 > --- a/arch/x86/kvm/lapic.c > +++ b/arch/x86/kvm/lapic.c > @@ -66,6 +66,7 @@ > #define APIC_DEST_NOSHORT 0x0 > #define APIC_DEST_MASK 0x800 > #define MAX_APIC_VECTOR 256 > +#define APIC_VECTORS_PER_REG 32 > > #define VEC_POS(v) ((v) & (32 - 1)) > #define REG_POS(v) (((v) >> 5) << 4) > @@ -208,25 +209,30 @@ static unsigned int apic_lvt_mask[APIC_LVT_NUM] = { > > static int find_highest_vector(void *bitmap) > { > - u32 *word = bitmap; > - int word_offset = MAX_APIC_VECTOR >> 5; > + int vec; > + u32 *reg; > > - while ((word_offset != 0) && (word[(--word_offset) << 2] == 0)) > - continue; > + for (vec = MAX_APIC_VECTOR - APIC_VECTORS_PER_REG; > + vec >= 0; vec -= APIC_VECTORS_PER_REG) { > + reg = bitmap + REG_POS(vec); > + if (*reg) > + return fls(*reg) - 1 + vec; > + } > > - if (likely(!word_offset && !word[0])) > - return -1; > - else > - return fls(word[word_offset << 2]) - 1 + (word_offset << 5); > + return -1; > } > > static u8 count_vectors(void *bitmap) > { > - u32 *word = bitmap; > - int word_offset; > + int vec; > + u32 *reg; > u8 count = 0; > - for (word_offset = 0; word_offset < MAX_APIC_VECTOR >> 5; ++word_offset) > - count += hweight32(word[word_offset << 2]); > + > + for (vec = 0; vec < MAX_APIC_VECTOR; vec += APIC_VECTORS_PER_REG) { > + reg = bitmap + REG_POS(vec); > + count += hweight32(*reg); > + } > + > return count; > } > > -- > 1.7.5.4 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Sep 05, 2012 at 07:30:01PM +0900, Takuya Yoshikawa wrote: > find_highest_vector() and count_vectors(): > - Instead of using magic values, define and use proper macros. > > find_highest_vector(): > - Remove likely() which is there only for historical reasons and not > doing correct branch predictions anymore. Using such heuristics > to optimize this function is not worth it now. Let CPUs predict > things instead. > > - Stop checking word[0] separately. This was only needed for doing > likely() optimization. > > - Use for loop, not while, to iterate over the register array to make > the code clearer. > > Note that we actually confirmed that the likely() did wrong predictions > by inserting debug code. > > Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> > Cc: Michael S. Tsirkin <mst@redhat.com> Applied, thanks. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index 18d149d..8ace252 100644 --- a/arch/x86/kvm/lapic.c +++ b/arch/x86/kvm/lapic.c @@ -66,6 +66,7 @@ #define APIC_DEST_NOSHORT 0x0 #define APIC_DEST_MASK 0x800 #define MAX_APIC_VECTOR 256 +#define APIC_VECTORS_PER_REG 32 #define VEC_POS(v) ((v) & (32 - 1)) #define REG_POS(v) (((v) >> 5) << 4) @@ -208,25 +209,30 @@ static unsigned int apic_lvt_mask[APIC_LVT_NUM] = { static int find_highest_vector(void *bitmap) { - u32 *word = bitmap; - int word_offset = MAX_APIC_VECTOR >> 5; + int vec; + u32 *reg; - while ((word_offset != 0) && (word[(--word_offset) << 2] == 0)) - continue; + for (vec = MAX_APIC_VECTOR - APIC_VECTORS_PER_REG; + vec >= 0; vec -= APIC_VECTORS_PER_REG) { + reg = bitmap + REG_POS(vec); + if (*reg) + return fls(*reg) - 1 + vec; + } - if (likely(!word_offset && !word[0])) - return -1; - else - return fls(word[word_offset << 2]) - 1 + (word_offset << 5); + return -1; } static u8 count_vectors(void *bitmap) { - u32 *word = bitmap; - int word_offset; + int vec; + u32 *reg; u8 count = 0; - for (word_offset = 0; word_offset < MAX_APIC_VECTOR >> 5; ++word_offset) - count += hweight32(word[word_offset << 2]); + + for (vec = 0; vec < MAX_APIC_VECTOR; vec += APIC_VECTORS_PER_REG) { + reg = bitmap + REG_POS(vec); + count += hweight32(*reg); + } + return count; }
find_highest_vector() and count_vectors(): - Instead of using magic values, define and use proper macros. find_highest_vector(): - Remove likely() which is there only for historical reasons and not doing correct branch predictions anymore. Using such heuristics to optimize this function is not worth it now. Let CPUs predict things instead. - Stop checking word[0] separately. This was only needed for doing likely() optimization. - Use for loop, not while, to iterate over the register array to make the code clearer. Note that we actually confirmed that the likely() did wrong predictions by inserting debug code. Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp> Cc: Michael S. Tsirkin <mst@redhat.com> --- arch/x86/kvm/lapic.c | 30 ++++++++++++++++++------------ 1 files changed, 18 insertions(+), 12 deletions(-)