On 1/16/2025 8:19 PM, Ethan Zhao wrote:
在 2025/1/17 9:21, H. Peter Anvin 写道:
On 1/16/25 16:37, Ethan Zhao wrote:
hpa suggested to introduce "switch_likely" for this kind of
optimization
on a switch statement, which is also easier to read. I measured it
with
a user space focus test, it does improve performance a lot. But
obviously there are still a lot of work to do.
Find a way to instruct compiler to pick the right hot branch
meanwhile make folks
reading happy... yup, a lot of work.
It's not that complicated, believe it or not.
/*
* switch(v) biased for speed in the case v == l
*
* Note: gcc is quite sensitive to the exact form of this
* expression.
*/
#define switch_likely(v,l) \
switch((__typeof__(v))__builtin_expect((v),(l)))
I tried this macro as following, but got something really *weird* from gcc.
+#define switch_likely(v,l) \
+ switch((__typeof__(v))__builtin_expect((v),(l)))
+
__visible noinstr void fred_entry_from_user(struct pt_regs *regs)
{
unsigned long error_code = regs->orig_ax;
+ unsigned short etype = regs->fred_ss.type & 0xf;
/* Invalidate orig_ax so that syscall_get_nr() works correctly */
regs->orig_ax = -1;
- switch (regs->fred_ss.type) {
+ switch_likely ((etype == EVENT_TYPE_EXTINT || etype ==
EVENT_TYPE_OTHER), etype) {
Just swap the 2 arguments, and it should be:
+ switch_likely (etype, EVENT_TYPE_OTHER) {
Probably also check __builtin_expect on
https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html.
case EVENT_TYPE_EXTINT:
return fred_extint(regs);
case EVENT_TYPE_NMI:
@@ -256,11 +260,12 @@ __visible noinstr void fred_entry_from_user(struct
pt_regs *regs)
__visible noinstr void fred_entry_from_kernel(struct pt_regs *regs)
{
unsigned long error_code = regs->orig_ax;
+ unsigned short etype = regs->fred_ss.type & 0xf;
/* Invalidate orig_ax so that syscall_get_nr() works correctly */
regs->orig_ax = -1;
- switch (regs->fred_ss.type) {
+ switch_likely (etype == EVENT_TYPE_EXTINT, etype) {
case EVENT_TYPE_EXTINT:
return fred_extint(regs);
case EVENT_TYPE_NMI:
Got the asm code as following:
objdump -d vmlinux.o | awk '/<fred_entry_from_user>:/{c=65} c&&c--'
00000000000015a0 <fred_entry_from_user>:
15a0: 0f b6 87 a6 00 00 00 movzbl 0xa6(%rdi),%eax
15a7: 48 8b 77 78 mov 0x78(%rdi),%rsi
15ab: 55 push %rbp
15ac: 48 c7 47 78 ff ff ff movq $0xffffffffffffffff,0x78(%rdi)
15b3: ff
15b4: 48 89 e5 mov %rsp,%rbp
15b7: 66 83 e0 0f and $0xf,%ax
15bb: 74 11 je 15ce <fred_entry_from_user+0x2e>
15bd: 66 83 f8 07 cmp $0x7,%ax
15c1: 74 0b je 15ce <fred_entry_from_user+0x2e>
15c3: e8 78 fc ff ff callq 1240 <fred_extint>
15c8: 5d pop %rbp
15c9: e9 00 00 00 00 jmpq 15ce <fred_entry_from_user+0x2e>
15ce: e8 4d fd ff ff callq 1320 <fred_bad_type>
15d3: 5d pop %rbp
15d4: e9 00 00 00 00 jmpq 15d9 <fred_entry_from_user+0x39>
15d9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
00000000000015e0 <__pfx_fred_entry_from_kernel>:
15e0: 90 nop
15e1: 90 nop
00000000000015f0 <fred_entry_from_kernel>:
15f0: 55 push %rbp
15f1: 48 8b 77 78 mov 0x78(%rdi),%rsi
15f5: 48 c7 47 78 ff ff ff movq $0xffffffffffffffff,0x78(%rdi)
15fc: ff
15fd: 48 89 e5 mov %rsp,%rbp
1600: f6 87 a6 00 00 00 0f testb $0xf,0xa6(%rdi)
1607: 75 0b jne 1614 <fred_entry_from_kernel+0x24>
1609: e8 12 fd ff ff callq 1320 <fred_bad_type>
160e: 5d pop %rbp
160f: e9 00 00 00 00 jmpq 1614 <fred_entry_from_kernel+0x24>
1614: e8 27 fc ff ff callq 1240 <fred_extint>
1619: 5d pop %rbp
161a: e9 00 00 00 00 jmpq 161f <fred_entry_from_kernel+0x2f>
161f: 90 nop
0000000000001620 <__pfx___fred_entry_from_kvm>:
1620: 90 nop
1621: 90 nop
Even the fred_entry_from_kernel() asm code doesn't look right.
*gcc version 8.5.0 20210514 (Red Hat 8.5.0-10) (GCC)*
**
*Did I screw up something ?*
**
*Thanks,*
*Ethan*
-hpa
