Re: [PATCH v4 3/6] perf: Shrink the size of the recursion counter.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-07-01 14:31:37 [+0200], Peter Zijlstra wrote:
> On Mon, Jun 24, 2024 at 05:15:16PM +0200, Sebastian Andrzej Siewior wrote:
> > There are four recursion counter, one for each context. The type of the
> > counter is `int' but the counter is used as `bool' since it is only
> > incremented if zero.
> > 
> > Reduce the type of the recursion counter to an unsigned char, keep the
> > increment/ decrement operation.
> 
> Does this actually matter? Aren't u8 memops encoded by longer
> instructions etc..

The goal here isn't to reduce the opcodes but to add it to task_struct
without making it larger by filling a hole.

But since you made me look at assembly:
old:
     316b:       65 48 8b 15 00 00 00    mov    %gs:0x0(%rip),%rdx        # 3173 <perf_swevent_get_recursion_context+0x33>
     3173:       1c ff                   sbb    $0xff,%al
     3175:       48 0f be c8             movsbq %al,%rcx
     3179:       48 8d 94 8a 00 00 00    lea    0x0(%rdx,%rcx,4),%rdx
     3180:       00
                         317d: R_X86_64_32S      .data..percpu+0x4c
     3181:       8b 0a                   mov    (%rdx),%ecx
     3183:       85 c9                   test   %ecx,%ecx
     3185:       75 0e                   jne    3195 <perf_swevent_get_recursion_context+0x55>
     3187:       c7 02 01 00 00 00       movl   $0x1,(%rdx)
^^^
     318d:       0f be c0                movsbl %al,%eax

new:
     2ff8:       1c ff                   sbb    $0xff,%al
     2ffa:       81 e2 00 01 ff 00       and    $0xff0100,%edx
     3000:       83 fa 01                cmp    $0x1,%edx
     3003:       1c ff                   sbb    $0xff,%al
     3005:       48 0f be d0             movsbq %al,%rdx
     3009:       48 8d 94 11 00 00 00    lea    0x0(%rcx,%rdx,1),%rdx
     3010:       00
                         300d: R_X86_64_32S      .data..percpu+0x4c
 
     3011:       80 3a 00                cmpb   $0x0,(%rdx)
     3014:       75 0b                   jne    3021 <perf_swevent_get_recursion_context+0x51>
     3016:       c6 02 01                movb   $0x1,(%rdx)
^^^
     3019:       0f be c0                movsbl %al,%eax
     301c:       e9 00 00 00 00          jmp    3021 <perf_swevent_get_recursion_context+0x51>

So we do even save a few bytes. We could avoid the "movsbl" at 3019 by
making the return type `unsigned char' ;)

Sebastian