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.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/events/callchain.c | 2 +-
 kernel/events/core.c      | 2 +-
 kernel/events/internal.h  | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 1273be84392cf..ad57944b6c40e 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -29,7 +29,7 @@ static inline size_t perf_callchain_entry__sizeof(void)
 				 sysctl_perf_event_max_contexts_per_stack));
 }
 
-static DEFINE_PER_CPU(int, callchain_recursion[PERF_NR_CONTEXTS]);
+static DEFINE_PER_CPU(u8, callchain_recursion[PERF_NR_CONTEXTS]);
 static atomic_t nr_callchain_events;
 static DEFINE_MUTEX(callchain_mutex);
 static struct callchain_cpus_entries *callchain_cpus_entries;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6256a9593c3da..f48ce05907042 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9781,7 +9781,7 @@ struct swevent_htable {
 	int				hlist_refcount;
 
 	/* Recursion avoidance in each contexts */
-	int				recursion[PERF_NR_CONTEXTS];
+	u8				recursion[PERF_NR_CONTEXTS];
 };
 
 static DEFINE_PER_CPU(struct swevent_htable, swevent_htable);
diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 5150d5f84c033..f9a3244206b20 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -208,7 +208,7 @@ arch_perf_out_copy_user(void *dst, const void *src, unsigned long n)
 
 DEFINE_OUTPUT_COPY(__output_copy_user, arch_perf_out_copy_user)
 
-static inline int get_recursion_context(int *recursion)
+static inline int get_recursion_context(u8 *recursion)
 {
 	unsigned char rctx = interrupt_context_level();
 
@@ -221,7 +221,7 @@ static inline int get_recursion_context(int *recursion)
 	return rctx;
 }
 
-static inline void put_recursion_context(int *recursion, int rctx)
+static inline void put_recursion_context(u8 *recursion, int rctx)
 {
 	barrier();
 	recursion[rctx]--;
-- 
2.45.2

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..

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

On Mon, Jul 01, 2024 at 02:56:43PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

Changelog failed to mention this crucial fact.