[PATCH] trace: adjust code layout in get_recursion_context

[PATCH] trace: adjust code layout in get_recursion_context

[Jesper Dangaard Brouer]

In an XDP redirect applications using tracepoint xdp:xdp_redirect to
diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
was consuming 2% CPU. This was reduced to 1.6% with this simple
change.

Looking at the annotated asm code, it was clear that the unlikely case
in_nmi() test was chosen (by the compiler) as the most likely
event/branch.  This small adjustment makes the compiler (gcc version
7.1.1 20170622 (Red Hat 7.1.1-3)) put in_nmi() as an unlikely branch.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 kernel/events/internal.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..56aa462760fa 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -208,12 +208,12 @@ static inline int get_recursion_context(int *recursion)
 {
 	int rctx;
 
-	if (in_nmi())
-		rctx = 3;
+	if (in_softirq())
+		rctx = 1;
 	else if (in_irq())
 		rctx = 2;
-	else if (in_softirq())
-		rctx = 1;
+	else if (in_nmi())
+		rctx = 3;
 	else
 		rctx = 0;
 

Re: [PATCH] trace: adjust code layout in get_recursion_context

[Peter Zijlstra]

On Tue, Aug 22, 2017 at 04:40:24PM +0200, Jesper Dangaard Brouer wrote:
> In an XDP redirect applications using tracepoint xdp:xdp_redirect to
> diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
> was consuming 2% CPU. This was reduced to 1.6% with this simple
> change.

It is also incorrect. What do you suppose it now returns when the NMI
hits a hard IRQ which hit during a Soft IRQ?

> @@ -208,12 +208,12 @@ static inline int get_recursion_context(int *recursion)
>  {
>  	int rctx;
>  
> +	if (in_softirq())
> +		rctx = 1;
>  	else if (in_irq())
>  		rctx = 2;
> +	else if (in_nmi())
> +		rctx = 3;
>  	else
>  		rctx = 0;
>  
> 

Re: [PATCH] trace: adjust code layout in get_recursion_context

[Peter Zijlstra]

On Tue, Aug 22, 2017 at 05:14:10PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 22, 2017 at 04:40:24PM +0200, Jesper Dangaard Brouer wrote:
> > In an XDP redirect applications using tracepoint xdp:xdp_redirect to
> > diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
> > was consuming 2% CPU. This was reduced to 1.6% with this simple
> > change.
> 
> It is also incorrect. What do you suppose it now returns when the NMI
> hits a hard IRQ which hit during a Soft IRQ?

Does this help any? I can imagine the compiler could struggle to CSE
preempt_count() seeing how its an asm thing.

---
 kernel/events/internal.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..e0b5b8fa83a2 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -206,13 +206,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs);
 
 static inline int get_recursion_context(int *recursion)
 {
+	unsigned int pc = preempt_count();
 	int rctx;
 
-	if (in_nmi())
+	if (pc & NMI_MASK)
 		rctx = 3;
-	else if (in_irq())
+	else if (pc & HARDIRQ_MASK)
 		rctx = 2;
-	else if (in_softirq())
+	else if (pc & SOFTIRQ_OFFSET)
 		rctx = 1;
 	else
 		rctx = 0;

Re: [PATCH] trace: adjust code layout in get_recursion_context

[Jesper Dangaard Brouer]

On Tue, 22 Aug 2017 17:20:25 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Aug 22, 2017 at 05:14:10PM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 22, 2017 at 04:40:24PM +0200, Jesper Dangaard Brouer wrote:  
> > > In an XDP redirect applications using tracepoint xdp:xdp_redirect to
> > > diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
> > > was consuming 2% CPU. This was reduced to 1.6% with this simple
> > > change.  
> > 
> > It is also incorrect. What do you suppose it now returns when the NMI
> > hits a hard IRQ which hit during a Soft IRQ?  
> 
> Does this help any? I can imagine the compiler could struggle to CSE
> preempt_count() seeing how its an asm thing.

Nope, it does not help (see assembly below, with perf percentages).

But I think I can achieve that I want by a simple unlikely(in_nmi()) annotation.

> ---
>  kernel/events/internal.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index 486fd78eb8d5..e0b5b8fa83a2 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -206,13 +206,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs);
>  
>  static inline int get_recursion_context(int *recursion)
>  {
> +	unsigned int pc = preempt_count();
>  	int rctx;
>  
> -	if (in_nmi())
> +	if (pc & NMI_MASK)
>  		rctx = 3;
> -	else if (in_irq())
> +	else if (pc & HARDIRQ_MASK)
>  		rctx = 2;
> -	else if (in_softirq())
> +	else if (pc & SOFTIRQ_OFFSET)

Hmmm... shouldn't this be SOFTIRQ_MASK?

>  		rctx = 1;
>  	else
>  		rctx = 0;

perf_swevent_get_recursion_context  /proc/kcore
       │
       │
       │    Disassembly of section load0:
       │
       │    ffffffff811465c0 <load0>:
 13.32 │      push   %rbp
  1.43 │      mov    $0x14d20,%rax
  5.12 │      mov    %rsp,%rbp
  6.56 │      add    %gs:0x7eec3b5d(%rip),%rax
  0.72 │      lea    0x34(%rax),%rdx
  0.31 │      mov    %gs:0x7eec5db2(%rip),%eax
  2.46 │      mov    %eax,%ecx
  6.86 │      and    $0x7fffffff,%ecx
  0.72 │      test   $0x100000,%eax
       │    ↓ jne    40
       │      test   $0xf0000,%eax
  0.41 │    ↓ je     5b
       │      mov    $0x8,%ecx
       │      mov    $0x2,%eax
       │    ↓ jmp    4a
       │40:   mov    $0xc,%ecx
       │      mov    $0x3,%eax
  2.05 │4a:   add    %rcx,%rdx
 16.60 │      mov    (%rdx),%ecx
  2.66 │      test   %ecx,%ecx
       │    ↓ jne    6d
  1.33 │      movl   $0x1,(%rdx)
  1.54 │      pop    %rbp
  4.51 │    ← retq
  3.89 │5b:   shr    $0x8,%ecx
  9.53 │      and    $0x1,%ecx
  0.61 │      movzbl %cl,%eax
  0.92 │      movzbl %cl,%ecx
  4.30 │      shl    $0x2,%rcx
 14.14 │    ↑ jmp    4a
       │6d:   mov    $0xffffffff,%eax
       │      pop    %rbp
       │    ← retq
       │      xchg   %ax,%ax



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

Re: [PATCH] trace: adjust code layout in get_recursion_context

[Jesper Dangaard Brouer]

On Tue, 22 Aug 2017 19:00:39 +0200
Jesper Dangaard Brouer <brouer@redhat.com> wrote:

> On Tue, 22 Aug 2017 17:20:25 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > On Tue, Aug 22, 2017 at 05:14:10PM +0200, Peter Zijlstra wrote:  
> > > On Tue, Aug 22, 2017 at 04:40:24PM +0200, Jesper Dangaard Brouer wrote:    
> > > > In an XDP redirect applications using tracepoint xdp:xdp_redirect to
> > > > diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
> > > > was consuming 2% CPU. This was reduced to 1.6% with this simple
> > > > change.    
> > > 
> > > It is also incorrect. What do you suppose it now returns when the NMI
> > > hits a hard IRQ which hit during a Soft IRQ?    
> > 
> > Does this help any? I can imagine the compiler could struggle to CSE
> > preempt_count() seeing how its an asm thing.  
> 
> Nope, it does not help (see assembly below, with perf percentages).
> 
> But I think I can achieve that I want by a simple unlikely(in_nmi()) annotation.

Like:

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..e1a7ac7bd686 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -208,7 +208,7 @@ static inline int get_recursion_context(int *recursion)
 {
        int rctx;
 
-       if (in_nmi())
+       if (unlikely(in_nmi()))
                rctx = 3;
        else if (in_irq())
                rctx = 2;

Testing this show I get the expected result. Although, the 2% is
reduced to 1.85% (and not 1.6% as before).  


perf_swevent_get_recursion_context  /proc/kcore
       │
       │    Disassembly of section load0:
       │
       │    ffffffff811465c0 <load0>:
  4.94 │      push   %rbp
  2.56 │      mov    $0x14d20,%rax
 14.81 │      mov    %rsp,%rbp
  3.47 │      add    %gs:0x7eec3b5d(%rip),%rax
  0.91 │      lea    0x34(%rax),%rdx
  1.46 │      mov    %gs:0x7eec5db2(%rip),%eax
  8.04 │      test   $0x100000,%eax
       │    ↓ jne    59
  3.11 │      test   $0xf0000,%eax
       │    ↓ jne    4d
  0.37 │      test   $0xff,%ah
  1.83 │      setne  %cl
  9.87 │      movzbl %cl,%eax
  2.01 │      movzbl %cl,%ecx
  1.65 │      shl    $0x2,%rcx
  4.39 │3c:   add    %rcx,%rdx
 29.62 │      mov    (%rdx),%ecx
  2.93 │      test   %ecx,%ecx
       │    ↓ jne    65
  0.55 │      movl   $0x1,(%rdx)
  2.56 │      pop    %rbp
  4.94 │    ← retq
       │4d:   mov    $0x8,%ecx
       │      mov    $0x2,%eax
       │    ↑ jmp    3c
       │59:   mov    $0xc,%ecx
       │      mov    $0x3,%eax
       │    ↑ jmp    3c
       │65:   mov    $0xffffffff,%eax
       │      pop    %rbp
       │    ← retq



> > ---
> >  kernel/events/internal.h | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> > index 486fd78eb8d5..e0b5b8fa83a2 100644
> > --- a/kernel/events/internal.h
> > +++ b/kernel/events/internal.h
> > @@ -206,13 +206,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs);
> >  
> >  static inline int get_recursion_context(int *recursion)
> >  {
> > +	unsigned int pc = preempt_count();
> >  	int rctx;
> >  
> > -	if (in_nmi())
> > +	if (pc & NMI_MASK)
> >  		rctx = 3;
> > -	else if (in_irq())
> > +	else if (pc & HARDIRQ_MASK)
> >  		rctx = 2;
> > -	else if (in_softirq())
> > +	else if (pc & SOFTIRQ_OFFSET)  
> 
> Hmmm... shouldn't this be SOFTIRQ_MASK?
> 
> >  		rctx = 1;
> >  	else
> >  		rctx = 0;  
> 
> perf_swevent_get_recursion_context  /proc/kcore
>        │
>        │
>        │    Disassembly of section load0:
>        │
>        │    ffffffff811465c0 <load0>:
>  13.32 │      push   %rbp
>   1.43 │      mov    $0x14d20,%rax
>   5.12 │      mov    %rsp,%rbp
>   6.56 │      add    %gs:0x7eec3b5d(%rip),%rax
>   0.72 │      lea    0x34(%rax),%rdx
>   0.31 │      mov    %gs:0x7eec5db2(%rip),%eax
>   2.46 │      mov    %eax,%ecx
>   6.86 │      and    $0x7fffffff,%ecx
>   0.72 │      test   $0x100000,%eax
>        │    ↓ jne    40
>        │      test   $0xf0000,%eax
>   0.41 │    ↓ je     5b
>        │      mov    $0x8,%ecx
>        │      mov    $0x2,%eax
>        │    ↓ jmp    4a
>        │40:   mov    $0xc,%ecx
>        │      mov    $0x3,%eax
>   2.05 │4a:   add    %rcx,%rdx
>  16.60 │      mov    (%rdx),%ecx
>   2.66 │      test   %ecx,%ecx
>        │    ↓ jne    6d
>   1.33 │      movl   $0x1,(%rdx)
>   1.54 │      pop    %rbp
>   4.51 │    ← retq
>   3.89 │5b:   shr    $0x8,%ecx
>   9.53 │      and    $0x1,%ecx
>   0.61 │      movzbl %cl,%eax
>   0.92 │      movzbl %cl,%ecx
>   4.30 │      shl    $0x2,%rcx
>  14.14 │    ↑ jmp    4a
>        │6d:   mov    $0xffffffff,%eax
>        │      pop    %rbp
>        │    ← retq
>        │      xchg   %ax,%ax
> 
> 
> 



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

[PATCH V2] trace: adjust code layout in get_recursion_context

[Jesper Dangaard Brouer]

In an XDP redirect applications using tracepoint xdp:xdp_redirect to
diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
was consuming 2% CPU. This was reduced to 1.85% with this simple
change.

Looking at the annotated asm code, it was clear that the unlikely case
in_nmi() test was chosen (by the compiler) as the most likely
event/branch.  This small adjustment makes the compiler (gcc version
7.1.1 20170622 (Red Hat 7.1.1-3)) put in_nmi() as an unlikely branch.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 kernel/events/internal.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..e1a7ac7bd686 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -208,7 +208,7 @@ static inline int get_recursion_context(int *recursion)
 {
 	int rctx;
 
-	if (in_nmi())
+	if (unlikely(in_nmi()))
 		rctx = 3;
 	else if (in_irq())
 		rctx = 2;

Re: [PATCH] trace: adjust code layout in get_recursion_context

[Peter Zijlstra]

On Tue, Aug 22, 2017 at 07:00:39PM +0200, Jesper Dangaard Brouer wrote:
> >  static inline int get_recursion_context(int *recursion)
> >  {
> > +	unsigned int pc = preempt_count();
> >  	int rctx;
> >  
> > -	if (in_nmi())
> > +	if (pc & NMI_MASK)
> >  		rctx = 3;
> > -	else if (in_irq())
> > +	else if (pc & HARDIRQ_MASK)
> >  		rctx = 2;
> > -	else if (in_softirq())
> > +	else if (pc & SOFTIRQ_OFFSET)
> 
> Hmmm... shouldn't this be SOFTIRQ_MASK?

No, that was actually broken, this is correct. See the comment near
__local_bh_disable_ip(). We use the low SOFTIRQ bit to indicate if we're
in softirq and the rest of the bits to 'disable' softirq.

The code should've been using in_serving_softirq().

> perf_swevent_get_recursion_context  /proc/kcore
>        │
>        │
>        │    Disassembly of section load0:
>        │
>        │    ffffffff811465c0 <load0>:
>  13.32 │      push   %rbp
>   1.43 │      mov    $0x14d20,%rax
>   5.12 │      mov    %rsp,%rbp
>   6.56 │      add    %gs:0x7eec3b5d(%rip),%rax
>   0.72 │      lea    0x34(%rax),%rdx
>   0.31 │      mov    %gs:0x7eec5db2(%rip),%eax
>   2.46 │      mov    %eax,%ecx
>   6.86 │      and    $0x7fffffff,%ecx
>   0.72 │      test   $0x100000,%eax
>        │    ↓ jne    40
>        │      test   $0xf0000,%eax
>   0.41 │    ↓ je     5b
>        │      mov    $0x8,%ecx
>        │      mov    $0x2,%eax
>        │    ↓ jmp    4a
>        │40:   mov    $0xc,%ecx
>        │      mov    $0x3,%eax
>   2.05 │4a:   add    %rcx,%rdx
>  16.60 │      mov    (%rdx),%ecx
>   2.66 │      test   %ecx,%ecx
>        │    ↓ jne    6d
>   1.33 │      movl   $0x1,(%rdx)
>   1.54 │      pop    %rbp
>   4.51 │    ← retq
>   3.89 │5b:   shr    $0x8,%ecx
>   9.53 │      and    $0x1,%ecx
>   0.61 │      movzbl %cl,%eax
>   0.92 │      movzbl %cl,%ecx
>   4.30 │      shl    $0x2,%rcx
>  14.14 │    ↑ jmp    4a
>        │6d:   mov    $0xffffffff,%eax
>        │      pop    %rbp
>        │    ← retq
>        │      xchg   %ax,%ax
> 


Bah, that's fairly disgusting code but I'm failing to come up with
anything significantly better with the current rules.

However, if we change the rules on what *_{enter,exit} do, such that we
always have all the bits set, we can. The below patch (100% untested)
gives me the following asm, which looks better (of course, given how
things go today, it'll both explode and be slower once you get it to
work).

Also, its not strictly correct in that while we _should_ not have nested
hardirqs, they _could_ happen and push the SOFTIRQ count out of the 2
bit 'serving' range.

In any case, something to play with...

$ objdump -dr defconfig-build/kernel/events/core.o | awk '/<[^>]*>:$/ {p=0} /<perf_swevent_get_recursion_context>:/ {p=1} {if (p) print $0}'
0000000000004450 <perf_swevent_get_recursion_context>:
    4450:       55                      push   %rbp
    4451:       48 c7 c6 00 00 00 00    mov    $0x0,%rsi
                        4454: R_X86_64_32S      .data..percpu+0x20
    4458:       48 89 e5                mov    %rsp,%rbp
    445b:       65 48 03 35 00 00 00    add    %gs:0x0(%rip),%rsi        # 4463 <perf_swevent_get_recursion_context+0x13>
    4462:       00 
                        445f: R_X86_64_PC32     this_cpu_off-0x4
    4463:       65 8b 15 00 00 00 00    mov    %gs:0x0(%rip),%edx        # 446a <perf_swevent_get_recursion_context+0x1a>
                        4466: R_X86_64_PC32     __preempt_count-0x4
    446a:       89 d0                   mov    %edx,%eax
    446c:       c1 e8 08                shr    $0x8,%eax
    446f:       83 e0 01                and    $0x1,%eax
    4472:       89 c1                   mov    %eax,%ecx
    4474:       83 c0 01                add    $0x1,%eax
    4477:       f7 c2 00 00 0f 00       test   $0xf0000,%edx
    447d:       0f 44 c1                cmove  %ecx,%eax
    4480:       81 e2 00 00 10 00       and    $0x100000,%edx
    4486:       83 fa 01                cmp    $0x1,%edx
    4489:       83 d8 ff                sbb    $0xffffffff,%eax
    448c:       48 63 d0                movslq %eax,%rdx
    448f:       48 8d 54 96 2c          lea    0x2c(%rsi,%rdx,4),%rdx
    4494:       8b 0a                   mov    (%rdx),%ecx
    4496:       85 c9                   test   %ecx,%ecx
    4498:       75 08                   jne    44a2 <perf_swevent_get_recursion_context+0x52>
    449a:       c7 02 01 00 00 00       movl   $0x1,(%rdx)
    44a0:       5d                      pop    %rbp
    44a1:       c3                      retq   
    44a2:       b8 ff ff ff ff          mov    $0xffffffff,%eax
    44a7:       5d                      pop    %rbp
    44a8:       c3                      retq   
    44a9:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)

---
diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index c683996110b1..480babbbc2a5 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -35,7 +35,7 @@ static inline void rcu_nmi_exit(void)
 #define __irq_enter()					\
 	do {						\
 		account_irq_enter_time(current);	\
-		preempt_count_add(HARDIRQ_OFFSET);	\
+		preempt_count_add(HARDIRQ_OFFSET+SOFTIRQ_OFFSET);	\
 		trace_hardirq_enter();			\
 	} while (0)
 
@@ -51,7 +51,7 @@ static inline void rcu_nmi_exit(void)
 	do {						\
 		trace_hardirq_exit();			\
 		account_irq_exit_time(current);		\
-		preempt_count_sub(HARDIRQ_OFFSET);	\
+		preempt_count_sub(HARDIRQ_OFFSET+SOFTIRQ_OFFSET);	\
 	} while (0)
 
 /*
@@ -65,7 +65,7 @@ static inline void rcu_nmi_exit(void)
 		lockdep_off();					\
 		ftrace_nmi_enter();				\
 		BUG_ON(in_nmi());				\
-		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
+		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET + SOFTIRQ_OFFSET);	\
 		rcu_nmi_enter();				\
 		trace_hardirq_enter();				\
 	} while (0)
@@ -75,7 +75,7 @@ static inline void rcu_nmi_exit(void)
 		trace_hardirq_exit();				\
 		rcu_nmi_exit();					\
 		BUG_ON(!in_nmi());				\
-		preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);	\
+		preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET + SOFTIRQ_OFFSET);	\
 		ftrace_nmi_exit();				\
 		lockdep_on();					\
 		printk_nmi_exit();				\
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index cae461224948..e8e7b759b196 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -50,7 +50,7 @@
 #define HARDIRQ_OFFSET	(1UL << HARDIRQ_SHIFT)
 #define NMI_OFFSET	(1UL << NMI_SHIFT)
 
-#define SOFTIRQ_DISABLE_OFFSET	(2 * SOFTIRQ_OFFSET)
+#define SOFTIRQ_DISABLE_OFFSET	(4 * SOFTIRQ_OFFSET)
 
 /* We use the MSB mostly because its available */
 #define PREEMPT_NEED_RESCHED	0x80000000
diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..981e4163e16c 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -206,16 +206,15 @@ static inline unsigned long perf_aux_size(struct ring_buffer *rb)
 
 static inline int get_recursion_context(int *recursion)
 {
-	int rctx;
-
-	if (in_nmi())
-		rctx = 3;
-	else if (in_irq())
-		rctx = 2;
-	else if (in_softirq())
-		rctx = 1;
-	else
-		rctx = 0;
+	unsigned int pc = preempt_count();
+	int rctx = 0;
+
+	if (pc & SOFTIRQ_OFFSET)
+		rctx++;
+	if (pc & HARDIRQ_MASK)
+		rctx++;
+	if (pc & NMI_MASK)
+		rctx++;
 
 	if (recursion[rctx])
 		return -1;

Re: [PATCH] trace: adjust code layout in get_recursion_context

[Ingo Molnar]

* Peter Zijlstra <peterz@infradead.org> wrote:

> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index 486fd78eb8d5..981e4163e16c 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -206,16 +206,15 @@ static inline unsigned long perf_aux_size(struct ring_buffer *rb)
>  
>  static inline int get_recursion_context(int *recursion)
>  {
> -	int rctx;
> -
> -	if (in_nmi())
> -		rctx = 3;
> -	else if (in_irq())
> -		rctx = 2;
> -	else if (in_softirq())
> -		rctx = 1;
> -	else
> -		rctx = 0;
> +	unsigned int pc = preempt_count();
> +	int rctx = 0;
> +
> +	if (pc & SOFTIRQ_OFFSET)
> +		rctx++;
> +	if (pc & HARDIRQ_MASK)
> +		rctx++;
> +	if (pc & NMI_MASK)
> +		rctx++;

Just a nit: if this ever gets beyond the proof of concent stage please rename 'pc' 
to something like 'count', because 'pc' stands for so many other things (program 
counter, etc.) which makes it all look a bit weird ...

Thanks,

	Ingo

[tip:perf/core] tracing, perf: Adjust code layout in get_recursion_context()

[tip-bot for Jesper Dangaard Brouer]

Commit-ID:  d0618410eced4eb092295fad10312a4545fcdfaf
Gitweb:     http://git.kernel.org/tip/d0618410eced4eb092295fad10312a4545fcdfaf
Author:     Jesper Dangaard Brouer <brouer@redhat.com>
AuthorDate: Tue, 22 Aug 2017 19:22:43 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 25 Aug 2017 11:04:18 +0200

tracing, perf: Adjust code layout in get_recursion_context()

In an XDP redirect applications using tracepoint xdp:xdp_redirect to
diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
was consuming 2% CPU. This was reduced to 1.85% with this simple
change.

Looking at the annotated asm code, it was clear that the unlikely case
in_nmi() test was chosen (by the compiler) as the most likely
event/branch.  This small adjustment makes the compiler (GCC version
7.1.1 20170622 (Red Hat 7.1.1-3)) put in_nmi() as an unlikely branch.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/150342256382.16595.986861478681783732.stgit@firesoul
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/events/internal.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 5377c59..843e970 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -208,7 +208,7 @@ static inline int get_recursion_context(int *recursion)
 {
 	int rctx;
 
-	if (in_nmi())
+	if (unlikely(in_nmi()))
 		rctx = 3;
 	else if (in_irq())
 		rctx = 2;