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

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