Messages in this thread Patch in this message | | | Date | Tue, 22 Aug 2017 19:10:37 +0200 | From | Jesper Dangaard Brouer <> | Subject | Re: [PATCH] trace: adjust code layout in get_recursion_context |
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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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