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