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Subject[RFC] tracepoint/jump_label overhead
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Date
The general admitted claim of a tracepoint being on x86 a single
instruction :

jmp +0

Is not always true.

For example in mm/slub.c, kmem_cache_alloc()

void *ret = slab_alloc(s, gfpflags, NUMA_NO_NODE, _RET_IP_);
trace_kmem_cache_alloc(_RET_IP_, ret, s->objsize, s->size, gfpflags);
return ret;

We can check compiler output and see that 4 extra instructions were
added because s->objsize & s->size are evaluated.

I noticed this in a perf session, because these 4 extra instructions
added some noticeable latency/cost.

c10e26a4: 8b 5d d8 mov -0x28(%ebp),%ebx
c10e26a7: 85 db test %ebx,%ebx
c10e26a9: 75 6d jne c10e2718 (doing the memset())
c10e26ab: 8b 76 0c mov 0xc(%esi),%esi // extra 1
c10e26ae: 8b 5d 04 mov 0x4(%ebp),%ebx // extra 2
c10e26b1: 89 75 f0 mov %esi,-0x10(%ebp) // extra 3
c10e26b4: 89 5d ec mov %ebx,-0x14(%ebp) // extra 4
c10e26b7: e9 00 00 00 00 jmp c10e26bc
c10e26bc: 8b 45 d8 mov -0x28(%ebp),%eax
c10e26bf: 83 c4 28 add $0x28,%esp
c10e26c2: 5b pop %ebx
c10e26c3: 5e pop %esi
c10e26c4: 5f pop %edi
c10e26c5: c9 leave


A fix would be to not declare an inline function but a macro...

#define trace_kmem_cache_alloc(...) \
if (static_branch(&__tracepoint_kmem_cache_alloc.key)) \
__DO_TRACE(&__tracepoint_kmem_cache_alloc, \
...

Anyone has some clever idea how to make this possible ?

Thanks




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