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SubjectRe: [RFC] tracepoint/jump_label overhead
On 11/17/2011 05:25 PM, Peter Zijlstra wrote:
> On Thu, 2011-11-17 at 04:55 +0100, Eric Dumazet wrote:
> > 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 ?

You could do that with a code generator, which I'm sure everyone will like.

> Right so you're not really supposed to use arguments that require
> evaluation in tracepoints, although I bet its common these days :/
>
> The problem here is that its 'hard' to pass s in and have the
> TP_fast_assign() thing do the dereference because of the sl[auo]b thing.
>

You could have sl[auo]b define a function or macro which tp_fast_assign
then uses to dereference its parameter.

--
error compiling committee.c: too many arguments to function



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