Messages in this thread | | | Date | Thu, 17 Nov 2011 17:38:36 +0200 | From | Avi Kivity <> | Subject | Re: [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
| |