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Date: Thu, 17 Nov 2011 10:56:17 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Eric Dumazet <eric.dumazet@...il.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...e.hu>,
Christoph Lameter <cl@...ux-foundation.org>,
rostedt <rostedt@...dmis.org>
Subject: Re: [RFC] tracepoint/jump_label overhead
* Peter Zijlstra (peterz@...radead.org) 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 ?
>
> 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.
I created a variant of tracepoints for userspace in my lttng-ust project
where I changed the invokation of a tracepoint to:
tracepoint(provider, name, arg1, arg2, arg3, ..., arg10);
This allows me to much with a the tracepoint call-site since it becomes
a macro. This macro uses the __tracepoint_provider_name() static inline
underneath.
I just took a go at implementing the behavior proposed by Eric Dumazet
in the lttng-ust tree, it seems to work out nicely! Now doing this at
kernel-level would imply changing the trace_system_event(..) call-sites
into trace(system, event, ...), trace(system_event, ...) or
tracepoint(system, event, ...). Is that something you are willing to
consider doing ?
Diff for lttng-ust:
diff --git a/include/lttng/tracepoint.h b/include/lttng/tracepoint.h
index 2ca4598..8e3b604 100644
--- a/include/lttng/tracepoint.h
+++ b/include/lttng/tracepoint.h
@@ -35,8 +35,11 @@ extern "C" {
* Tracepoints should be added to the instrumented code using the
* "tracepoint()" macro.
*/
-#define tracepoint(provider, name, args...) \
- __trace_##provider##___##name(args)
+#define tracepoint(provider, name, args...) \
+ do { \
+ if (caa_unlikely(__tracepoint_##provider##___##name.state)) \
+ __trace_##provider##___##name(args); \
+ } while (0)
/*
* it_func[0] is never NULL because there is at least one element in the array
@@ -148,13 +151,6 @@ extern "C" {
#define _TP_ARGS_PROTO_DATA(...) _TP_PROTO_DATA_N(_TP_NARGS(0, ##__VA_ARGS__), ##__VA_ARGS__)
#define _TP_ARGS_VARS_DATA(...) _TP_VARS_DATA_N(_TP_NARGS(0, ##__VA_ARGS__), ##__VA_ARGS__)
-#define __CHECK_TRACE(provider, name, proto, args) \
- do { \
- if (caa_unlikely(__tracepoint_##provider##___##name.state)) \
- __DO_TRACE(&__tracepoint_##provider##___##name, \
- TP_PARAMS(proto), TP_PARAMS(args)); \
- } while (0)
-
/*
* Make sure the alignment of the structure in the __tracepoints section will
* not add unwanted padding between the beginning of the section and the
@@ -164,8 +160,8 @@ extern "C" {
extern struct tracepoint __tracepoint_##provider##___##name; \
static inline void __trace_##provider##___##name(proto) \
{ \
- __CHECK_TRACE(provider, name, TP_PARAMS(data_proto), \
- TP_PARAMS(data_args)); \
+ __DO_TRACE(&__tracepoint_##provider##___##name, \
+ TP_PARAMS(data_proto), TP_PARAMS(data_args)); \
} \
static inline int \
__register_trace_##provider##___##name(void (*probe)(data_proto), void *data) \
--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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