linux-kernel - Re: [RFC] tracepoint/jump

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Date:	Thu, 17 Nov 2011 16:25:20 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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

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.
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