linux-kernel - [RFC] tracepoint/jump

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Date:	Thu, 17 Nov 2011 04:55:04 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: [RFC] tracepoint/jump_label overhead

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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