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Message-ID: <20090907154824.GA1085@Krystal>
Date: Mon, 7 Sep 2009 11:48:24 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To: Jason Baron <jbaron@...hat.com>
Cc: linux-kernel@...r.kernel.org, roland@...hat.com, rth@...hat.com,
mingo@...e.hu
Subject: Re: [PATCH 0/4] RFC: jump label - (tracepoint optimizations)
* Jason Baron (jbaron@...hat.com) wrote:
> hi,
>
> Problem:
>
> Currenly, tracepoints are implemented using a conditional. The conditional
> check requires checking a global variable for each tracepoint. Although,
> the overhead of this check is small, it increases under memory pressure. As we
> increase the number of tracepoints in the kernel this may become more of an
> issue. In addition, tracepoints are often dormant (disabled), and provide no
> direct kernel functionality. Thus, it is highly desirable to reduce their
> impact as much as possible. Mathieu Desnoyers has already suggested a number of
> requirements for a solution to this issue.
>
Hi Jason,
Thanks for working on this. It's a useful idea that's just been sitting
there for too long now. Please see comments below,
> Solution:
>
> In discussing this problem with Roland McGrath and Richard Henderson, we came
> up with a new 'asm goto' statement that allows branching to a label. Thus, this
> patch set introdues a 'STATIC_JUMP_IF()' macro as follows:
>
> #ifdef HAVE_STATIC_JUMP
>
> #define STATIC_JUMP_IF(tag, label, cond) \
> asm goto ("1:" /* 5-byte insn */ \
> P6_NOP5 \
Hrm, be careful there. P6_NOP5 is not always a single instruction. If
you are preempted in the middle of it, bad things could happen, even
with stop_machine, if you iret in the middle the of the new jump
instruction. It could cause an illegal instruction fault. You should use
an atomic nop5. I think the function tracer already does, since I
told Steven about this exact issue.
> ".pushsection __jump_table, \"a\" \n\t" \
> _ASM_PTR "1b, %l[" #label "], %c0 \n\t" \
> ".popsection \n\t" \
> : : "i" (__sjstrtab_##tag) : : label)
>
Supporting multiple labels to create a real jump table would be a
nice-to-have as future enhancement too. This could be called
STATIC_JUMP_TABLE(). Actually, the STATIC_JUMP_IF could probably be
implemented in terms of STATIC_JUMP_TABLE.
> #else
>
> #define STATIC_JUMP_IF(tag, label, cond) \
> if (unlikely(cond)) \
> goto label;
>
Hrm, however, it's not clear to me how the STATIC_JUMP_TABLE() fallback
would look like. In the case of STATIC_JUMP_IF, it's a simple if (),
which makes support of compilers lacking static jump support easy. We
could probably use a switch statement to replace the STATIC_JUMP_TABLE
though.
> #endif /* !HAVE_STATIC_JUMP */
>
>
> which can be used as:
>
> STATIC_JUMP_IF(trace, trace_label, jump_enabled);
> printk("not doing tracing\n");
> if (0) {
> trace_label:
> printk("doing tracing: %d\n", file);
> }
>
Hrm. Is there any way to make this a bit prettier ? Given modifications
are made to gcc anyway...
Maybe:
static_jump_if (trace, jump_enabled) {
...
} else {
...
}
And for the jump table:
static_jump_table (trace, jump_select) {
case 0: ...
break;
case 1: ...
break;
default:
...
}
> ---------------------------------------
>
> Thus, if 'HAVE_STATIC_JUMP' is defined (which will depend ultimately on the
> existence of 'asm goto' in the compiler version), we simply have a no-op
> followed by a jump around the dormant (disabled) tracing code.
Hrm, why don't we collapse that into a single 5-bytes jump instruction
instead ?
> The
> 'STATIC_JUMP_IF()' macro, produces a 'jump_table' which has the following
> format:
>
> [instruction address] [jump target] [tracepoint name]
>
> Thus, to enable a tracepoint, we simply patch the 'instruction address' with
> a jump to the 'jump target'. The current implementation is using ftrace
> infrastructure to accomplish the patching, which uses 'stop_machine'. In
> subsequent versions, we will update the mechanism to use more efficient
> code patching techniques.
>
> I've tested the performance of this using 'get_cycles()' calls around the
> tracepoint call sites. For an Intel Core 2 Quad cpu (in cycles, averages):
>
> idle after tbench run
> ---- ----------------
> old code 32 88
> new code 2 4
>
>
> The performance improvement can be reproduced very reliably (using patch 4
> in this series) on both Intel and AMD hardware.
>
> In terms of code analysis the current code for the disabled case is a 'cmpl'
> followed by a 'je' around the tracepoint code. so:
>
> cmpl - 83 3d 0e 77 87 00 00 - 7 bytes
> je - 74 3e - 2 bytes
>
> total of 9 instruction bytes.
>
> The new code is a 'nopl' followed by a 'jmp'. Thus:
>
> nopl - 0f 1f 44 00 00 - 5 bytes
> jmp - eb 3e - 2 bytes
>
> total of 7 instruction bytes.
>
> So, the new code also accounts for 2 less bytes in the instruction cache per tracepoint.
>
With a single 5-bytes jump, this would account for a 5 bytes total,
which is 4 bytes less.
Thanks,
Mathieu
> here's a link to the gcc thread introducing this feature:
>
> http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
>
> Todo:
>
> -convert the patching to a more optimal implementation (not using stop machine)
> -expand infrastructure for modules
> -other use cases?
> -mark the trace_label section with 'cold' attributes
> -add module support
> -add support for other arches (besides x86)
>
> thanks,
>
> -Jason
>
> arch/x86/kernel/ftrace.c | 36 +++++
> arch/x86/kernel/test_nx.c | 3 +
> include/asm-generic/vmlinux.lds.h | 11 ++
> include/linux/ftrace.h | 3 +
> include/linux/jump_label.h | 45 ++++++
> include/linux/tracepoint.h | 50 +++++---
> kernel/Makefile | 2 +-
> kernel/jump_label.c | 271 +++++++++++++++++++++++++++++++++++++
> kernel/trace/trace_workqueue.c | 2 +
> kernel/tracepoint.c | 134 ++++++++++++++++++
> 10 files changed, 540 insertions(+), 17 deletions(-)
> create mode 100644 include/linux/jump_label.h
> create mode 100644 kernel/jump_label.c
>
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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