linux-kernel - Re: [PATCH 04/15] ftrace: pack event structures.

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Date:	Tue, 08 Mar 2011 18:30:03 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"David S. Miller" <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, mrubin@...gle.com,
	David Sharp <dhsharp@...gle.com>
Subject: Re: [PATCH 04/15] ftrace: pack event structures.

David (Miller),

I'm going back through this patch set and looking to incorporate it into
2.6.39. But I know how touchy sparc is with "packed" data structures. Do
you see this patch as hurting sparc?

I wonder if we should change this to something like:

#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
# define do_event_packed __attirbute__((packed))
#else
# define do_event_packed
#endif

and use "do_event_packed" instead?


On Fri, 2010-12-03 at 16:13 -0800, David Sharp wrote:
> Ftrace event structures have a 12-byte struct trace_entry at the beginning.
> If the structure is aligned, this means that if the first field is 64-bits,
> there will be 4 bytes of padding. Ironically, due to the 4-byte ringbuffer
> header, this will make 64-bit writes unaligned, if the ring buffer position
> is currently 64-bit aligned:
> 4(rb)+12(ftrace)+4(pad) = 20;  20%8 = 4

Note, on 64bit archs without CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, the
ring buffer forces the 8 byte header, to keep the data portion of the
event load 8byte aligned.

-- Steve

> 
> Adding __attribute__((packed)) to the event structures removes the extra
> space from the trace events, and actually improves alignment of trace
> events with a first field that is 64-bits.
> 
> About 65 tracepoints have a 4-byte pad at offset 12:
>  # find events -name format | xargs -n1 awk '
>    $1=="name:" {name=$2}
>    $1=="format:"{FS="\t"}
>    $3=="offset:12;" && $4=="size:4;"{okay=1}
>    $3=="offset:16;" && !okay {print name}' | wc -l
> 65
> 
> With all 'syscalls' and 'timer' events enabled, this results in a 5%
> improvement in a simple 512MB read benchmark with warm caches.
> 
> Tested:
> 
> setup:
>  # echo 1 >events/syscalls/enable
>  # echo 1 >events/timer/enable
>  # echo 0 > tracing_enabled
> off:
>  # for n in $(seq 10) ; do \
>    time dd if=/dev/hda3 of=/dev/null bs=1K count=512K ; \
>    done
> on:
>  # for n in $(seq 10) ; do \
>    echo 1 >tracing_enabled; \
>    time dd if=/dev/hda3 of=/dev/null bs=1K count=512K ; \
>    echo 0 >tracing_enabled; \
>    echo > trace; \
>    done
> 
> real time      mean/median/stdev
> w/o patch off: 1.1679/1.164/0.0169
> w/o patch on : 1.9432/1.936/0.0274
> w/  patch off: 1.1715/1.159/0.0431
> w/  patch on : 1.8425/1.836/0.0138
> "on" delta: -0.1007 --> -5.2%
> 
> Google-Bug-Id: 2895627
> 
> Signed-off-by: David Sharp <dhsharp@...gle.com>
> ---
>  include/trace/ftrace.h |    5 +++--
>  kernel/trace/trace.h   |    2 +-
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index a9377c0..51d1f52 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -48,7 +48,8 @@
>  #define __array(type, item, len)	type	item[len];
>  
>  #undef __dynamic_array
> -#define __dynamic_array(type, item, len) u32 __data_loc_##item;
> +#define __dynamic_array(type, item, len) \
> +    u32 __data_loc_##item __attribute__((aligned(4)));
>  
>  #undef __string
>  #define __string(item, src) __dynamic_array(char, item, -1)
> @@ -62,7 +63,7 @@
>  		struct trace_entry	ent;				\
>  		tstruct							\
>  		char			__data[0];			\
> -	};								\
> +	} __attribute__((packed));					\
>  									\
>  	static struct ftrace_event_class event_class_##name;
>  
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 9021f8c..2e80433 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -60,7 +60,7 @@ enum trace_type {
>  	struct struct_name {					\
>  		struct trace_entry	ent;			\
>  		tstruct						\
> -	}
> +	} __attribute__((packed))
>  
>  #undef TP_ARGS
>  #define TP_ARGS(args...)	args


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