[ltt-dev] [PATCH 07/13] implement fmt align

[Mathieu Desnoyers]

* Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> 
> we need know the align for accessing the binary data from
> event.(skip the padding data, calculate offset).
> 
> It can also be used for write-side.
> 

Yes, but I prefer not, because I like to do a single pass in the format
string to calculate both the type sizes individually and the largest
alignment. It makes sense to implement this for the read-side though.

Please see comments below,

> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> ---
>  b/ltt/ltt-serialize.c      |   60 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/ltt-tracer.h |    9 ++++++
>  2 files changed, 69 insertions(+)
> diff --git a/include/linux/ltt-tracer.h b/include/linux/ltt-tracer.h
> index ece6819..b49cac2 100644
> --- a/include/linux/ltt-tracer.h
> +++ b/include/linux/ltt-tracer.h
> @@ -311,6 +316,9 @@ static inline unsigned int ltt_align(size_t align_drift, size_t size_of_type)
>  	size_t alignment = min(sizeof(void *), size_of_type);
>  	return (alignment - align_drift) & (alignment - 1);
>  }
> +
> +extern unsigned int ltt_fmt_largest_align(size_t align_drift, const char *fmt);
> +
>  /* Default arch alignment */
>  #define LTT_ALIGN
>  
> @@ -327,6 +335,12 @@ static inline unsigned int ltt_align(size_t align_drift,
>  	return 0;
>  }
>  
> +static inline unsigned int ltt_fmt_largest_align(size_t align_drift,
> +		const char *fmt)
> +{
> +	return 0;
> +}
> +
>  #define LTT_ALIGN __attribute__((packed))
>  
>  static inline int ltt_get_alignment(void)
> diff --git a/ltt/ltt-serialize.c b/ltt/ltt-serialize.c
> index 5c34899..deb9106 100644
> --- a/ltt/ltt-serialize.c
> +++ b/ltt/ltt-serialize.c
> @@ -531,6 +538,66 @@
>  }
>  EXPORT_SYMBOL_GPL(ltt_serialize_printf);
>  
> +#ifdef CONFIG_LTT_ALIGNMENT
> +
> +unsigned int ltt_fmt_largest_align(size_t align_drift, const char *fmt)
> +{
> +	char trace_size = 0, c_size = 0;
> +	enum ltt_type trace_type = LTT_TYPE_NONE, c_type = LTT_TYPE_NONE;
> +	unsigned long attributes = 0;
> +	int largest_align = 1;
> +
> +	for (; *fmt ; ++fmt) {
> +		switch (*fmt) {
> +		case '#':
> +			/* tracetypes (#) */
> +			++fmt;			/* skip first '#' */
> +			if (*fmt == '#')	/* Escaped ## */
> +				break;
> +			attributes = 0;
> +			fmt = parse_trace_type(fmt, &trace_size, &trace_type,
> +				&attributes);
> +
> +			largest_align = max_t(int, largest_align, trace_size);
> +			if (largest_align >= ltt_get_alignment())
> +				goto exit;
> +			break;
> +		case '%':
> +			/* c types (%) */
> +			++fmt;			/* skip first '%' */
> +			if (*fmt == '%')	/* Escaped %% */
> +				break;
> +			fmt = parse_c_type(fmt, &c_size, &c_type, NULL);
> +			/*
> +			 * Output c types if no trace types has been
> +			 * specified.
> +			 */
> +			if (!trace_size)
> +				trace_size = c_size;
> +			if (trace_type == LTT_TYPE_NONE)
> +				trace_type = c_type;
> +			if (c_type == LTT_TYPE_STRING)
> +				trace_type = LTT_TYPE_STRING;
> +
> +			largest_align = max_t(int, largest_align, trace_size);
> +			if (largest_align >= ltt_get_alignment())
> +				goto exit;
> +
> +			trace_size = 0;
> +			c_size = 0;
> +			trace_type = LTT_TYPE_NONE;
> +			c_size = LTT_TYPE_NONE;
> +			break;
> +		}
> +	}
> +
> +exit:
> +	return (largest_align - align_drift) & (largest_align - 1);

Hrm, I think there is something wrong here.

On x86_32, the max alignment is sizeof(u32).

However, if you get a u64 type here, the largest_align will become
sizeof(u64). This is why, in ltt_vtrace(), I do a 

        largest_align = 1;      /* must be non-zero for ltt_align */
        data_size = ltt_get_data_size(&closure, serialize_private,
                                        &largest_align, fmt, &args_copy);
        largest_align = min_t(int, largest_align, sizeof(void *));

So we should document in the ltt_fmt_largest_align header to always do a
largest_align = min_t(int, largest_align, sizeof(void *)); on the return
value (and check that we do it) or we simply do this operation before
returning.

Mathieu

> +}
> +EXPORT_SYMBOL_GPL(ltt_fmt_largest_align);
> +
> +#endif
> +
>  /*
>   * Calculate data size
>   * Assume that the padding for alignment starts at a sizeof(void *) address.
> 
> 
> 
> 
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-- 
Mathieu Desnoyers
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