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

[Lai Jiangshan]

Mathieu Desnoyers wrote:
> * 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

I haven't seen this line:
			if (largest_align >= ltt_get_alignment())
				goto exit;

We shouldn't hardcode "sizeof(void *)", we should use ltt_get_alignment().


> 
>> +}
>> +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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>