[ltt-dev] [PATCH 03/13] read event header from ltt-relay buffer

[Mathieu Desnoyers]

* Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> 
> we implement ltt_read_event_header() read event header from ltt-relay
> buffer. it does the work oppositely to ltt_write_event_header().
> 

While I'm at it.. just want to tell you the global design looks very
good. Let's continue on the small details...

> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> ---
>  ltt-tracer.h |   59 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 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
> @@ -492,6 +506,65 @@ static inline size_t ltt_write_event_header(struct ltt_trace_struct *trace,
>  	return buf_offset;
>  }
>  
> +/*
> + * ltt_read_event_header
> + * buf_offset must aligned on 32 bits
> + */
> +static inline size_t ltt_read_event_header( struct rchan_buf *buf,

Whitespace above before struct.
This will need some kerneldocs-style header eventually.

> +		long buf_offset, u64 *tsc, u32 *event_size, u16 *eID,
> +		unsigned int *rflags)
> +{
> +	struct ltt_event_header header;
> +	u16 small_size;
> +
> +	ltt_relay_read(buf, buf_offset, &header, sizeof(header));
> +	buf_offset += sizeof(header);
> +

Random question :

On what architecture do you test this ? You might want to force the

ltt/Kconfig

config LTT_ALIGNMENT
        bool "Align Linux Trace Toolkit Traces"
        default y if !HAVE_EFFICIENT_UNALIGNED_ACCESS
        help
          This option enables dynamic alignment of data in buffers. The
          alignment is made on the smallest size between architecture size
          and the size of the value to be written.

          Dynamically calculating the offset of the data has a performance cost,
          but it is more efficient on some architectures (especially 64 bits) to
          align data than to write it unaligned.

Because many arch define HAVE_EFFICIENT_UNALIGNED_ACCESS, especially
x86. And that might be a source of bug. (this is the role of the
ltt_align() macro).

Mathieu

> +	*event_size = INT_MAX;
> +	*eID = header.id_time >> LTT_TSC_BITS;
> +	*tsc = header.id_time & LTT_TSC_MASK;
> +
> +	switch (*eID) {
> +	case 29:
> +		*rflags = LTT_RFLAG_ID_SIZE_TSC;
> +		ltt_relay_read(buf, buf_offset, eID, sizeof(u16));
> +		buf_offset += sizeof(u16);
> +		ltt_relay_read(buf, buf_offset, &small_size, sizeof(u16));
> +		buf_offset += sizeof(u16);
> +		if (small_size == 0xFFFFU) {
> +			ltt_relay_read(buf, buf_offset, event_size, sizeof(u32));
> +			buf_offset += sizeof(u32);
> +		} else
> +			*event_size = small_size;
> +		buf_offset += ltt_align(buf_offset, sizeof(u64));
> +		ltt_relay_read(buf, buf_offset, tsc, sizeof(u64));
> +		buf_offset += sizeof(u64);
> +		break;
> +	case 30:
> +		*rflags = LTT_RFLAG_ID_SIZE;
> +		ltt_relay_read(buf, buf_offset, eID, sizeof(u16));
> +		buf_offset += sizeof(u16);
> +		ltt_relay_read(buf, buf_offset, &small_size, sizeof(u16));
> +		buf_offset += sizeof(u16);
> +		if (small_size == 0xFFFFU) {

Hrm, well, in the previous email I told you we should not hardcode
"16".. and I notice that I've hardcoded 0xFFFU in
ltt_write_event_header... Well ideally this one should also become a
define, for both ltt_write_event_header and ltt_read_event_header.

Mathieu

> +			ltt_relay_read(buf, buf_offset, event_size, sizeof(u32));
> +			buf_offset += sizeof(u32);
> +		} else
> +			*event_size = small_size;
> +		break;
> +	case 31:
> +		*rflags = LTT_RFLAG_ID;
> +		ltt_relay_read(buf, buf_offset, eID, sizeof(u16));
> +		buf_offset += sizeof(u16);
> +		break;
> +	default:
> +		*rflags = 0;
> +		break;
> +	}
> +
> +	return buf_offset;
> +}
> +
>  /* Lockless LTTng */
>  
>  /* Buffer offset macros */
> 
> 
> 
> 
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-- 
Mathieu Desnoyers
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