On Wed, Dec 02, 2015 at 01:38:19PM +0000, Wang Nan wrote: > This sloution requires user program (perf) do more things. At least > following things and limitations should be considered: > > 1. Before reading such ring buffer, perf must ensure all events which > may output to it is already stopped, so the 'head' pointer it get > is the end of the last record.
Right, this is tricky, this would not allow two snapshots to happen back to back since that would then result in a bunch of missed events. Aside from this issue its a rather nice idea. > 2. We must ensure all events attached this ring buffer has > 'PERF_SAMPLE_SIZE' selected. That can be easily enforced. > 3. There must no tracking events output to this ring buffer. That is rather unfortunate, we'd best fix that up. > 4. 2 bytes extra space is required for each record. 8, perf records must be 8 byte aligned and sized. > Further improvement can be taken: > > 1. If PERF_SAMPLE_SIZE is selected, we can avoid outputting the event > size in header. Which eliminate extra space cose; That would mandate you always parse the stream backwards. Which seems rather unfortunate. Also, no you cannot recoup the extra space, see the alignment and size requirement. > 2. We can find a way to append size information for tracking events > also. The !sample records you mean? Yes those had better have them too. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
