Messages in this thread | | | Date | Thu, 3 Dec 2015 18:31:49 +0800 | From | "Wangnan (F)" <> | Subject | Re: [RFC PATCH] perf/core: Put size of a sample at the end of it |
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On 2015/12/3 18:08, Peter Zijlstra wrote: > 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.
Thank you for your attitude. We can start consider it seriously.
Now I'm working on perf side code to make a workable prototype.
>> 2. We must ensure all events attached this ring buffer has >> 'PERF_SAMPLE_SIZE' selected. > That can be easily enforced.
Yes.
>> 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.
So does it means we need to pad before outputing size?
>> 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.
That's good. We don't need to consider this :)
Before receiving your comment I'm thinking about modifying DEFINE_OUTPUT_COPY() to write first sizeof(header) bytes at the end of reserved area, so it would work automatically for every possible events.
>> 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.
Thank you.
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