Messages in this thread | | | Date | Mon, 18 Nov 2013 10:01:17 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/5] perf record: mmap output file - v5 |
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On Fri, Nov 15, 2013 at 09:41:53AM -0700, David Ahern wrote: > On 11/13/13, 4:34 AM, Peter Zijlstra wrote: > >>>one option here is not allow page faults and system wide system calls. > >>>system wide tracing needs mmap; page faults for a task can use write(). > >>>I left that option in case something like this came up. > >> > >>So maybe splice() sounds like the right long term solution after all? :-/ > > > >Right until you put a tracepoint (kprobe) somewhere in whatever function > >is used to transfer a single page into/from a splice pipe. > > > >You can always screw yourself over using this stuff, no exceptions. > > > > What now? Can we add the mmap path as an option?
I'd say an option is always a possibility, but someone please try what happens if you use stupid large events (dwarf stack copies) on PERF_COUNT_SW_PAGE_FAULTS (.period=1) while recording with mmap().
The other option is to simply disallow PERF_SAMPLE_STACK_USER for that event.
Personally I think 8k copies for every event are way stupid anyway, that's a metric ton of data at a huge cost.
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