Messages in this thread | | | Date | Tue, 3 Dec 2013 10:26:18 -0800 | Subject | Re: [RFC PATCH tip 0/5] tracing filters with BPF | From | Alexei Starovoitov <> |
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On Tue, Dec 3, 2013 at 7:33 AM, Steven Rostedt <rostedt@goodmis.org> wrote: > On Tue, 3 Dec 2013 10:16:55 +0100 > Ingo Molnar <mingo@kernel.org> wrote: > > >> So, to do the math: >> >> tracing 'all' overhead: 95 nsecs per event >> tracing 'eth5 + old filter' overhead: 157 nsecs per event >> tracing 'eth5 + BPF filter' overhead: 54 nsecs per event >> >> So via BPF and a fairly trivial filter, we are able to reduce tracing >> overhead for real - while old-style filters. > > Yep, seems that BPF can do what I wasn't able to do with the normal > filters. Although, I haven't looked at the code yet, I'm assuming that > the BPF works on the parameters passed into the trace event. The normal > filters can only process the results of the trace (what's being > recorded) not the parameters of the trace event itself. To get what's > recorded, we need to write to the buffer first, and then we decided if > we want to keep the event or not and discard the event from the buffer > if we do not. > > That method does not reduce overhead at all, and only adds to it, as > Alexei's tests have shown. The purpose of the filter was not to reduce > overhead, but to reduce filling the buffer with needless data.
Precisely. Assumption is that filters will filter out majority of the events. So filter takes pt_regs as input, has to interpret them and call bpf_trace_printk if it really wants to store something for the human to see. We can extend bpf trace filters to return true/false to indicate whether TP_printk-format specified as part of the event should be printed as well, but imo that's unnecessary. When I was using bpf filters to debug networking bits I didn't need that printk format of the event. I only used event as an entry point, filtering out things and printing different fields vs initial event. More like what developers do when they sprinkle trace_printk/dump_stack through the code while debugging.
the only inconvenience so far is to know how parameters are getting into registers. on x86-64, arg1 is in rdi, arg2 is in rsi,... I want to improve that after first step is done. In the proposed patches bpf_context == pt_regs at the event entry point. Would be cleaner to have struct {arg1,arg2,…} as bpf_context instead. But that needed more code and I wanted to keep the first patch to the minimum. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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