Messages in this thread | | | Subject | Re: [PATCH net-next 2/8] perf, bpf: allow bpf programs attach to tracepoints | From | Alexei Starovoitov <> | Date | Tue, 5 Apr 2016 11:09:30 -0700 |
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On 4/5/16 7:18 AM, Peter Zijlstra wrote: > On Mon, Apr 04, 2016 at 09:52:48PM -0700, Alexei Starovoitov wrote: >> introduce BPF_PROG_TYPE_TRACEPOINT program type and allow it to be >> attached to tracepoints. > > More specifically the perf tracepoint handler, not tracepoints directly.
yes. perf tracepoint handler only. There is no attempt here to attach to ftrace tracepoint handler.
>> The tracepoint will copy the arguments in the per-cpu buffer and pass >> it to the bpf program as its first argument. > >> The layout of the fields can be discovered by doing >> 'cat /sys/kernel/debug/tracing/events/sched/sched_switch/format' >> prior to the compilation of the program with exception that first 8 bytes >> are reserved and not accessible to the program. This area is used to store >> the pointer to 'struct pt_regs' which some of the bpf helpers will use: >> +---------+ >> | 8 bytes | hidden 'struct pt_regs *' (inaccessible to bpf program) >> +---------+ >> | N bytes | static tracepoint fields defined in tracepoint/format (bpf readonly) >> +---------+ >> | dynamic | __dynamic_array bytes of tracepoint (inaccessible to bpf yet) >> +---------+ >> >> Not that all of the fields are already dumped to user space via perf ring buffer >> and some application access it directly without consulting tracepoint/format. > > We call those apps broken..
yes. uapi/linux/perf_event.h lines 742-749 are pretty clear about it: "In other words, PERF_SAMPLE_RAW contents are not an ABI"
>> Same rule applies here: static tracepoint fields should only be accessed >> in a format defined in tracepoint/format. The order of fields and >> field sizes are not an ABI. > > >> @@ -56,8 +57,9 @@ perf_trace_##call(void *__data, proto) \ >> sizeof(u64)); \ >> __entry_size -= sizeof(u32); \ >> \ >> - entry = perf_trace_buf_prepare(__entry_size, \ >> - event_call->event.type, &__regs, &rctx); \ >> + event_type = prog ? TRACE_EVENT_TYPE_MAX : event_call->event.type; \ >> + entry = perf_trace_buf_prepare(__entry_size, event_type, \ >> + &__regs, &rctx); \ >> if (!entry) \ >> return; \ >> \ >> @@ -67,6 +69,14 @@ perf_trace_##call(void *__data, proto) \ >> \ >> { assign; } \ >> \ >> + if (prog) { \ >> + *(struct pt_regs **)entry = __regs; \ >> + if (!trace_call_bpf(prog, entry) || hlist_empty(head)) { \ >> + perf_swevent_put_recursion_context(rctx); \ >> + return; \ > > So if the prog 'fails' you consume the entry,
I wouldn't call it 'fails' ;) The interpretation of return code from bpf program is defined in kernel/trace/bpf_trace.c as: * 0 - return from kprobe (event is filtered out) * 1 - store kprobe event into ring buffer * Other values are reserved and currently alias to 1
so the above !trace_call_bpf() check matches existing bpf+kprobe behavior.
> >> + } \ >> + memset(&entry->ent, 0, sizeof(entry->ent)); \ > > But if not, you destroy it and then feed it to perf?
yes. If bpf prog returns 1 the buffer goes into normal ring-buffer with all perf_event attributes and so on. So far there wasn't a single real use case where we went this path. Programs always do aggregation inside and pass stuff to user space either via bpf maps or via bpf_perf_event_output() helper. I wanted to keep perf_trace_xx() calls to be minimal in .text size so memset above is one x86 instruction, but I don't mind replacing this memset with a call to a helper function that will do: local_save_flags(flags); tracing_generic_entry_update(entry, flags, preempt_count()); entry->type = type; Then whether bpf attached or not the ring buffer will see the same raw tracepoint entry. You think it's cleaner?
> >> + } \ >> perf_trace_buf_submit(entry, __entry_size, rctx, __addr, \ >> __count, __regs, head, __task); \ >> } > > >> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c >> index 7a68afca8249..7ada829029d3 100644 >> --- a/kernel/trace/trace_event_perf.c >> +++ b/kernel/trace/trace_event_perf.c >> @@ -284,6 +284,9 @@ void *perf_trace_buf_prepare(int size, unsigned short type, >> *regs = this_cpu_ptr(&__perf_regs[*rctxp]); >> raw_data = this_cpu_ptr(perf_trace_buf[*rctxp]); >> >> + if (type == TRACE_EVENT_TYPE_MAX) >> + return raw_data; >> + >> /* zero the dead bytes from align to not leak stack to user */ >> memset(&raw_data[size - sizeof(u64)], 0, sizeof(u64)); > > What's this hunk do? Why can you skip this stuff for BPF attached > events?
that hunk is skipping init of first 8 bytes, which are occupied by 'struct trace_entry' normally and are inited as: local_save_flags(flags); tracing_generic_entry_update(entry, flags, preempt_count()); entry->type = type; that adds extra overhead that bpf progs don't need. If bpf needs current pid, it calls bpf_get_current_pid_tgid() helper. local_save_flags() is also quite slow x86 insn that is not needed. If programs would need to read flags, we can introduce new helper to read it. These 8 bytes are instead used to store hidden 'struct pt_regs' pointer which is invisible to bpf programs directly and used by two bpf helpers: bpf_get_stackid() and bpf_perf_event_output() which need pt_regs. See patch 4/8
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