On Mon, Jun 3, 2019 at 5:44 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote: > > On 06/04/2019 01:54 AM, Alexei Starovoitov wrote: > > On Mon, Jun 3, 2019 at 4:48 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote: > >> On 06/04/2019 01:27 AM, Alexei Starovoitov wrote: > >>> On Mon, Jun 3, 2019 at 3:59 PM Matt Mullins <mmullins@xxxxxx> wrote: > >>>> > >>>> If these are invariably non-nested, I can easily keep bpf_misc_sd when > >>>> I resubmit. There was no technical reason other than keeping the two > >>>> codepaths as similar as possible. > >>>> > >>>> What resource gives you worry about doing this for the networking > >>>> codepath? > >>> > >>> my preference would be to keep tracing and networking the same. > >>> there is already minimal nesting in networking and probably we see > >>> more when reuseport progs will start running from xdp and clsbpf > >>> > >>>>> Aside from that it's also really bad to miss events like this as exporting > >>>>> through rb is critical. Why can't you have a per-CPU counter that selects a > >>>>> sample data context based on nesting level in tracing? (I don't see a discussion > >>>>> of this in your commit message.) > >>>> > >>>> This change would only drop messages if the same perf_event is > >>>> attempted to be used recursively (i.e. the same CPU on the same > >>>> PERF_EVENT_ARRAY map, as I haven't observed anything use index != > >>>> BPF_F_CURRENT_CPU in testing). > >>>> > >>>> I'll try to accomplish the same with a percpu nesting level and > >>>> allocating 2 or 3 perf_sample_data per cpu. I think that'll solve the > >>>> same problem -- a local patch keeping track of the nesting level is how > >>>> I got the above stack trace, too. > >>> > >>> I don't think counter approach works. The amount of nesting is unknown. > >>> imo the approach taken in this patch is good. > >>> I don't see any issue when event_outputs will be dropped for valid progs. > >>> Only when user called the helper incorrectly without BPF_F_CURRENT_CPU. > >>> But that's an error anyway. > >> > >> My main worry with this xchg() trick is that we'll miss to export crucial > >> data with the EBUSY bailing out especially given nesting could increase in > >> future as you state, so users might have a hard time debugging this kind of > >> issue if they share the same perf event map among these programs, and no > >> option to get to this data otherwise. Supporting nesting up to a certain > >> level would still be better than a lost event which is also not reported > >> through the usual way aka perf rb. > > > > I simply don't see this 'miss to export data' in all but contrived conditions. > > Say two progs share the same perf event array. > > One prog calls event_output and while rb logic is working > > another prog needs to start executing and use the same event array > > Correct. > > > slot. Today it's only possible for tracing prog combined with networking, > > but having two progs use the same event output array is pretty much > > a user bug. Just like not passing BPF_F_CURRENT_CPU. > > I don't see the user bug part, why should that be a user bug? because I suspect that 'struct bpf_event_entry' is not reentrable (even w/o issues with 'struct perf_sample_data'). Even if we always use 'struct perf_sample_data' on stack, I suspect the same 'struct bpf_event_entry' cannot be reused in the nested way. > It's the same > as if we would say that sharing a BPF hash map between networking programs > attached to different hooks or networking and tracing would be a user bug > which it is not. One concrete example would be cilium monitor where we > currently expose skb trace and drop events a well as debug data through > the same rb. This should be usable from any type that has perf_event_output > helper enabled (e.g. XDP and tc/BPF) w/o requiring to walk yet another per > cpu mmap rb from user space. sure. those are valid use cases, but in all cases bpf_event_output() on particular 'struct bpf_event_entry' will end before another one will begin, no?
