On Thu, Jun 2, 2022 at 6:52 PM John Mazzie <john.p.mazzie@xxxxxxxxx> wrote: > > Thanks for the help. fentry/fexit seem to be working to get every > event when tracing both events. > > I do have one question about how fentry/fexit work in regards to the > function parameters. fexit can access the function parameters in > addition to the return value. Let's say the parameters are pointers > whose value changes between entry and exit on the probed function. Are > the parameters being accessed on entry or exit in fexit. My assumption > would be exit, so I could access the modified values. Is that correct? > The data I'm pulling appears like it might just be the entry > (non-configured) values. For fexit programs values of registers corresponding to input arguments is stored before the function call. If function updates those register it won't be reflected. > > On Tue, May 24, 2022 at 6:40 PM Andrii Nakryiko > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > On Tue, May 24, 2022 at 9:12 AM John Mazzie <john.p.mazzie@xxxxxxxxx> wrote: > > > > > > After thinking about this more, maybe it's more to do with the > > > interrupt handler for nvme_complete_rq. > > > > > > In this situation, sometimes the handler for nvme_setup_cmd could be > > > interrupted to handle nvme_complete_rq and in this situation the > > > nvme_complete_rq handler wouldn't run. because the nvme_setup_cmd > > > handler is not complete. > > > > > > Is this understanding correct? > > > > Yes. > > > > > I'm assuming there is no real > > > workaround for this situation, so we may just need to accept some > > > missed events. > > > > Try using fentry/fexit programs instead. They use different reentrancy > > protection which is at a per-program level. > > > > > > > > John > > > > > > On Sat, May 21, 2022 at 11:52 AM John Mazzie <john.p.mazzie@xxxxxxxxx> wrote: > > > > > > > > In this case is a BPF program the individual handler of a tracepoint, > > > > or in my context, a BPF program my compiled program that traces both > > > > tracepoints? We aren't running any other BPF tracing during these > > > > tests besides my program counting these 2 tracepoints. > > > > > > > > In my program I have 2 handlers, one for > > > > tracepoint:nvme:nvme_setup_cmd and another for > > > > tracepoint:nvme:nvme_complete_rq. I've created a PERCPU_HASH map for > > > > each handler (unique map for each) to use that keeps track of each > > > > time the handler is invoked. The only thing that handler is doing in > > > > each case is incrementing the count value in the map. Though I do > > > > filter by device on each tracepoint. If I comment out the > > > > nvme_setup_cmd code the nvme_complete_rq does get the correct count. > > > > > > > > The user side of my program just prints the values for each of these > > > > maps on a 10 second increment. > > > > > > > > I can share my code to make this easier, is it preferred that I upload > > > > my code to github and share the link in this thread? > > > > > > > > I agree that your suggestion could be my issue, but I just want to > > > > make sure we're on the same page since I'm less familiar with the > > > > internals of BPF. > > > > > > > > Thanks, > > > > John > > > > > > > > On Fri, May 20, 2022 at 7:10 PM Andrii Nakryiko > > > > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > > > > > > > On Wed, May 18, 2022 at 2:35 PM John Mazzie <john.p.mazzie@xxxxxxxxx> wrote: > > > > > > > > > > > > My group at Micron is using BPF and love the tracing capabilities it > > > > > > provides. We are mainly focused on the storage subsystem and BPF has > > > > > > been really helpful in understanding how the storage subsystem > > > > > > interacts with our drives while running applications. > > > > > > > > > > > > In the process of developing a tool using BPF to trace the nvme > > > > > > driver, we ran into an issue with some missing events. I wanted to > > > > > > check to see if this is possibly a bug/limitation that I'm hitting or > > > > > > if it's expected behavior with heavy tracing. We are trying to trace 2 > > > > > > trace points (nvme_setup_cmd and nvme_complete_rq) around 1M times a > > > > > > second. > > > > > > We noticed if we just trace one of the two, we see all the expected > > > > > > events, but if we trace both at the same time, the nvme_complete_rq > > > > > > > > > > kprobe programs have per-CPU reentrancy protection. That is, if some > > > > > BPF kprobe/tracepoint program is running and something happens (e.g., > > > > > BPF program calls some kernel function that has another BPF program > > > > > attached to it, or preemption happens and another BPF program is > > > > > supposed to run) that would trigger another BPF program, then that > > > > > nested BPF program invocation will be skipped. > > > > > > > > > > This might be what happens in your case. > > > > > > > > > > > misses events. I am using two different percpu_hash maps to count both > > > > > > events. One for setup and another for complete. My expectation was > > > > > > that tracing these events would affect performance, somewhat, but not > > > > > > miss events. Ultimately the tool would be used to trace nvme latencies > > > > > > at the driver level by device and process. > > > > > > > > > > > > My tool was developed using libbpf v0.7, and I've tested on Rocky > > > > > > Linux 8.5 (Kernel 4.18.0), Ubuntu 20.04 (Kernel 5.4) and Fedora 36 > > > > > > (Kernel 5.17.6) with the same results. > > > > > > > > > > > > Thanks, > > > > > > John Mazzie > > > > > > Principal Storage Solutions Engineer > > > > > > Micron Technology, Inc.
