Hi,
I have an issue with kretprobes on a GCE instance. The VM has only one
CPU and the kernel is configured without preempt:
/sys/devices/system/cpu/possible
0
# cat /boot/config-4.4.0-66-generic |grep CONFIG_PREEMPT
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
In this case, my understanding was there could be only one active
kretprobe (maxactive=1):
/* Pre-allocate memory for max kretprobe instances */
if (rp->maxactive <= 0) {
#ifdef CONFIG_PREEMPT
rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
#else
rp->maxactive = num_possible_cpus();
#endif
}
However, some kretprobes get lost. I added a bpf_trace_printk at the
very beginning of both kprobe__tcp_v4_connect and
kretprobe__tcp_v4_connect and I get the following log in trace_pipe:
scope-19720 [000] d... 497.513538: : kprobe__tcp_v4_connect -
pid_tgid 84662395358472
scope-19720 [000] d... 497.513703: : kretprobe__tcp_v4_connect -
pid_tgid 84662395358472
curl-19834 [000] d... 497.516090: : kprobe__tcp_v4_connect -
pid_tgid 85186381368698
scope-19720 [000] d... 497.516345: : kprobe__tcp_v4_connect -
pid_tgid 84662395358472
scope-19821 [000] d... 497.519377: : kprobe__tcp_v4_connect -
pid_tgid 84662395358573
scope-19821 [000] d... 497.521792: : kprobe__tcp_v4_connect -
pid_tgid 84662395358573
curl-19834 [000] d... 497.529933: : kretprobe__tcp_v4_connect -
pid_tgid 85186381368698
scope-19790 [000] d... 497.624384: : kprobe__tcp_v4_connect -
pid_tgid 84658100391246
scope-19790 [000] dN.. 497.624592: : kretprobe__tcp_v4_connect -
pid_tgid 84658100391246
Most of the time it works fine, but here above there is a succession
of 4 kprobes with only one kretprobe (pid 85186381368698).
Is there an explanation for the lost kretprobes? I don't see the
function tcp_v4_connect sleeping.
Is there a way to define a ebpf-kretprobe with a higher maxactive?
Cheers,
Alban
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