Messages in this thread Patch in this message | | | Date | Mon, 15 Jun 2015 11:03:22 +0200 | From | Daniel Wagner <> | Subject | Re: latency histogram with BPF |
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On 06/12/2015 07:17 PM, Alexei Starovoitov wrote: > On 6/12/15 7:33 AM, Daniel Wagner wrote: >> On 06/12/2015 08:12 AM, Daniel Wagner wrote: >> Attaching kprobes to trace_preempt_[on|off] works fine. Empty BPF >> programs connected to the probes is no problem as well. So I changed the >> BPF program to use only arrays instead of hash tables. No crash anymore. > > yes. I've tried that too. arrays work fine indeed. > >> I suspect the hash table code will call trace_preempt_[off|on] >> eventually and that is not going to fly. > > The recursive calls into bpf programs are detected and prevented. > That's ok. I've tested attaching kprobes to kmalloc/kfree and > from the program do hash_map->update_elem->kmalloc which triggers > recursive call into the same program. All works fine. > There is something else here.
If the first map is an array all is fine too. So it seems it need two hash tables to trigger it.
@@ -11,7 +11,7 @@ #include "bpf_helpers.h"
struct bpf_map_def SEC("maps") my_map = { - .type = BPF_MAP_TYPE_HASH, + .type = BPF_MAP_TYPE_ARRAY, .key_size = sizeof(unsigned int), .value_size = sizeof(u64), .max_entries = 1024, @@ -45,9 +45,10 @@ SEC("kprobe/trace_preempt_off") int bpf_prog1(struct pt_regs *ctx) { int cpu = bpf_get_smp_processor_id(); - u64 ts = bpf_ktime_get_ns(); + u64 *ts = bpf_map_lookup_elem(&my_map, &cpu);
- bpf_map_update_elem(&my_map, &cpu, &ts, BPF_ANY); + if (ts) + *ts = bpf_ktime_get_ns();
return 0; }
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