LTTng » LTTng-UST

tracing "hello" with kernel tracing enabled lets us discover that sched_getcpu, supposed to be a vDSO, actually triggers a system call:

[16:05:32.991027867] (+0.000000909) ust_tests_hello:tptest: { 1 }, { intfield = 9, intfield2 = 0x9, longfield = 9, netintfield = 9, netintfieldhex = 0x9, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2 }
[16:05:32.991029728] (+0.000001861) sys_getcpu: { 1 }, { cpup = 0x7FFF1B010350, nodep = 0x0, tcache = 0x7FC76A03E7D8 }
[16:05:32.991033220] (+0.000003492) exit_syscall: { 1 }, { ret = 0 }
[16:05:32.991034256] (+0.000001036) ust_tests_hello:tptest: { 1 }, { intfield = 10, intfield2 = 0xA, longfield = 10, netintfield = 10, netintfieldhex = 0xA, arrfield1 = [ [0] = 1, [1] = 2, [2] = 3 ], arrfield2 = "test", _seqfield1_length = 4, seqfield1 = [ [0] = 116, [1] = 101, [2] = 115, [3] = 116 ], _seqfield2_length = 4, seqfield2 = "test", stringfield = "test", floatfield = 2222, doublefield = 2 }
[16:05:32.991036204] (+0.000001948) sys_getcpu: { 1 }, { cpup = 0x7FFF1B010350, nodep = 0x0, tcache = 0x7FC76A03E7D8 }
[16:05:32.991036936] (+0.000000732) exit_syscall: { 1 }, { ret = 0 }

It will need to be investigated at the glibc and kernel level, and maybe we will have to implement our own wrapper over the vDSO.