[lttng-dev] Segfault at v_read() called from lib_ring_buffer_try_reserve_slow() in LTTng-UST traced app - CPU/VMware dependent

[David OShea]

Hi Mathieu,

> -----Original Message-----
> From: Mathieu Desnoyers [mailto:mathieu.desnoyers at efficios.com]
> Sent: Tuesday, 13 January 2015 2:06 AM
> To: David OShea
> Cc: lttng-dev
> Subject: Re: [lttng-dev] Segfault at v_read() called from
> lib_ring_buffer_try_reserve_slow() in LTTng-UST traced app - CPU/VMware
> dependent
[...]
> > > Is it possible that this is an issue in LTTng, or should I work out
> how the
> > > kernel works out which CPU it is running on and then look into
> whether
> > > there
> > > are any VMware bugs in this area?
> >
> > This appears to be very likely a VMware bug. /proc/cpuinfo should
> show
> > 4 CPUs (and sysconf(_SC_NPROCESSORS_CONF) should return 4) if the
> current
> > CPU number can be 0, 1, 2, 3 throughout execution.

/proc/cpuinfo shows two CPUs:

processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 26
model name      : Intel(R) Xeon(R) CPU           X7550  @ 2.00GHz
stepping        : 4
microcode       : 8
cpu MHz         : 1995.000
cache size      : 18432 KB
physical id     : 0
siblings        : 1
core id         : 0
cpu cores       : 1
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 11
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology tsc_reliable nonstop_tsc aperfmperf unfair_spinlock pni ssse3 cx16 sse4_1 sse4_2 popcnt hypervisor lahf_lm ida dts
bogomips        : 3990.00
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management:

processor       : 1
vendor_id       : GenuineIntel
cpu family      : 6
model           : 26
model name      : Intel(R) Xeon(R) CPU           X7550  @ 2.00GHz
stepping        : 4
microcode       : 8
cpu MHz         : 1995.000
cache size      : 18432 KB
physical id     : 2
siblings        : 1
core id         : 0
cpu cores       : 1
apicid          : 2
initial apicid  : 2
fpu             : yes
fpu_exception   : yes
cpuid level     : 11
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology tsc_reliable nonstop_tsc aperfmperf unfair_spinlock pni ssse3 cx16 sse4_1 sse4_2 popcnt hypervisor lahf_lm ida dts
bogomips        : 3990.00
clflush size    : 64
cache_alignment : 64
address sizes   : 40 bits physical, 48 bits virtual
power management:

> You might want to look at the sysconf(3) manpage, especially the parts
> about
> _SC_NPROCESSORS_CONF and _SC_NPROCESSORS_ONLN. My guess is that vmware
> is lying
> about the number of "possible" CPUs (_SC_NPROCESSORS_CONF).

_SC_NPROCESSORS_CONF = 2
_SC_NPROCESSORS_ONLN = 2

Thanks for the pointers, I will look into possible VMware bugs.

Out of curiosity, what happens if I happened to have a system with hot-pluggable CPUs - does _SC_NPROCESSORS_CONF reflect the maximum number of CPUs I can insert, and that is how many LTTng will support?

Thanks,
David

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