ltt/branches/poly/doc/developer/lttng-xen.txt

   1
   2
   3 LTTng for Xen
   4
   5 Design document
   6
   7 Mathieu Desnoyers, November 2006
   8
   9 Last modification :
  10
  11 Only allow a single channel. Directory consumed by a lttd-xen client through
  12 hypercalls. This design choice is made because there are no RCU lists and there
  13 is no link between the number of CPUs seen by Xen and number of CPUs in the
  14 dom0.
  15
  16 Does better than Xentrace :
  17 Variable size records
  18 Easily extensible event description
  19 // not planned : missing RCU list. Multiple active traces
  20 Reader uses poll()-like mechanism instead of polling the buffers each 100ms.
  21
  22
  23 * Dom0 lttd-xen process
  24
  25 Does an hypercall to map the buffers.
  26 Does an hypercall to poll for data, put to sleep by the OS.
  27 Uses write from the mapped buffers to a file.
  28
  29
  30 * Xen
  31
  32 tb_control (hypercalls seen from dom0 OS)
  33 - trace_create
  34 Map a page for control information, return it for mapping by user space. (rw)
  35 Map multiple pages for buffers. return them for mapping by user space.
  36 (shared, read-only for consumer)
  37 - trace_start
  38 Activate tracing in control information.
  39 - trace_stop
  40 Stop tracing in control information.
  41 - trace_destroy
  42 wait for pending writers (is the hypervisor preemptible ? not full.)
  43   Preemption : not full -> spinlock does not disable preemption : would deadlock
  44   for the non irq_disable case.
  45   volountary : poll call -> volountary. explicit __schedule call., do_yield().
  46 Quiescent state detection.
  47   for_each_domain( d )
  48     for_each_vcpu( d, v )
  49       vcpu_sleep_sync(v);
  50 destroy the channels
  51
  52 Events coming from xen :
  53 - Buffer switch
  54   through virq sent to dom0
  55
  56
  57
  58 * Dom0 Linux OS
  59
  60 Receive virq upon buffer switch.
  61 xen_wakeup_readers handler : wakes up the readers for the channel.
  62
  63 Hypercalls are autonomous from LTTng linux kernel tracing.
  64
  65 on lttngxentrace create, hypercall channel_create
  66 on lttngxentrace start, hypercall channel_start
  67 on lttngxentrace stop, hypercall channel_stop
  68 on lttngxentrace destroy, hypercall channel_destroy
  69
  70
  71 * Facilities
  72
  73 Export a facilities per cpu channel too.
  74
  75
  76 Q1 : In Xen preemptible ? Full ? Volountary ? Volountary X
  77 Q2 : How to synchronize to wait for writers to finish writing ? X
  78
  79
  80 Number of CPUs in dom0 != number of CPUs in Xen.
  81 CPU hotplug in kernel tracing ? Done.
  82
  83