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 (increment refcount)
  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 unmap buffer, decrement refcount.
  29
  30
  31 * Xen
  32
  33 tb_control (hypercalls seen from dom0 OS)
  34 (hypercalls done by lttctl-xen)
  35 - trace_create
  36 Map a page for control information, return it for mapping by user space. (rw)
  37 Map multiple pages for buffers. return them for mapping by user space.
  38 (shared, read-only for consumer)
  39 increment refcount
  40 - trace_start
  41 Activate tracing in control information.
  42 - trace_stop
  43 Stop tracing in control information.
  44 - trace_destroy
  45 wait for pending writers (is the hypervisor preemptible ? not full.)
  46   Preemption : not full -> spinlock does not disable preemption : would deadlock
  47   for the non irq_disable case.
  48   volountary : poll call -> volountary. explicit __schedule call., do_yield().
  49 Quiescent state detection.
  50   for_each_domain( d )
  51     for_each_vcpu( d, v )
  52       vcpu_sleep_sync(v);
  53 decrement refcount : destroy the channels
  54
  55 Events coming from xen :
  56 - Buffer switch
  57   through virq sent to dom0
  58 - wakes up poll on the buffers FD.
  59
  60
  61 * Dom0 Linux OS
  62
  63 Receive virq upon buffer switch.
  64 xen_wakeup_readers handler : wakes up the readers for the channel.
  65
  66 Hypercalls are autonomous from LTTng linux kernel tracing.
  67
  68 on lttngxentrace create, hypercall channel_create
  69 on lttngxentrace start, hypercall channel_start
  70 on lttngxentrace stop, hypercall channel_stop
  71 on lttngxentrace destroy, hypercall channel_destroy
  72
  73
  74 * Facilities
  75
  76 Export a facilities per cpu channel too.
  77
  78
  79 Q1 : In Xen preemptible ? Full ? Volountary ? Volountary X
  80 Q2 : How to synchronize to wait for writers to finish writing ? X
  81
  82
  83 Number of CPUs in dom0 != number of CPUs in Xen.
  84 CPU hotplug in kernel tracing ? Done.
  85
  86