[Qemu-devel] [PATCH] gdb command: qemu iohandlers

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Add a gdb command to print the current set of IOHandlers and
if one of them is a thread yielding for data print the backtrace.

Useful for debugging why an incoming migration has stalled, e.g.

  ----
  {fd_read = 0x7fd4c8e40d00 <fd_coroutine_enter>, fd_write = 0x0, opaque =
  0x7fd4b8bfeb00, next = {le_next = 0x7fd4cac81b00, le_prev =
  0x7fd4c93d2bd0 <io_handlers>}, fd = 22, pollfds_idx = 0, deleted =
  false}
  #0  qemu_coroutine_switch (from_=from_@entry=0x7fd4cbb33c00,
  to_=to_@entry=0x7fd4c8b13a90,
      action=action@entry=COROUTINE_YIELD) at coroutine-ucontext.c:177
  #1  0x00007fd4c8e40507 in qemu_coroutine_yield () at
  qemu-coroutine.c:145
  #2  0x00007fd4c8e40e75 in yield_until_fd_readable (fd=22) at
  qemu-coroutine-io.c:90
  #3  0x00007fd4c8df347f in qemu_rdma_block_for_wrid
  (rdma=rdma@entry=0x7fd4b8c7e010,
      wrid_requested=wrid_requested@entry=2000,
  byte_len=byte_len@entry=0x0) at migration/rdma.c:1510
  #4  0x00007fd4c8df388f in qemu_rdma_post_send_control
  (rdma=rdma@entry=0x7fd4b8c7e010, buf=buf@entry=0x0,
      head=head@entry=0x7fd4b8bfed00) at migration/rdma.c:1608
  #5  0x00007fd4c8df4b8e in qemu_rdma_exchange_recv (rdma=0x7fd4b8c7e010,
  head=0x7fd4b8bfed50, expecting=3)
      at migration/rdma.c:1814
  #6  0x00007fd4c8df5089 in qemu_rdma_get_buffer (opaque=0x7fd4cba34950,
  buf=0x7fd4cc24fd20 "TR\022",
      pos=<optimized out>, size=32768) at migration/rdma.c:2611
  #7  0x00007fd4c8df000d in qemu_fill_buffer (f=f@entry=0x7fd4cc24fcf0) at
  migration/qemu-file.c:214
  #8  0x00007fd4c8df08d4 in qemu_peek_byte (f=f@entry=0x7fd4cc24fcf0,
  offset=offset@entry=0)
      at migration/qemu-file.c:447
  #9  0x00007fd4c8c2cad1 in qemu_loadvm_state (f=f@entry=0x7fd4cc24fcf0)
  at /root/qemu-world3/migration/savevm.c:1128
  #10 0x00007fd4c8ded895 in process_incoming_migration_co
  (opaque=0x7fd4cc24fcf0) at migration/migration.c:145
  #11 0x00007fd4c8e4112a in coroutine_trampoline (i0=<optimized out>,
  i1=<optimized out>) at coroutine-ucontext.c:80
  #12 0x00007fd4c453e0f0 in ?? () from /lib64/libc.so.6
  #13 0x00007ffee263e870 in ?? ()
  #14 0x0000000000000000 in ?? ()
  ----
  {fd_read = 0x7fd4c8e3ace0 <sigfd_handler>, fd_write = 0x0, opaque = 0x5,
  next = {le_next = 0x0, le_prev = 0x7fd4cb78a3d8}, fd = 5, pollfds_idx =
  1, deleted = false}

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 scripts/qemu-gdb.py | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/scripts/qemu-gdb.py b/scripts/qemu-gdb.py
index 6c7f4fb..2670625 100644
--- a/scripts/qemu-gdb.py
+++ b/scripts/qemu-gdb.py
@@ -102,6 +102,30 @@ class CoroutineCommand(gdb.Command):
         coroutine_pointer = gdb.parse_and_eval(argv[0]).cast(gdb.lookup_type('CoroutineUContext').pointer())
         bt_jmpbuf(coroutine_pointer['env']['__jmpbuf'])
 
+class IOhandlersCommand(gdb.Command):
+    '''Display a backtrace for io handlers'''
+    def __init__(self):
+        gdb.Command.__init__(self, 'qemu iohandlers', gdb.COMMAND_DATA,
+                             gdb.COMPLETE_NONE)
+
+    def invoke(self, arg, from_tty):
+        cur = gdb.parse_and_eval('io_handlers.lh_first')
+        sym_fd_coroutine_enter = gdb.parse_and_eval('fd_coroutine_enter')
+
+        while not isnull(cur):
+            entry = cur.dereference()
+            gdb.write('----\n%s\n' % entry)
+
+            # For functions we know, extract the coroutine pointer and backtrace
+            if cur['fd_read'] == sym_fd_coroutine_enter:
+                # yield_until_fd_readable
+                coptr = (cur['opaque'].cast(gdb.lookup_type('FDYieldUntilData').pointer()))['co']
+                coptr = coptr.cast(gdb.lookup_type('CoroutineUContext').pointer())
+                bt_jmpbuf(coptr['env']['__jmpbuf'])
+
+            cur = cur['next']['le_next']
+        gdb.write('----\n')
+
 class MtreeCommand(gdb.Command):
     '''Display the memory tree hierarchy'''
     def __init__(self):
@@ -161,4 +185,5 @@ class MtreeCommand(gdb.Command):
 
 QemuCommand()
 CoroutineCommand()
+IOhandlersCommand()
 MtreeCommand()
-- 
2.4.3