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[Xen-devel] [PATCH 00/10] libxl: switch driver to use a single libxl_ctx

This series is a follow up to


It goes a step further and changes the libxl driver to use one,
driver-wide libxl_ctx.  Currently the libxl driver has one driver-wide
ctx for operations that are not domain-specific and a ctx for each
domain.  This approach was necessary back in the old Xen4.1 libxl days,
but with the newer libxl it is more of a hinderance than benefit.
Ian Jackson suggested moving to a single ctx while discussing some
deadlocks and assertions encountered in the libxl driver when under
load from tests such as OpenStack Tempest.

Making such a change involves quite a bit of code movement.  I've tried
to split that up into a reviewable series,  the result of which are the
9 patches that follow.  I've ran this through all of my automated tests
as well as some hacky tests I created to reproduce failures revealed by

One downside of moving to a single ctx is losing the per-domain log
files.  Currently, a single log stream can be associated with ctx, hence
all logging from libxl will go to a single file.  Ian is going to
investigate possibilities to accommodate per-domain log files in libxl,
but in the meantime folks using Xen are accustomed to a single
log file from the xend days.

I've been testing this series on xen-unstable and Xen 4.4.1 + commits
2ffeb5d7, 4b9143e4, 5a968257, 60ce518a, 66bff9fd, 77a1bf37, f49f9b41,
6b5a5bba, 93699882d, f1335f0d, and 8bc64413.  Results are much better
than before applying the series, but I do notice a stuck hypercall
after many (hundreds) concurrent domain create/destroy operations.
The single libxl_ctx is locked in the callpath, essentially deadlocking
the driver.

Thread 1 (Thread 0x7f0649a198c0 (LWP 2235)):
 0  0x00007f0645272397 in ioctl () from /lib64/libc.so.6
 1  0x00007f0645d8e353 in linux_privcmd_hypercall (xch=<optimized out>,
    h=<optimized out>, hypercall=<optimized out>) at xc_linux_osdep.c:134
 2  0x00007f0645d854b8 in do_xen_hypercall (xch=xch@entry=0x7f0630039390, 
    hypercall=hypercall@entry=0x7fffd53f80e0) at xc_private.c:249
 3  0x00007f0645d86aa4 in do_sysctl (sysctl=sysctl@entry=0x7fffd53f8080, 
    xch=xch@entry=0x7f0630039390) at xc_private.h:281
 4  xc_sysctl (xch=xch@entry=0x7f0630039390,
    sysctl=sysctl@entry=0x7fffd53f8170) at xc_private.c:656
 5  0x00007f0645d7bfbf in xc_domain_getinfolist (xch=0x7f0630039390, 
    first_domain=first_domain@entry=119, max_domains=max_domains@entry=1, 
    info=info@entry=0x7fffd53f8260) at xc_domain.c:382
 6  0x00007f0645fabca6 in domain_death_xswatch_callback
    (egc=0x7fffd53f83f0, w=<optimized out>, wpath=<optimized out>,
    epath=<optimized out>) at libxl.c:1041
 7  0x00007f0645fd75a8 in watchfd_callback (egc=0x7fffd53f83f0,
    ev=<optimized out>, fd=<optimized out>, events=<optimized out>,
    revents=<optimized out>) at libxl_event.c:515
 8  0x00007f0645fd8ac3 in libxl_osevent_occurred_fd (ctx=<optimized out>, 
    for_libxl=<optimized out>, fd=<optimized out>,
    events_ign=<optimized out>, revents_ign=<optimized out>) at
 9  0x00007f063a23402c in libxlFDEventCallback (watch=454, fd=33,
    vir_events=1, fd_info=0x7f0608007e70) at libxl/libxl_driver.c:123

There is no hint in any logs or dmesg suggesting a cause for the stuck
hypercall.  Any suggestions for further debugging tips appreciated.

Jim Fehlig (10):
  libxl: remove redundant calls to libxl_evdisable_domain_death
  libxl: use libxl_ctx passed to libxlConsoleCallback
  libxl: use driver-wide ctx in fd and timer event handling
  libxl: Move setup of child processing code to driver initialization
  libxl: move event registration to driver initialization
  libxl: use global libxl_ctx in event handler
  libxl: remove unnecessary libxlDomainEventsRegister
  libxl: make libxlDomainFreeMem static
  libxl: remove per-domain libxl_ctx
  libxl: change libxl log stream to ERROR log level

 src/libxl/libxl_conf.c      |   2 +-
 src/libxl/libxl_domain.c    | 438 ++++++---------------------------------
 src/libxl/libxl_domain.h    |  27 +--
 src/libxl/libxl_driver.c    | 484 +++++++++++++++++++++++++++++++-------------
 src/libxl/libxl_migration.c |  17 +-
 5 files changed, 426 insertions(+), 542 deletions(-)


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