• Juergen Gross's avatar
    xen: optimize xenbus driver for multiple concurrent xenstore accesses · fd8aa909
    Juergen Gross authored
    Handling of multiple concurrent Xenstore accesses through xenbus driver
    either from the kernel or user land is rather lame today: xenbus is
    capable to have one access active only at one point of time.
    Rewrite xenbus to handle multiple requests concurrently by making use
    of the request id of the Xenstore protocol. This requires to:
    - Instead of blocking inside xb_read() when trying to read data from
      the xenstore ring buffer do so only in the main loop of
    - Instead of doing writes to the xenstore ring buffer in the context of
      the caller just queue the request and do the write in the dedicated
      xenbus thread.
    - Instead of just forwarding the request id specified by the caller of
      xenbus to xenstore use a xenbus internal unique request id. This will
      allow multiple outstanding requests.
    - Modify the locking scheme in order to allow multiple requests being
      active in parallel.
    - Instead of waiting for the reply of a user's xenstore request after
      writing the request to the xenstore ring buffer return directly to
      the caller and do the waiting in the read path.
    Additionally signal handling was optimized by avoiding waking up the
    xenbus thread or sending an event to Xenstore in case the addressed
    entity is known to be running already.
    As a result communication with Xenstore is sped up by a factor of up
    to 5: depending on the request type (read or write) and the amount of
    data transferred the gain was at least 20% (small reads) and went up to
    a factor of 5 for large writes.
    In the end some more rough edges of xenbus have been smoothed:
    - Handling of memory shortage when reading from xenstore ring buffer in
      the xenbus driver was not optimal: it was busy looping and issuing a
      warning in each loop.
    - In case of xenstore not running in dom0 but in a stubdom we end up
      with two xenbus threads running as the initialization of xenbus in
      dom0 expecting a local xenstored will be redone later when connecting
      to the xenstore domain. Up to now this was no problem as locking
      would prevent the two xenbus threads interfering with each other, but
      this was just a waste of kernel resources.
    - An out of memory situation while writing to or reading from the
      xenstore ring buffer no longer will lead to a possible loss of
      synchronization with xenstore.
    - The user read and write part are now interruptible by signals.
    Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
    Signed-off-by: default avatarBoris Ostrovsky <boris.ostrovsky@oracle.com>
xenbus_comms.c 11.2 KB