Re: [libvirt] [PATCH v2] nodedev: Document the udevEventHandleThread

On 11/2/18 3:48 AM, Erik Skultety wrote: > On Thu, Nov 01, 2018 at 01:48:39PM -0400, John Ferlan wrote: >> Commit cdbe1332 neglected to document the API. So let's add some >> details about the algorithm and why it was used to help future >> readers understand the issues encountered. Based largely off a >> description provided by Erik Skultety <eskultet(a)redhat.com&gt;. > > Oh, I missed ^this last sentence. Since you're blaming the commit already and > the description based on a mailing list thread will not be part of the log, you > should IMHO drop it. > >> >> NB: Management of the processing udev device notification is a >> delicate balance between the udev process, the scheduler, and when >> exactly the data from/for the socket is received. The balance is >> particularly important for environments when multiple devices are >> added into the system more or less simultaneously such as is done >> for mdev. > > "or SRIOV" > > Note: I can't really remember what I used for reproducer, mdevs or a SRIOV > card. > >> In these cases scheduler blocking on the udev recv() call > > I don't think scheduler is blocking anywhere, it's rather old libudev blocking > on the recv call ;) > >> is more noticeable. It's expected that future devices will follow >> similar algorithms. Even though the algorithm does present some >> challenges for older OS's (such as Centos 6), trying to rewrite >> the algorithm to fit both models would be more complex and involve >> pulling the monitor object out of the private data lockable object >> and would need to be guarded by a separate lock. Devising such an >> algorithm to work around issues with older OS's at the expense >> of more modern OS algorithms in newer event processing code may >> result in unexpected issues, so the choice is to encourage use >> of newer OS's with newer udev event processing code. >> >> Signed-off-by: John Ferlan <jferlan(a)redhat.com&gt; >> --- >> >> v1: https://www.redhat.com/archives/libvir-list/2018-October/msg01053.html >> >> Changes are from code review with some minor tweaks/editing as I >> went along. Mistakes are my own! >> >> src/node_device/node_device_udev.c | 28 ++++++++++++++++++++++++++++ >> 1 file changed, 28 insertions(+) >> >> diff --git a/src/node_device/node_device_udev.c b/src/node_device/node_device_udev.c >> index 22897591de..f2c2299d4d 100644 >> --- a/src/node_device/node_device_udev.c >> +++ b/src/node_device/node_device_udev.c >> @@ -1591,6 +1591,26 @@ udevEventMonitorSanityCheck(udevEventDataPtr priv, >> } >> >> >> +/** >> + * udevEventHandleThread >> + * @opaque: unused >> + * >> + * Thread to handle the udevEventHandleCallback processing when udev >> + * tells us there's a device change for us (add, modify, delete, etc). >> + * >> + * Once notified there is data to be processed, the actual @device >> + * data retrieval by libudev may be delayed due to how threads are >> + * scheduled. In fact, the event loop could be scheduled earlier than >> + * the handler thread, thus potentially emitting the very same event >> + * the handler thread is currently trying to process, simply because >> + * the data hadn't been retrieved from the socket. >> + * > > ... > >> + * NB: Usage of event based socket algorithm causes some issues with >> + * older platforms such as CentOS 6 in which the data socket is opened > > ^Sounds a bit too generic, as an event based algorithm is not forced to open a > socket without O_NONBLOCK - just put something like "older platforms' libudev > opens sockets without the NONBLOCK flag which might cause issues with event > based algorithm" or something alike in there. > * NB: Some older distros, such as CentOS 6, libudev opens sockets * without the NONBLOCK flag which might cause issues with event * based algorithm. Although the issue can be mitigated by resetting * priv->dataReady for each event found; however, the scheduler issues * would still come into play.