Ping? On Wednesday, December 10, 2014, Umut Tezduyar Lindskog <u...@tezduyar.com> wrote:
> On Mon, Dec 8, 2014 at 8:09 PM, Lennart Poettering > <lenn...@poettering.net <javascript:;>> wrote: > > On Sun, 30.11.14 14:38, Umut Tezduyar Lindskog (u...@tezduyar.com > <javascript:;>) wrote: > > > >> Hi, > >> > >> We are experiencing an unbreakable loop in manager_dispatch_gc_queue. > >> Problem happens when systemd runs in sysV compatibility mode (Porky > >> enables this). > >> > >> Seems like manager_dispatch_gc_queue's while loop gets stuck and seems > >> like unit_gc_sweep cannot make a decision about the unit. As a result, > >> it marks the unit with offset_unsure and adds the unit back to gc > >> queue. > >> > >> If I am reading the code correctly recursive unit_gc_sweep will never > >> be able to remove the unit from the gc queue if it is referenced by > >> another unit and if another unit is referenced by the unit. > >> > >> A is referenced by B > >> B is referenced by A > > > > So in this case first A will be processed by the GC sweep, it will > > follow the link to B while setting the state to IN_PATH and invoke the > > GC sweep on that. B will then be set to IN_PATH too. GC sweep now > > follows its link back, and up at A again, but this time return quickly > > because its state is set to IN_PATH. Due to this, it will then set B's > > state to UNSURE, and return to A, which in effect will now be set to > > UNSURE too. Now, we return into GC queue dispatch call, which will > > notice that it is UNSURE and uprgade that to BAD, and kill it because > > there's nothin in the unit's dependency network that is clearly a > > GOOD, and hence should be removed. > > > > The essence of cycle breaking here is really in > > manager_dispatch_gc_queue() which uprgades UNSURE to BAD in the end. I > > am not seeing how this could end up in an endless loop hence. > > I have debugged it more and as you have said there is no bug in code > but it takes so long to go out of unit_gc_sweep I thought there is a > forever loop. > > Attached is my patch on 216 and > > https://drive.google.com/file/d/0B_uiALgWpGXtZ0VidURxSnVhcDA/view?usp=sharing > is a part of the log after patch. > > It has been 3 hours since I issued "systemctl isolate" and according > to the logs I can see that garbage collection logic is making it's way > back up. I guess it will eventually resolve itself but after so many > hours. > > (Search for "- -" and it is happening every 300.000 > lines) > > Problem seemed to be introduced on "95ed329" - Move handling of sysv > initscripts to a generator. > > This is totally due to how sysV generator is linking services but I > think slowness on GC can happen on a complex system with many units > linked with each other. > > Thoughts? > Umut > > > > >> > >> We have this circular referenced by dependency between units and I am > >> quite sure they are due to sysV compatibility. > >> > >> I know that systemd does not allow circular dependency between units > >> (ex, wants, or after) but do we allow circular referenced by > >> dependency? If so, then it is expected that manager_dispatch_gc_queue > >> gets stuck. > >> > >> We can reproduce it on 216/217 when we isolate a target. > >> > >> Note: Line > http://cgit.freedesktop.org/systemd/systemd/tree/src/core/manager.c?id=941a643569dc6b53d0b334276d2a3cc0ed159e88#n875 > >> should be before > >> > http://cgit.freedesktop.org/systemd/systemd/tree/src/core/manager.c?id=941a643569dc6b53d0b334276d2a3cc0ed159e88#n872 > >> since unit_gc_sweep() sets the u->in_gc_queue = true if it cannot make > >> a decision and we set it back to false. > > > > This is intended. After the sweep returned back to the anchor we can > > make our decision: either add the unit to the cleanup queue in which > > case it should removed from the GC queue, or it is reinstantated as > > a good unit that should continue to exist, in which case it should be > > removed from the GC queue too. > > > > Can't see a bug here... > > > > Can you elaborate on how precisely you are encountering the GC loop? > > > > Lennart > > > > -- > > Lennart Poettering, Red Hat >
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