Can pid be reused ?

[Loic Dachary <loic@dachary.org>]

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Hi,

Something strange happens on fedora20 with linux 3.11.10-301.fc20.x86_64. Running make -j8 check on https://github.com/ceph/ceph/pull/2750 a process gets killed from time to time. For instance it shows as

TEST_erasure_crush_stripe_width: 124: stripe_width=4096
TEST_erasure_crush_stripe_width: 125: ./ceph osd pool create pool_erasure 12 12 erasure
*** DEVELOPER MODE: setting PATH, PYTHONPATH and LD_LIBRARY_PATH ***
./test/mon/osd-pool-create.sh: line 120: 27557 Killed                  ./ceph osd pool create pool_erasure 12 12 erasure
TEST_erasure_crush_stripe_width: 126: ./ceph --format json osd dump
TEST_erasure_crush_stripe_width: 126: tee osd-pool-create/osd.json

in the test logs. Note the 27557 Killed . I originally thought it was because some ulimit was crossed and set them to very generous / unlimited hard / soft thresholds.

core file size          (blocks, -c) 0                                                                                     
data seg size           (kbytes, -d) unlimited                                                                             
scheduling priority             (-e) 0                                                                                     
file size               (blocks, -f) unlimited                                                                             
pending signals                 (-i) 515069                                                                                
max locked memory       (kbytes, -l) unlimited                                                                             
max memory size         (kbytes, -m) unlimited                                                                             
open files                      (-n) 400000                                                                                
pipe size            (512 bytes, -p) 8                                                                                     
POSIX message queues     (bytes, -q) 819200                                                                                
real-time priority              (-r) 0                                                                                     
stack size              (kbytes, -s) unlimited                                                                             
cpu time               (seconds, -t) unlimited                                                                             
max user processes              (-u) unlimited                                                                             
virtual memory          (kbytes, -v) unlimited                                                                             
file locks                      (-x) unlimited    

Benoit Canet suggested that I installed systemtap ( https://www.sourceware.org/systemtap/wiki/SystemtapOnFedora ) and ran https://sourceware.org/systemtap/examples/process/sigkill.stp to watch what was sending the kill signal. It showed the following:

...
SIGKILL was sent to ceph-osd (pid:27557) by vstart_wrapper. uid:1001
SIGKILL was sent to python (pid:27557) by vstart_wrapper. uid:1001
....

which suggests that pid 27557 used by ceph-osd was reused for the python script that was killed above. Because the script that kills daemons is very agressive and kill -9 the pid to check if it really is dead

https://github.com/ceph/ceph/blob/giant/src/test/mon/mon-test-helpers.sh#L64

it explains the problem.

However, as Dan Mick suggests, reusing pid quickly could break a number of things and it is a surprising behavior. Maybe something else is going on. A loop creating processes sees their pid increasing and not being reused.

Any idea about what is going on would be much appreciated :-)

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre



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