#systemtap on 2020-06-19 — irc logs at freenode.irclog.whitequark.org

2015-11-12 23:18 fche changed the topic of #systemtap to: http://sourceware.org/systemtap; email systemtap@sourceware.org if answers here not timely, conversations may be logged

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09:03 <agentzh> fche: v5 patch is ready and tested: https://gist.github.com/agentzh/3e8267cfcf111563f7d7da17d474e5cc

09:04 <agentzh> i spent hours testing various different macro settings affected by this patch :P

09:04 <agentzh> it should be solid now.

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11:32 <fche> agentzh, yeah that looks good to me

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11:59 <zamba> i'm attempting to get systemtap working on my ubuntu 20.04, but getting the following: ERROR: Couldn't insert module '/tmp/stapQdncqZ/stap_4686639b05b6bd6593af2199b04f2013_26378.ko': Operation not permitted

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12:35 <fche> does dmesg offer any information?

12:36 <fche> zamba, ^^

12:38 <zamba> [772759.025631] Lockdown: staprun: unsigned module loading is restricted; see man kernel_lockdown.7

12:38 <zamba> ah

12:39 <fche> yeah

12:40 <fche> so probably need module signing goo as per secureboot https://sourceware.org/systemtap/wiki/SecureBoot

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12:48 <zamba> oh...

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17:08 <agentzh> fche: cool, thanks

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19:24 <agentzh> fche: noted one strange thing, even a single thread of test, the stream mode would see frequent lock contention on the inode.

19:24 <agentzh> *even in

19:24 <agentzh> that's strange...

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19:31 <agentzh> bulk mode is fine though, even in a 30-thread test.

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20:16 <agentzh> fche: okay, in the single-thread test, enabling relay_flush() is necessary to avoid data loss in the end (stream mode).

20:16 <agentzh> i'll just use inode_lock() to protect the original relay_flush() call instead of removing it.

20:16 <agentzh> that context should allow sleep, right?

20:16 <fche> probably; should verify.

20:16 <agentzh> okay, will check the backtrace

20:17 <agentzh> also, i need to turn down_write_trylock into a true spinlock.

20:17 <agentzh> otherwise the locks are frequently lost even under a single thread.

20:17 <agentzh> in stream mode.

20:17 <fche> that'd be a different lock then

20:18 <agentzh> i still don't understand why stream mode has so much lock contention even for a single thread.

20:18 <agentzh> i'll use a bounded hot loop to try down_write_trylock() repeatedly.

20:18 <agentzh> it seems working for me.

20:18 <agentzh> as from my v1 patch.

20:18 <agentzh> if you don't object.

20:19 <agentzh> [65827.681940] got inode lock: i=14367: cycles: 538094

20:19 <fche> ok, will need to see / think about that patch

20:19 <agentzh> the worst case for obtaining the inode lock is like this

20:19 <agentzh> sure, will prepare v.

20:19 <fche> considered stap tracing that inode locking activity?

20:19 <agentzh> *v6

20:20 <agentzh> sounds like a good idea

20:20 <agentzh> :)

20:20 * agentzh is too used to printk now.

20:21 <agentzh> will try tracing the locking and unlocking funcs.

20:21 <agentzh> hopefully they are not inlined...

20:24 <agentzh> fche: the relay_flush() is called with this kernel backtrace: https://gist.github.com/agentzh/1b69ae02864c6bac455081a76da73e23

20:25 <agentzh> it should allow sleeping?

20:25 <agentzh> looks like syscall context.

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20:36 <fche> probably ....

20:36 <fche> run it on a lockdep/rawhide kernel to help chekc

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21:06 <agentzh> fche: yeah, that would be helpful.

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22:06 <agentzh> fche: alas. stap has compatibility issues with the latest fedora rawhide. even hello world .stp won't compile: https://sourceware.org/bugzilla/show_bug.cgi?id=26142

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23:38 <agentzh> fche: ah, i just used stap to trace stap's relay lock contention. the inode locks which ko fail to obtain are all later released by this kernel + user backtrace: https://gist.github.com/agentzh/48d9d93b3bcebff0d0591a9586056b88

23:39 <agentzh> i'd say most of them.

23:43 <agentzh> some of the samples are staprun's stp_main_loop() function which sends the STP_STOP command to the control channel under "case STP_REQUEST_EXIT:".

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