Messages in this thread | | | Date | Mon, 30 Mar 2020 11:08:55 -0400 | From | Steven Rostedt <> | Subject | Re: ftrace not showing the process names for all processes on syscall events |
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On Mon, 30 Mar 2020 14:28:01 +0000 David Laight <David.Laight@ACULAB.COM> wrote:
> I've just updated one of my systems to 5.6.0-rc7+ (end of last week). > ftrace in showing <...>-3179 in the system call events for a couple > of threads of the active processes. > Other threads of the same processes are fine. > The scheduler process switch events also show the full name. > > Is this a known regression?
Well, that code hasn't changed in years. But can you explain more of what you did? Was this the "trace" file, or "trace_pipe" file? The command names are cached in an array (see /sys/kernel/tracing/saved_cmdlines) of the size that is defined by the saved_cmdlines_size file.
The cmdlines get updated via the sched_switch and sched_waking trace events. The update is protected by a spinlock, which is only taken with a "trylock", if the lock fails, then it does not get updated (we don't want to hold back the running code just to cache the name of an event), but it will try at the next sched event until it succeeds. This means that under strong contention, it may fail to cache certain names.
This is not a regression, it's really just the work load that can cause event names to be missed. I could work on something that if you have the sched events enabled, that the output side could do its own caching to get better results.
-- Steve
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