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tuned-ppd: Fix inotify watch for performance degradation #753

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tuned-ppd: Fix inotify watch for performance degradation #753

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@msizanoen1 msizanoen1 commented Feb 26, 2025 •
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tuned-ppd: Fix inotify watch for performance degradation
0e8d3bb 
Ensure the parameters for PerformanceDegradedEventHandler are passed in
the correct order.
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zacikpa commented Feb 27, 2025

Oh, wow, good catch, thanks!

@msizanoen1
tuned-ppd: Fix indentation
bbbeed5
@msizanoen1
tuned-ppd: Pin virtual files in memory for inotify
bdf6b4d 
The inodes backing the kernfs virtual files for the attributes we watch
may be dropped by the kernel due to memory exhaustion or explicit user
action (sysctl vm.drop_caches). This causes inotify events to not be
delivered as kernfs_notify() simply drops the event if the inode cannot
be found in the kernel caches.

Fix this by explicitly pinning the attributes virtual files in memory to
ensure we receive inotify events.
@msizanoen1 msizanoen1 requested a review from zacikpa March 16, 2025 08:23
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zacikpa commented Mar 18, 2025

@msizanoen1, I'm not able to reproduce the inode caching issue without the last commit applied. Could you please clarify how you're reproducing it and on what system? I tried writing "3" to /proc/sys/vm/drop_caches, but inotify events were still delivered properly to tuned-ppd after that.

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I was able to reproduce this on a ThinkPad X1 Carbon 9th gen running Fedora 41, against the dytc_lapmode trigger specifically. I just put it on my lap, wait until dytc_lapmode is 1, run sysctl vm.drop_caches=3, close the lid, move it to a stable surface, then reopen it. After reopening the lid dytc_lapmode should be 0 but GNOME Settings still reports that the lap sensor is triggered and /var/log/tuned/tuned-ppd.log doesn't report any changes.

The specific problematic kernel line of code is https://github.com/torvalds/linux/blob/76b6905c11fd3c6dc4562aefc3e8c4429fefae1e/fs/kernfs/file.c#L926 (should be kernfs_get_inode(info->sb, kn) instead) but it's arguable whether this is a kernel bug considering that using inotify on sysfs is not officially supported and changes to attributes should be listened to with poll() instead.

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msizanoen1 commented Mar 18, 2025 •
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Please note that the dytc_lapmode file must not be open by any process on the system (check with lsof before running sysctl vm.drop_caches=3) for this to be reproducible.

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zacikpa approved these changes Mar 18, 2025
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Thanks for the quick response and detailed instructions! I was able to reproduce the issue and can confirm that keeping the files open resolves it.

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