Red Hat Bugzilla – Bug 244187
hald slows down system clock on systems with apm
Last modified: 2013-03-05 22:51:07 EST
Description of problem:
hald reads /proc/apm every 2 seconds, which causes the system clock to slow down.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Stop hald.
2. System clock runs normally.
3. Restart hald.
4. System clock runs slow.
Hald slows down the system clock to such an extent and with such a large jitter
that ntp is no longer able to keep the system time synchronized, resulting in
massive clock drifts (i.e. hours). On my system, the slowdown was about 10 hours
Hald shouldn't slow down the system clock.
Actually, this is really a kernel bug, because any user can cat /proc/apm, which
has the same effect. I have filed a bug report against the kernel package too
(see https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=244177). However, this
is not a new bug in the kernel and when it was reported against kernel 2.2.18
over six years ago, the thread ended with Erik Mouw suggesting caching the
results of /proc/apm and Alan Cox saying this is a userspace issue:
In other words, unless the kernel people can fix the problem with /proc/apm,
userspace programs shouldn't read /proc/apm that frequently.
Commenting out lines 391 and 392 in hald/linux/apm.c (i.e. so read_from_apm is
no longer called) works around the problem.
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We need /proc/apm for the battery and AC detection. It sounds like a kernel bug
that needs fixing, not HAL. Sorry.