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Bug #1159

Missing documentation: before Linux 4.17, suspending the system while tracing is active can result in incorrect event record timestamps

Added by Philippe Proulx 11 months ago. Updated 11 months ago.

Status:
Needs feedback
Priority:
Low
Target version:
Start date:
04/20/2018
Due date:
% Done:

0%

Estimated time:

Description

From LWN:

The CLOCK_MONOTONIC and CLOCK_BOOTTIME clocks used to differ only in that the latter is fast-forwarded after a suspend-and-resume cycle. As of 4.17, CLOCK_MONOTONIC is also moved forward to reflect the time that the system spent suspended. As a result, the two timers are now identical and have been unified within the kernel. Among other things, that change eliminates a potentially surprising behavior wherein the offset between the monotonic and realtime clocks would change after a resume. Thomas Gleixner noted: "There might be side effects in applications, which rely on the (unfortunately) well documented behaviour of the MONOTONIC clock, but the downsides of the existing behaviour are probably worse.

We should document that LTTng can produce incorrect traces when the target system is suspended while tracing is active if the system runs Linux 4.16 or less.

Mathieu Desnoyers suggested that we put this in lttng-regenerate(1)'s man page. Is this the appropriate location? Should the warning exist in all command man pages (as a common limitation at the end of the page)? lttng-create(1)'s man page could also be an appropriate location.

History

#1

Updated by Philippe Proulx 11 months ago

  • Priority changed from Normal to Low

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