** Changed in: linux (Ubuntu Xenial)
Status: In Progress => Fix Committed
** Changed in: linux (Ubuntu Bionic)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1877858
Title:
Improve TSC refinement (and calibration) reliability
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Xenial:
Fix Committed
Status in linux source package in Bionic:
Fix Committed
Bug description:
[Impact]
* We received a report recently of a missing TSC refinement across multiple
reboots of a server, in an Intel Skylake-based processor. This was only
reproducible in Bionic pre-5.0.
* After checking kernel commits, we came up with 2 commits that
largely improve the situation: a786ef152cdc ("x86/tsc: Make
calibration refinement more robust")
[git.kernel.org/linus/a786ef152cdc] and 604dc9170f24 ("x86/tsc: Use
CPUID.0x16 to calculate missing crystal frequency")
[git.kernel.org/linus/604dc9170f24]. We hereby request SRU for both of
them.
* The first commit contains improvement in comments and in an offset to match
more recent (fast) machines, but the important part is a retry mechanism in the
TSC refinement (in case it fails due to some disturbance on TSC read, like
NMIs/SMIs).
* The second commit is an improvement in TSC calibration for Skylake (and
some other models), by checking a register instead of relying on table-based
hardcoded values.
* A note for Xenial (kernel 4.4): the second patch would require the
inclusion of more commits, so given the "maturity" of this release
(and the fact kernel 4.15 is an HWE for Xenial), I've kept it out of
Xenial, backporting only the first and more important patch for 4.4 .
[Test case]
* Unfortunately there's not an easy way to test the effectiveness of the
commits, specially the refinement improvement.
* The user that reported us the missing refinements was able to test
300 reboots with a regular Bionic kernel (and it reproduced the issue
at least once), whereas when they tested with Bionic kernel + both
hereby proposed commits, the problem didn't happen.
* Regarding the calibration commit, it was well-tested by community
using multiple machines and checking the TSC calibration read vs.
tables present in instlatx64.atw.hu .
[Regression potential]
* We consider the regression potential low, specially due to the nature of
the patches: the first is basically a retry mechanism (and some improvement in
an offset to reflect more recent machines), and the 2nd is an improvement for
TSC calibration on some platforms (that are currently hardcoded in a
table-based way in kernel). Also, the patches are present upstream for a while
and I couldn't find any fixes for them.
* An hypothetical regression from the 2nd patch could be in TSC
precision calculation, which refinement itself might as well
circumvent. From the first patch, a bug in code is the one
hypothetical regression I could think.
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