Ubuntu
linux package

Hardy - Use MFENCE/LFENCE instead of CPUID

Bug #249135 reported by Tim Gardner
10
Affects Status Importance Assigned to Milestone
linux (Ubuntu)
Invalid
Undecided
Unassigned
Hardy
Fix Released
Medium
Tim Gardner

Bug Description

Hi,

There have been a series of patches committed to the mainline kernel recently that address a performance issue for gettimeofday when running on hypervisors that enable hardware assisted virtualization. The non-ideal performance occurs because a CPUID instruction is used to serialize the pipeline before RDTSC, and when using hardware virtualization, CPUID always exits to the hypervisor.

The code in question also exists in the ubuntu-hardy tree.

The fix is to use MFENCE/LFENCE instead of CPUID. This series of patches backports those commits for current ubuntu's git tree.

Please have a look.

Thanks,
Alok

Revision history for this message
Tim Gardner (timg-tpi) wrote :

These 6 commits migrate from CPUID to MFENCE/LFENCE:

http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=12d503a50efbfccebd31f3bf0d6365043d8d3ea1
http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=b312a5f161978b4c41afe12261c17763e1e480b3
http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=f4c08bc613b9ec5b7e65ee8503dcc5672ee9cb5c
http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=0fe07cb0805ffb42225164ce5ce49de5af8b4120
http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=d6bcc67937ecea5c9cee274615c60d159e0dec6f
http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=7cceb5df7cb20d12d2bfcb89595ca2eb663732e2

Changed in linux:
assignee: nobody → timg-tpi
importance: Undecided → Medium
milestone: none → ubuntu-8.04.2
status: New → Fix Committed
Steve Langasek (vorlon)
Changed in linux:
assignee: nobody → timg-tpi
importance: Undecided → Medium
milestone: none → ubuntu-8.04.2
status: New → In Progress
milestone: ubuntu-8.04.2 → ubuntu-8.10-beta
Revision history for this message
Steve Langasek (vorlon) wrote :

Accepted into -proposed, please test and give feedback here. Please see https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you in advance!

Changed in linux:
milestone: ubuntu-8.04.2 → none
status: In Progress → Fix Committed
Revision history for this message
Steve Langasek (vorlon) wrote :

Marking this as verification-failed because of bug #251344, a regression between 2.6.24-19 and 2.6.24-20. I don't know if that's tied to this change, but seems as good as any for tracking...

Revision history for this message
Tim Gardner (timg-tpi) wrote :

Looks like this should fix the missing CPU feature:

http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=27c7fd7b06c2f9b4941f99b310280d3a3fb27991

Revision history for this message
Jouni Mettala (jouni-mettala) wrote :

Since some intel cpu:s are affected too should arch/x86/kernel/cpu/intel.c be patched too?

Something like:
- if (cpu_has_xmm)
+ if (cpu_has_xmm2)
      set_bit(X86_FEATURE_LFENCE_RDTSC, c->x86_capability);

to /arch/x86/kernel/cpu/intel.c

Revision history for this message
Tim Gardner (timg-tpi) wrote :

See https://bugs.edge.launchpad.net/ubuntu/hardy/+source/linux/+bug/251344/comments/104

Revision history for this message
Martin Pitt (pitti) wrote :

Accepted into -proposed, please test and give feedback here. Please see https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you in advance!

Revision history for this message
Martin Pitt (pitti) wrote :

Regression verified as fixed in bug 251334.

Revision history for this message
Martin Pitt (pitti) wrote :

linux 2.6.24-21 copied to hardy-updates.

Changed in linux:
status: Fix Committed → Fix Released
Revision history for this message
Steve Langasek (vorlon) wrote :

This bug is still only marked as 'fix committed' for intrepid and beyond, since last July. Can someone confirm that it's fixed in intrepid/jaunty?

Tim Gardner (timg-tpi)
Changed in linux (Ubuntu):
assignee: Tim Gardner (timg-tpi) → nobody
importance: Medium → Undecided
milestone: ubuntu-8.10-beta → none
status: Fix Committed → Invalid
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