** Changed in: linux (Ubuntu Vivid)
Status: Confirmed => Fix Committed
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https://bugs.launchpad.net/bugs/1410741
Title:
VDSO clock function may be miss-optimised leading to hangs on SMP
systems
Status in linux package in Ubuntu:
Fix Committed
Status in linux source package in Trusty:
Confirmed
Status in linux source package in Utopic:
Confirmed
Status in linux source package in Vivid:
Fix Committed
Bug description:
commit 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba
Author: Andy Lutomirski <l...@amacapital.net>
Date: Sun Dec 21 08:57:46 2014 -0800
x86, vdso: Use asm volatile in __getcpu
In Linux 3.18 and below, GCC hoists the lsl instructions in the
pvclock code all the way to the beginning of __vdso_clock_gettime,
slowing the non-paravirt case significantly. For unknown reasons,
presumably related to the removal of a branch, the performance issue
is gone as of
e76b027e6408 x86,vdso: Use LSL unconditionally for vgetcpu
but I don't trust GCC enough to expect the problem to stay fixed.
There should be no correctness issue, because the __getcpu calls in
__vdso_vlock_gettime were never necessary in the first place.
Note to stable maintainers: In 3.18 and below, depending on
configuration, gcc 4.9.2 generates code like this:
9c3: 44 0f 03 e8 lsl %ax,%r13d
9c7: 45 89 eb mov %r13d,%r11d
9ca: 0f 03 d8 lsl %ax,%ebx
This patch won't apply as is to any released kernel, but I'll send a
trivial backported version if needed.
Fixes: 51c19b4f5927 x86: vdso: pvclock gettime support
Cc: sta...@vger.kernel.org # 3.8+
Cc: Marcelo Tosatti <mtosa...@redhat.com>
Acked-by: Paolo Bonzini <pbonz...@redhat.com>
Signed-off-by: Andy Lutomirski <l...@amacapital.net>
===
break-fix: 51c19b4f5927f5a646e93d69f73c7e89ea14e737
1ddf0b1b11aa8a90cef6706e935fc31c75c406ba
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