The patch below does not apply to the 3.10-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.
thanks, greg k-h ------------------ original commit in Linus's tree ------------------ >From 1ddf0b1b11aa8a90cef6706e935fc31c75c406ba Mon Sep 17 00:00:00 2001 From: Andy Lutomirski <[email protected]> Date: Sun, 21 Dec 2014 08:57:46 -0800 Subject: [PATCH] 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: [email protected] # 3.8+ Cc: Marcelo Tosatti <[email protected]> Acked-by: Paolo Bonzini <[email protected]> Signed-off-by: Andy Lutomirski <[email protected]> diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h index e7e9682a33e9..f556c4843aa1 100644 --- a/arch/x86/include/asm/vgtod.h +++ b/arch/x86/include/asm/vgtod.h @@ -80,9 +80,11 @@ static inline unsigned int __getcpu(void) /* * Load per CPU data from GDT. LSL is faster than RDTSCP and - * works on all CPUs. + * works on all CPUs. This is volatile so that it orders + * correctly wrt barrier() and to keep gcc from cleverly + * hoisting it out of the calling function. */ - asm("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); + asm volatile ("lsl %1,%0" : "=r" (p) : "r" (__PER_CPU_SEG)); return p; } -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
