Good description. I'm leaving the whole email in for reference, see below:
On Fri, Jan 18, 2013 at 10:05:41PM +0100, Olivier Doucet wrote: > Hello, > > I think I found a performance issue in kernel. This problem was > introduced in 3.2.6 and is affecting all version, including 3.7.1 > (latest tested). > > I measured several kernel builds with a homebrew LAMP platform > benchmark (250 runs, average value kept). > > Kernel 3.2.0 was on production so far, and I tried an upgrade to 3.7.1 > (latest kernel at that time). No software other than the kernel was > modified. Kernel was built with the same .config file (new options > left with default value). > Difference in performance was quite huge : > Kernel 3.7.1 : ~ 3250 queries / second > Kernel 3.2.0 : ~ 4300 queries / second > > Yes, this is a 25% performance drop ... > > To narrow things down, I tested several kernels : > 3.2.0 to 3.2.5 : OK > 3.2.6, 3.2.11, 3.2.28, 3.2.36, 3.7.1 : PERFORMANCE DROP > On faulty kernels, performance drop is always the same. > > At this step, I know that bug was introduced in version 3.2.6. I then > used git bisect (amazing tool btw) to find the faulty commit : > f51d67a64f32cd81ea8b67ca964fb7cf7e783b2e PM / QoS: CPU C-state > breakage with PM Qos change > > Next test : I used a 3.2.6 and reverted this patch : performance was > back to normal. > I also reverted this patch on 3.2.36 (latest 3.2.X), and performance > was also OK. I was unable to revert this patch on 3.7.1 (structure of > the source file /include/linux/pm_qos.h changed too heavily). > > Kernel was built on a x86_64 platform with binutils 2.19.1 and gcc 4.4.0. > .config file used : https://gist.github.com/4567342 > Benchmark was run on a dual CPU INTEL L5630 with 4GB of RAM > > What can I do to resolve this problem ? Btw, the commit-id you've given is the stable commit-id but this is still ok, the mainline commit is mentioned in the commit message and is: commit d020283dc694c9ec31b410f522252f7a8397e67d Author: Venkatesh Pallipadi <ve...@google.com> Date: Fri Feb 3 22:22:25 2012 +0100 PM / QoS: CPU C-state breakage with PM Qos change So, let's invite the parties from the commit to CC, see what they have to say. Thanks. -- Regards/Gruss, Boris. Sent from a fat crate under my desk. Formatting is fine. -- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
