On 05/18/2013 05:21 AM, Rafael J. Wysocki wrote: > On Saturday, May 18, 2013 01:20:10 AM Srivatsa S. Bhat wrote: >> On 05/17/2013 11:47 PM, Olivier Doucet wrote: >>> Hello, >>> >>> This performance penalty is still present in kernel 3.9.2. And >>> CONFIG_PM cannot be deactivated anymore. >>> >>> I was able to make a working 3.9.2 (meaning with no penalty) with >>> following config and patch : >>> CONFIG_PM=y >>> CONFIG_PM_SLEEP=y >>> CONFIG_PM_SLEEP_SMP=y >>> CONFIG_CPU_IDLE=y >>> CONFIG_ACPI=y >>> CONFIG_ACPI_PROCESSOR=y >>> >>> Patch : https://gist.github.com/odoucet/5600630 >>> [...] >> So, to summarize my thoughts: >> - IMHO there is no regression here, you just depended on a bug included >> in 3.2.0 (which made it behave like idle=poll with CONFIG_PM=n) and >> started your comparisons from there. The later kernels (3.2.6+) got >> that bug fixed which is why you saw "performance drops". >> >> - As much as we would like to do it, we can't set the value of >> PM_QOS_CPU_DMA_LAT_DEFAULT_VALUE to 0 when CONFIG_PM=n, because >> CONFIG_PM doesn't encompass all power-management features (which is >> a pity). Doing that would need a big overhaul of all the relevant >> Kconfigs, which might or might not be worth the effort. (Because, who >> says that CONFIG_PM=n kernels are supposed to eat power like crazy??) > > I think it *is* worth the effort. We could drop some CONFIG_PM* options in > the > process which would simplify things quite a bit too. >
Ah, ok.. >> So here is my suggestion - use the interfaces provided by the kernel to >> fix your problem: >> - you can give idle=poll in the kernel command line, >> - OR you can echo 0 > /dev/cpu_dma_latency >> >> Irrespective of your kernel configuration options (CONFIG_PM=y/n), the >> CPUs will not enter deep idle states, giving you the performance >> improvement that you are looking for. > > Thanks a lot for the very clear explanation of this! > No problem! :-) Regards, Srivatsa S. Bhat -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
