Bug 84741

Summary: power usage rises a lot after resume from ram when using maxcpus=X
Product: ACPI Reporter: Arkadiusz Miskiewicz (arekm)
Component: BIOSAssignee: Lan Tianyu (tianyu.lan)
Status: CLOSED WILL_NOT_FIX    
Severity: normal CC: alan, lenb, rui.zhang
Priority: P1    
Hardware: All   
OS: Linux   
Kernel Version: 3.17-rc5 Subsystem:
Regression: No Bisected commit-id:

Description Arkadiusz Miskiewicz 2014-09-17 12:25:25 UTC 
Kernel: 3.17rc5 (but also 3.14, 3.10 and probably all other)
hw: Dell XPS 15 9530
cpu: i7-4702HQ


This CPU has 4 normal cores and 4 HT cores.

I was using maxcpus=4 on this machine. After resume from ram machine immediately eats ~10W more.

Initially I thought this is some Intel GPU bug and reported
https://bugs.freedesktop.org/show_bug.cgi?id=83914

But it looks like after resume all 8 cores are up, eating power while linux kernel only manages first 4. The reset 4 cores are only eating power and producing heat.

dmesg:
https://bugs.freedesktop.org/attachment.cgi?id=106351

How to reproduce:
- boot with maxcpus=4 (with minimal userspace for example)
- measure power usage with powertop
- suspend to ram, resume from ram
- measure power usage again, it rises ~10W

How to workaround the problem, online CPUS and then make then offline again:
for cpu in $(grep -l 0 /sys/devices/system/cpu/cpu*/online); do
    echo 1 > $cpu
    sleep 1
    echo 0 > $cpu
done

After this power usage is back to normal.

So, I guess if maxcpus is being used linux kernel should apply some action to other, offline cores, too to avoid them being power hungry.
Comment 1 Arkadiusz Miskiewicz 2014-09-17 12:26:36 UTC 
"all 8 cores are up" - by writting that I mean hardware is up/eating power but kernel still thinks these are offline etc.
Comment 2 Len Brown 2014-11-06 05:14:16 UTC 
maxcpus= cmdline causes Linux to ignore those cpus
both at boot-time, and at resume-time.

In both cases, they are left in whatever state the BIOS leaves them in.
Hopefully that is a low power state, but in this case, it seems the
BIOS is not leaving them in a low power state on resume from suspend.

Not immediately clear that it is Linux's job to fix this issue.