On Monday 30 March 2015 03:51 PM, Michael Ellerman wrote: > On Sun, 2015-22-03 at 04:42:59 UTC, "Shreyas B. Prabhu" wrote: >> Fastsleep is one of the idle state which cpuidle subsystem currently >> uses on power8 machines. In this state L2 cache is brought down to a >> threshold voltage. Therefore when the core is in fastsleep, the >> communication between L2 and L3 needs to be fenced. But there is a bug >> in the current power8 chips surrounding this fencing. >> >> OPAL provides a workaround which precludes the possibility of hitting >> this bug. But running with this workaround applied causes checkstop >> if any correctable error in L2 cache directory is detected. Hence OPAL >> also provides a way to undo the workaround. >> >> In the existing implementation, workaround is applied by the last thread >> of the core entering fastsleep and undone by the first thread waking up. >> But this has a performance cost. These OPAL calls account for roughly >> 4000 cycles everytime the core has to enter or wakeup from fastsleep. >> >> This patch introduces a sysfs attribute (fastsleep_workaround_state) >> to choose the behavior of this workaround. >> >> By default, fastsleep_workaround_state = 0. In this case, workaround >> is applied/undone everytime the core enters/exits fastsleep. >> >> fastsleep_workaround_state = 1. In this case the workaround is applied >> once on all the cores and never undone. This can be triggered by >> echo 1 > /sys/devices/system/cpu/fastsleep_workaround_state >> >> For simplicity this attribute can be modified only once. Implying, once >> fastsleep_workaround_state is changed to 1, it cannot be reverted to >> the default state. > > This sounds good, although the name is a bit vague. > > Just calling it "state" doesn't make it clear what 0 and 1 mean. > I think better would be "fastsleep_workaround_active" ? > > Though even that is a bit wrong, because 0 doesn't really mean it's not > active, > it means it's not *permanently* active. > > So another option would be to make it a string attribute, with the initial > state being eg. "dynamic" and then maybe "applied" for the applied state? > How about "fastsleep_workaround_permanent", with default value = 0. User can make workaround permanent by echoing 1 to it.
I'll post out V4 with the suggested changes. Thanks, Shreyas -- 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/
