On 07/31/2014 08:55 PM, Saravana Kannan wrote: > On 07/31/2014 03:58 PM, Prarit Bhargava wrote: >> >> >> On 07/31/2014 06:13 PM, Saravana Kannan wrote: >>> On 07/31/2014 02:08 PM, Prarit Bhargava wrote: >>>> >>>> >>>> On 07/31/2014 04:38 PM, Saravana Kannan wrote: >>>>> On 07/31/2014 01:30 PM, Prarit Bhargava wrote: >>>>>> >>>>>> >>>>>> On 07/31/2014 04:24 PM, Saravana Kannan wrote: >>>>>>> >>>>>>> Prarit, >>>>>>> >>>>>>> I'm not an expert on sysfs locking, but I would think the specific sysfs >>>>>>> lock >>>>>>> would depend on the file/attribute group. So, can you please try to >>>>>>> hotplug a >>>>>>> core in/out (to trigger the POLICY_EXIT) and then read a sysfs file >>>>>>> exported by >>>>>>> the governor? scaling_governor doesn't cut it since that file is not >>>>>>> removed on >>>>>>> policy exit event to governor. If it's ondemand, try reading/write it's >>>>>>> sampling >>>>>>> rate file. >>>>>> >>>>>> Thanks Saravana -- will do. I will get back to you shortly on this. >>>>>> >>>>> >>>>> Thanks. Btw, in case you weren't already aware of it. You'll have to >>>>> hoplug >>>>> out >>>>> all the CPUs in a cluster to trigger a POLICY_EXIT for that >>>>> cluster/policy. >>>> >>>> Yep -- the affected_cpus file should show all the cpus in the policy IIRC. >>>> One >>>> of the systems I have has 1 cpu/policy and has 48 threads so the >>>> POLICY_EXIT is >>>> called. >>>> >>>> I'll put something like >>>> >>>> while [1]; >>>> do >>>> echo ondemand > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor >>>> cat /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate >>>> echo 20000 > /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate >>>> cat /sys/devices/system/cpu/cpufreq/ondemand/sampling_rate >>>> echo 0 > /sys/devices/system/cpu/cpu1/online >>>> sleep 1 >>>> echo 1 > /sys/devices/system/cpu/cpu1/online >>>> sleep 1 >>>> done >>>> >>> >>> The actual race can only happen with 2 threads. I'm just trying to trigger a >>> lockdep warning here. >> >> I ran the above in two separate terminals with cpuset -c 0 and cpuset -c 1 to >> multi-thread it all. No deadlock or LOCKDEP trace after about 1/2 hour, so I >> think we're in the clear on that concern. >> > > I wasn't convinced. So, I took some help from Stephen to test it. > > It's been a while, so I didn't remember the original issue clearly when I gave > you some test suggestions. Now that I looked at the code more closely, I have > a > proper way to reproduce the original issue. > > Nack for this patch for 2 reasons: > 1. You seem to have accidentally removed a GOV_STOP in your patch. We > definitely > can't do that. This broke changing governors and that's why your patch didn't > cause any issues. Because all your governor echos were failing.
Yes, I mentioned that yesterday in the thread. > 2. When we fixed that and actually tried a proper test (not the one I gave > you), > we reproduced the original issue. > > To reproduce original issue: > Preconditions: > * lockdep is enabled > * governor per policy is enabled Okay. I have that ... > > Steps: > 1. Set governor to ondemand. > 2. Cat one of the ondemand sysfs files. > 3. Change governor to conservative. > > When you do that, there's an AB, BA dead lock issue with one thread trying to > cat a governor sysfs file and another thread trying to change governors. Okay, I'm going to try this too... P. > > -Saravana > -- 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/
