On Tue, Feb 2, 2016 at 6:01 PM, Juri Lelli juri.lelli@arm.com wrote:
Hi Rafael,
On 02/02/16 17:35, Rafael J. Wysocki wrote:
On Tue, Feb 2, 2016 at 4:47 PM, Juri Lelli juri.lelli@arm.com wrote:
Hi Viresh,
On 02/02/16 16:27, Viresh Kumar wrote:
Until now, governors (ondemand/conservative) were using the 'global-attr' or 'freq-attr', depending on the sysfs location where we want to create governor's directory.
The problem is that, in case of 'freq-attr', we are forced to use show()/store() present in cpufreq.c, which always take policy->rwsem.
And because of that we were facing some ABBA lockups during governor callback event CPUFREQ_GOV_POLICY_EXIT. And so we were dropping the rwsem right before calling governor callback for CPUFREQ_GOV_POLICY_EXIT event.
That caused further problems and it never worked perfectly.
This patch attempts to fix that by creating separate sysfs-ops for cpufreq governors.
Because things got much simplified now, we don't need separate show/store callbacks for governor-for-system and governor-per-policy cases.
Signed-off-by: Viresh Kumar viresh.kumar@linaro.org
This patch cleans things up a lot, that's good.
One thing I'm still concerned about, though: don't we need some locking in place for some of the store operations on governors attributes? Are store_{ignore_nice_load, sampling_down_fact, etc} safe without locking?
That would require some investigation I suppose.
It seems that we can call them from different cpus concurrently.
Yes, we can.
One quick-and-dirty way of dealing with that might be to introduce a "sysfs lock" into struct dbs_data and hold that around the invocation of gattr->store() in the sysfs_ops's ->store callback.
There is value in trying to solve this issue by using some of the existing locks, IMHO.
Some value - maybe. I'm not sure how much of it, though.
Finer-grained locking is generally easier to follow, because the locks tend to be used for specific purposes only.
Can't we actually try to use the policy->rwsem (or one of the core locks) + wait_for_completion approach as we do in cpufreq core?
No. Too many things depend on that lock already and some of them work by accident rather than by design.
Thanks, Rafael