On Wed, Feb 3, 2016 at 2:32 AM, Saravana Kannan <skan...@codeaurora.org> wrote: > On 02/02/2016 05:07 PM, Rafael J. Wysocki wrote: >> >> On Wed, Feb 3, 2016 at 12:42 AM, Rafael J. Wysocki <raf...@kernel.org> >> wrote: >>> >>> On Tue, Feb 2, 2016 at 11:21 PM, Saravana Kannan <skan...@codeaurora.org> >>> wrote: >>>> >>>> On 02/02/2016 11:40 AM, Rafael J. Wysocki wrote: >>>>> >>>>> >>>>> On Tue, Feb 2, 2016 at 6:01 PM, Juri Lelli <juri.le...@arm.com> wrote: >>>>>> >>>>>> >> >> [cut] >> >>>> >>>> I also don't like this patch because it forces governors to either >>>> implement >>>> their own macros and management of their attributes or force them to use >>>> the >>>> governor structs that come with cpufreq_governor.h. cpufreq_governor.h >>>> IMHO >>>> is very ondemand and conservative governor specific and is very >>>> irrelevant >>>> for sched-dvfs or any other governors (hint hint). >>>> >>>> The only time this ABBA locking is an issue is when governor are >>>> changing >>>> and trying to add/remove attributes. That can easily be checked in >>>> store_governor and dealt with without holding the policy rwsem if the >>>> governors can provide their per sys and per policy attribute arrays as >>>> part >>>> of registering themselves. >>>> >>>> I'm sorry that I just keep talking about the idea and not sending out >>>> the >>>> patches. >>> >>> >>> I think you have a point, though. >>> >>> The deadlock really is specific to the governors using the code in >>> cpufreq_governor.c. >> >> >> That said no other governors in the tree use any sysfs attributes for >> tunables AFAICS and the out-of-the tree ones are out of interest here. > > > But if we are expecting sched dvfs to come in, why make it worse for it. It > would be completely pointless to try and shoehorn sched dvfs to use > cpufreq_governor.c
Well, do you honestly think that using the existing stuff in it would be a good idea? If not, then why it matters at all? >> Also the deadlock happens if one of the tunable attributes is accessed >> while we're trying to remove it which very well may happen on read >> access too. > > Isn't this THE deadlock we are talking about? The removal of the attributes > only happen when governors are changes and we send a POLICY_EXIT and or all > the cores are hotplugged out. It generally happens when the "old" governor is going away, whatever the reason. > And my suggestion would work just as well there. > > Why are you prefixing your sentence with "Also"? Is there some other case > I'm not considering? Say someone is reading sampling_rate for a policy with 1 CPU in it and someone else is taking the CPU offline. The governor EXIT code path (that will trigger as a result) will try to remove the sampling_rate attribute and (if it does that under policy->rwsem) it'll wait for the read access to finish. Where exactly would you put the deadlock prevention in this case? Thanks, Rafael
