Messages in this thread | | | Date | Fri, 29 Jan 2016 16:57:02 -0800 | From | Saravana Kannan <> | Subject | Re: [RFC PATCH 00/19] cpufreq locking cleanups and documentation |
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On 01/11/2016 09:35 AM, Juri Lelli wrote: > Hi all, > > In the context of the ongoing discussion about introducing a simple platform > energy model to guide scheduling decisions (Energy Aware Scheduling [1]) > concerns have been expressed by Peter about the component in charge of driving > clock frequency selection (Steve recently posted an update of such component > [2]): https://lkml.org/lkml/2015/8/15/141. > > The problem is that, with this new approach, cpufreq core functions need to be > accessed from scheduler hot-paths and the overhead associated with the current > locking scheme might result to be unsustainable. > > Peter's proposed approach of using RCU logic to reduce locking overhead seems > reasonable, but things may not be so straightforward as originally thought. The > very first thing I actually realized when I started looking into this is that > it was hard for me to understand which locking mechanism was protecting which > data structure. As mostly a way to build a better understanding of the current > cpufreq locking scheme and also as preparatory work for implementing RCU logic, > I came up with this set of patches. In fact, at this stage, I would like each > patch to be considered as a question I'm asking rather than a proposed change, > thus the RFC tag for the series; with the intent of documenting current locking > scheme and modifying it a bit in order to make RCU logic implementation easier. > Actually, as you'll soon notice, I didn't really start from scratch. Mike > shared with me some patches he has been developing while looking at the same > problem. I've given Mike attribution for the patches that I took unchanged from > him, with thanks for sharing his findings with me. > > High level description of patches: > > o [01-04] cleanup and move code around to make things (hopefully) cleaner > o [05-14] insert lockdep assertions and fix uncovered erroneous situations > o [15-18] remove overkill usage of locking mechanism > o 19 adds documentation for the cleaned up locking scheme > > With Viresh' tests [3] on both arm TC2 and arm64 Juno boards I'm not seeing > anything bad happening. However, coverage is really small (as is my personal > confidence of not breaking things for other confs :-)). > > This set is based on top of linux-pm/linux-next as of today and it is also > available from here: > > git://linux-arm.org/linux-jl.git upstream/cpufreq_cleanups > > Comments, concerns and rants are the primary goal of this posting; I'm thus > looking forward to them. > > Best, > > - Juri > > [1] https://lkml.org/lkml/2015/7/7/754 > [2] https://lkml.org/lkml/2015/12/9/35 > [3] https://git.linaro.org/people/viresh.kumar/cpufreq-tests.git > > Juri Lelli (16): > cpufreq: kill for_each_policy > cpufreq: bring data structures close to their locks > cpufreq: assert locking when accessing cpufreq_policy_list > cpufreq: always access cpufreq_policy_list while holding > cpufreq_driver_lock > cpufreq: assert locking when accessing cpufreq_governor_list > cpufreq: fix warning for cpufreq_init_policy unlocked access to > cpufreq_governor_list > cpufreq: fix warning for show_scaling_available_governors unlocked > access to cpufreq_governor_list > cpufreq: assert policy->rwsem is held in cpufreq_set_policy > cpufreq: assert policy->rwsem is held in __cpufreq_governor > cpufreq: fix locking of policy->rwsem in cpufreq_init_policy > cpufreq: fix locking of policy->rwsem in cpufreq_offline_prepare > cpufreq: fix locking of policy->rwsem in cpufreq_offline_finish > cpufreq: remove useless usage of cpufreq_governor_mutex in > __cpufreq_governor > cpufreq: hold policy->rwsem across CPUFREQ_GOV_POLICY_EXIT > cpufreq: stop checking for cpufreq_driver being present in > cpufreq_cpu_get > cpufreq: documentation: document locking scheme > > Michael Turquette (3): > cpufreq: do not expose cpufreq_governor_lock > cpufreq: merge governor lock and mutex > cpufreq: remove transition_lock > > Documentation/cpu-freq/core.txt | 44 +++++++++++++ > drivers/cpufreq/cpufreq.c | 132 +++++++++++++++++++++++-------------- > drivers/cpufreq/cpufreq_governor.h | 2 - > include/linux/cpufreq.h | 5 -- > 4 files changed, 125 insertions(+), 58 deletions(-) >
Juri,
I haven't looked at the cpufreq-tests, but I doubt they do hotplug testing where they remove all the CPUs of a policy (to trigger a policy exit).
Can you please add that to your testing? I wouldn't be surprised if some of your clean ups would cause a dead lock. This clean up series is definitely appreciated, but I think the patch series might still be missing some patches that are needed to make things work without deadlocking.
I'll try to do a deeper analysis/review/testing, but kinda hard pressed on time here.
Thanks, Saravana
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