Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH V3 1/5] cpufreq: ondemand: Drop unnecessary locks from update_sampling_rate() | Date | Wed, 28 Oct 2015 06:54:27 +0100 |
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On Wednesday, October 28, 2015 10:14:51 AM Viresh Kumar wrote: > On 28-10-15, 05:05, Rafael J. Wysocki wrote: > > On Tuesday, October 13, 2015 01:39:01 PM Viresh Kumar wrote: > > > 'timer_mutex' is required to sync work-handlers of policy->cpus. > > > update_sampling_rate() is just canceling the works and queuing them > > > again. This isn't protecting anything at all in update_sampling_rate() > > > and is not gonna be of any use. > > > > > > Even if a work-handler is already running for a CPU, > > > cancel_delayed_work_sync() will wait for it to finish. > > > > > > Drop these unnecessary locks. > > > > > > Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com> > > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > > > > I'm queuing this up for 4.4, although I think that the changelog is not right. > > > > While at it, what are the race conditions the lock is protecting against? > > In cases where a single policy controls multiple CPUs, a timer is > queued for every cpu present in policy->cpus. When we reach the timer > handler (which can be on multiple CPUs together) on any CPU, we trace > CPU load for all policy->cpus and update the frequency accordingly.
That would be in dbs_timer(), right?
> The lock is for protecting multiple CPUs to do the same thing > together, as only its required to be done by a single CPU. Once any > CPUs handler has completed, it updates the last update time and drops > the mutex. At that point of time, other blocked handler (if any) check > the last update time and return early.
Well, that would mean we only needed to hold the lock around the need_load_eval() evaluation in dbs_timer() if I'm not mistaken.
We also should acquire it around updates of the sampling rate, which essentially is set_sampling_rate().
Is there any reason to acquire it in cpufreq_governor_limits(), then, for example?
> And then there are enough minute things that can go wrong if multiple > CPUs do the load evaluation and freq-update at the same time, apart > from it being an time wasting effort. > > And so I still think that the commit log isn't that bad. The > timer_mutex lock isn't required in other parts of the governor, they > are just for synchronizing the work-handlers of CPUs belonging to the > same policy.
I agree that it doesn't serve any purpose in the piece of code you're removing it from (which is why I agree with the patch), but the changelog is incomplete and confusing.
Thanks, Rafael
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