On 03/19/2014 08:18 PM, Srivatsa S. Bhat wrote:
On 03/19/2014 07:05 PM, Viresh Kumar wrote:
On 19 March 2014 17:45, Srivatsa S. Bhat srivatsa.bhat@linux.vnet.ibm.com wrote:
diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
bool transition_ongoing; /* Tracks transition status */struct mutex transition_lock;wait_queue_head_t transition_wait;Similar to what I have done in my last version, why do you need transition_ongoing and transition_wait? Simply work with transition_lock? i.e. Acquire it for the complete transition sequence.
We *can't* acquire it for the complete transition sequence in case of drivers that do asynchronous notification, because PRECHANGE is done in one thread and POSTCHANGE is done in a totally different thread! You can't acquire a lock in one task and release it in a different task. That would be a fundamental violation of locking.
That's why I introduced the wait queue to help us create a "flow" which encompasses 2 different, but co-ordinating tasks. You simply can't do that elegantly by using plain locks alone.
By the way, note the updated changelog in my patch. It includes a brief overview of the synchronization design, which is copy-pasted below for reference. I forgot to mention this earlier!
-----
This patch introduces a set of synchronization primitives to serialize frequency transitions, which are to be used as shown below:
cpufreq_freq_transition_begin();
//Perform the frequency change
cpufreq_freq_transition_end();
The _begin() call sends the PRECHANGE notification whereas the _end() call sends the POSTCHANGE notification. Also, all the necessary synchronization is handled within these calls. In particular, even drivers which set the ASYNC_NOTIFICATION flag can also use these APIs for performing frequency transitions (ie., you can call _begin() from one task, and call the corresponding _end() from a different task).
The actual synchronization underneath is not that complicated:
The key challenge is to allow drivers to begin the transition from one thread and end it in a completely different thread (this is to enable drivers that do asynchronous POSTCHANGE notification from bottom-halves, to also use the same interface).
To achieve this, a 'transition_ongoing' flag, a 'transition_lock' mutex and a wait-queue are added per-policy. The flag and the wait-queue are used in conjunction to create an "uninterrupted flow" from _begin() to _end(). The mutex-lock is used to ensure that only one such "flow" is in flight at any given time. Put together, this provides us all the necessary synchronization.
Regards, Srivatsa S. Bhat