Re: [PATCH v8 15/19] locking/rwsem: Adaptive disabling of reader optimistic spinning

From: Waiman Long
Date: Tue Jun 04 2019 - 13:32:43 EST

On 6/4/19 5:10 AM, Peter Zijlstra wrote:
> On Mon, May 20, 2019 at 04:59:14PM -0400, Waiman Long wrote:
>> Reader optimistic spinning is helpful when the reader critical section
>> is short and there aren't that many readers around. It makes readers
>> relatively more preferred than writers. When a writer times out spinning
>> on a reader-owned lock and set the nospinnable bits, there are two main
>> reasons for that.
>> 1) The reader critical section is long, perhaps the task sleeps after
>> acquiring the read lock.
>> 2) There are just too many readers contending the lock causing it to
>> take a while to service all of them.
>> In the former case, long reader critical section will impede the progress
>> of writers which is usually more important for system performance.
>> In the later case, reader optimistic spinning tends to make the reader
>> groups that contain readers that acquire the lock together smaller
>> leading to more of them. That may hurt performance in some cases. In
>> other words, the setting of nonspinnable bits indicates that reader
>> optimistic spinning may not be helpful for those workloads that cause it.
>> Therefore, any writers that have observed the setting of the writer
>> nonspinnable bit for a given rwsem after they fail to acquire the lock
>> via optimistic spinning will set the reader nonspinnable bit once they
>> acquire the write lock. Similarly, readers that observe the setting
>> of reader nonspinnable bit at slowpath entry will also set the reader
>> nonspinnable bit when they acquire the read lock via the wakeup path.
> So both cases set the _reader_ nonspinnable bit?