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?
Yes. -Longman
