On Sat, Feb 11, 2017 at 11:35:41AM +0900, Tejun Heo wrote: > Hello, Paul. > > On Fri, Feb 10, 2017 at 01:21:58PM -0800, Paul E. McKenney wrote: > > So RCU's expedited grace periods have been using workqueues for a > > little while, and things seem to be working. But as usual, I worry... > > Is this use subject to some sort of deadlock where RCU's workqueue cannot > > start running until after a grace period completes, but that grace > > period is the one needing the workqueue? Note that there are ways to > > set up your kernel so that all RCU grace periods are expedited. > > > > Should I be worried? If not, what prevents this from being a problem, > > especially given that workqueue handlers are allowed to wait for RCU > > grace periods to complete? > > A per-cpu (normal) workqueue's concurrency is regulated automatically > so that there are at least one worker running for the worker pool on a > given CPU. > > Let's say there are two work items queued on a workqueue. The first > one is something which will do synchronize_rcu() and the second is the > expedited grace period work item. When the first one runs > synchronize_rcu(), it'd block. If there are no other work items > running at the time, workqueue will dispatch another worker so that > there's at least one actively running, which in this case will be the > expedited rcu grace period work item. > > The dispatching of a new worker can be delayed by two things - memory > pressure preventing creation of a new worker and the workqueue hitting > maximum concurrency limit. > > If expedited RCU grace period is something that memory reclaim path > may depend on, the workqueue that it executes on should have > WQ_MEM_RECLAIM set, which will guarantee that there's at least one > worker (across all CPUs) which is ready to serve the work items on > that workqueue regardless of memory pressure. > > The latter, concurrency limit, would only matter if the RCU work items > use system_wq. system_wq's concurrency limit is very high (512 per > CPU), but it is theoretically possible to fill all up with work items > doing synchronize_rcu() with the expedited RCU work item scheduled > behind it. The system would already be in a very messed up state > outside the RCU situation tho.
Thank you for the information! So if I am to continue using workqueues for expedited RCU grace periods, I believe that need to do the following: 1. Use alloc_workqueue() to create my own WQ_MEM_RECLAIM workqueue. 2. Rework my workqueue handler to avoid blocking waiting for the expedited grace period to complete. I should be able to do a small number of timed wait, but if I actually wait for the grace period to complete, I might end up hogging the reserved items. (Or does my workqueue supply them for me? If so, so much the better!) 3. Concurrency would not be a problem -- there can be no more four work elements in flight across both possible flavors of expedited grace periods. Anything I am missing here? Thanx, Paul