Michal Hocko wrote: > On Fri 23-10-15 19:36:30, Tejun Heo wrote: > > Hello, Michal. > > > > On Fri, Oct 23, 2015 at 10:33:16AM +0200, Michal Hocko wrote: > > > Ohh, OK I can see wq_worker_sleeping now. I've missed your point in > > > other email, sorry about that. But now I am wondering whether this > > > is an intended behavior. The documentation says: > > > > This is. > > > > > WQ_MEM_RECLAIM > > > > > > All wq which might be used in the memory reclaim paths _MUST_ > > > have this flag set. The wq is guaranteed to have at least one > > > execution context regardless of memory pressure. > > > > > > Which doesn't seem to be true currently, right? Now I can see your patch > > > > It is true. > > > > > to introduce WQ_IMMEDIATE but I am wondering which WQ_MEM_RECLAIM users > > > could do without WQ_IMMEDIATE? I mean all current workers might be > > > looping in the page allocator and it seems possible that WQ_MEM_RECLAIM > > > work items might be waiting behind them so they cannot help to relieve > > > the memory pressure. This doesn't sound right to me. Or I am completely > > > confused and still fail to understand what is WQ_MEM_RECLAIM supposed to > > > be used for. > > > > It guarantees that there always is enough execution resource to > > execute a work item from that workqueue. > > OK, strictly speaking the rescuer is there but it is kind of pointless > if it doesn't fire up and do a work. > > > The problem here is not lack > > of execution resource but concurrency management misunderstanding the > > situation. > > And this sounds like a bug to me. > > > This also can be fixed by teaching concurrency management > > to be a bit smarter - e.g. if a work item is burning a lot of CPU > > cycles continuously or pool hasn't finished a work item over a certain > > amount of time, automatically ignore the in-flight work item for the > > purpose of concurrency management; however, this sort of inter-work > > item busy waits are so extremely rare and undesirable that I'm not > > sure the added complexity would be worthwhile. > > Don't we have some IO related paths which would suffer from the same > problem. I haven't checked all the WQ_MEM_RECLAIM users but from the > name I would expect they _do_ participate in the reclaim and so they > should be able to make a progress. Now if your new IMMEDIATE flag will > guarantee that then I would argue that it should be implicit for > WQ_MEM_RECLAIM otherwise we always risk a similar situation. What would > be a counter argument for doing that? WQ_MEM_RECLAIM only guarantees that a "struct task_struct" is preallocated in order to avoid failing to allocate it on demand due to a GFP_KERNEL allocation? Is this correct? WQ_CPU_INTENSIVE only guarantees that work items don't participate in concurrency management in order to avoid failing to wake up a "struct task_struct" which will process the work items? Is this correct? Is Michal's question "does it make sense to use WQ_MEM_RECLAIM without WQ_CPU_INTENSIVE"? In other words, any "struct task_struct" which calls rescuer_thread() must imply WQ_CPU_INTENSIVE in order to avoid failing to wake up due to being participated in concurrency management? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>