On Wed, Dec 15, 2021 at 01:49:28PM +0000, Matthew Wilcox wrote: > On Wed, Dec 15, 2021 at 11:44:49AM +0100, Peter Zijlstra wrote: > > On Tue, Dec 14, 2021 at 07:46:25PM -0800, Peter Oskolkov wrote: > > > > > Anyway, I'll test your patchset over the next week or so and let you > > > know if anything really needed is missing (other than waking an idle > > > server if there is one on a worker wakeup; this piece is definitely > > > needed). > > > > Right, so the problem I'm having is that a single idle server ptr like > > before can trivially miss waking annother idle server. > > > > Suppose: > > > > umcg::idle_server_tid_ptr > > > > Then the enqueue_and_wake() thing from the last patch would: > > > > idle_server_tid = xchg((pid_t __user *)self->idle_server_tid_ptr, 0); > > > > to consume the tid, and then use that to enqueue and wake. But what if a > > second wakeup happens right after that? There might be a second idle > > server, but we'll never find it, because userspace hasn't had time to > > update the field again. > > > > Alternatively, we do a linked list of servers, but then every such > > wakeup needs to iterate the whole list, looking for one that has > > UMCG_TF_IDLE set, or something like that, but that lookup is bad for > > performance. > > > > So I'm really not sure what way to go yet. > > 1. Linked lists are fugly and bad for the CPU. Absolutely.. although a stack might work, except for that ABA issue (and contention). > 2. I'm not sure how big the 'N' in 'M:N' is supposed to be. Might be > one per hardware thread? So it could be hundreds-to-thousands, > depending on the scale of system. Typically yes, one server task per hardware thread. Now, I'm also fairly sure you don't want excessive cross-node traffic for this stuff, so that puts a limit on things as well. > 3. The interface between user-kernel could be an array of idle tids, > maybe 16 entries long (16 * 4 = 64 bytes, just one cacheline). As a > server finishes work, it looks for a 0 tid in the batch and stores > its tid in the slot (cmpxchg, I guess, since the array will be shared > between processes). If there are no free slots in the array, then we > definitely have 16 threads already waiting for work, so it can park itself > in whatever data structure userspace wants to use to manage idle servers. > It's up to userspace to decide when to repopulate the array of available > servers from its data structure of idle servers. Right, a tid array might work. Could even have userspace specify the length, then it can do the trade-offs all on it's own. Either a fixed location for each server and a larger array, or clever things, whatever they want. I suppose I'll code up the variable length array, we have space for that.
