Messages in this thread | | | Date | Wed, 15 Dec 2021 11:44:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 0/3] sched: User Managed Concurrency Groups |
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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.
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