On Tue, Jul 28, 2020 at 05:43:02PM -0700, Sagi Grimberg wrote: > > > > Dynamically allocating each one is possible but not very scalable. > > > > > > The question is if there is some way, we can do this with on-stack > > > or a single on-heap rcu_head or equivalent that can achieve the same > > > effect. > > > > If the hctx structures are guaranteed to stay put, you could count > > them and then do a single allocation of an array of rcu_head structures > > (or some larger structure containing an rcu_head structure, if needed). > > You could then sequence through this array, consuming one rcu_head per > > hctx as you processed it. Once all the callbacks had been invoked, > > it would be safe to free the array. > > > > Sounds too simple, though. So what am I missing? > > We don't want higher-order allocations... So: (1) We don't want to embed the struct in the hctx because we allocate so many of them that this is non-negligable to add for something we typically never use. (2) We don't want to allocate dynamically because it's potentially huge. As long as we're using srcu for blocking hctx's, I think it's "pick your poison". Alternatively, Ming's percpu_ref patch(*) may be worth a look. * https://www.spinics.net/lists/linux-block/msg56976.html1
