[PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Keith Busch]

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