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

On 2020/7/29 12:39, Sagi Grimberg wrote:

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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

I'm not opposed to having this. Will require some more testing
as this affects pretty much every driver out there..

If we are going with a lightweight percpu_ref, can we just do
it also for non-blocking hctx and have a single code-path?
.

I tried to optimize the patch，support for non blocking queue and
blocking queue.
See next email.

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