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Subject: Re: [PATCH v5 1/2] blk-mq: add tagset quiesce interface
To:     paulmck@kernel.org
Cc:     Ming Lei <ming.lei@redhat.com>, Christoph Hellwig <hch@lst.de>,
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From:   Sagi Grimberg <sagi@grimberg.me>
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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...