[PATCH v3 1/2] blk-mq: add async quiesce interface

[Ming Lei]

On Mon, Jul 27, 2020 at 11:33:43AM +0800, Chao Leng wrote:
> 
> 
> On 2020/7/27 10:08, Ming Lei wrote:
> > > It is at the end and contains exactly what is needed to synchronize. Not
> > The sync is simply single global synchronize_rcu(), and why bother to add
> > extra >=40bytes for each hctx.
> > 
> > > sure what you mean by reuse hctx->srcu?
> > You already reuses hctx->srcu, but not see reason to add extra rcu_synchronize
> > to each hctx for just simulating one single synchronize_rcu().
> 
> To sync srcu together, the extra bytes must be needed, seperate blocking
> and non blocking queue to two hctx may be a not good choice.
> 
> There is two choice: the struct rcu_synchronize is added in hctx or in srcu.
> Though add rcu_synchronize in srcu has a  weakness: the extra bytes is
> not need if which do not need batch sync srcu, I still think it's better
> for the SRCU to provide the batch synchronization interface.

The 'struct rcu_synchronize' can be allocated from heap or even stack(
if no too many NSs), which is just one shot sync and the API is supposed
to be called in slow path. No need to allocate it as long lifetime variable.
Especially 'struct srcu_struct' has already been too fat.


Thanks, 
Ming