Re: [PATCH 2/5] blk-mq: rename hctx_lock & hctx_unlock

[Ming Lei <ming.lei@redhat.com>]

On Mon, Nov 22, 2021 at 09:53:53AM +0200, Sagi Grimberg wrote:
> 
> > -static inline void hctx_unlock(struct blk_mq_hw_ctx *hctx, int srcu_idx)
> > -	__releases(hctx->srcu)
> > +static inline void queue_unlock(struct request_queue *q, bool blocking,
> > +		int srcu_idx)
> > +	__releases(q->srcu)
> >   {
> > -	if (!(hctx->flags & BLK_MQ_F_BLOCKING))
> > +	if (!blocking)
> >   		rcu_read_unlock();
> >   	else
> > -		srcu_read_unlock(hctx->queue->srcu, srcu_idx);
> > +		srcu_read_unlock(q->srcu, srcu_idx);
> 
> Maybe instead of passing blocking bool just look at srcu_idx?
> 
> 	if (srcu_idx < 0)
> 		rcu_read_unlock();
> 	else
> 		srcu_read_unlock(q->srcu, srcu_idx);

This way needs to initialize srcu_idx in each callers.

> 
> Or look if the queue has srcu allocated?
> 
> 	if (!q->srcu)
> 		rcu_read_unlock();
> 	else
> 		srcu_read_unlock(q->srcu, srcu_idx);

This way is worse since q->srcu may involve one new cacheline fetch.

hctx->flags is always hot, so it is basically zero cost to check it.


Thanks,
Ming