Re: [PATCH 1/5] blk-mq: move srcu from blk_mq_hw_ctx to request_queue

Ming Lei
2021-11-22T13:08:56Z

On Mon, Nov 22, 2021 at 09:48:35AM +0200, Sagi Grimberg wrote:
>
> > > > + bool alloc_srcu;
> > >
> > > I found the following statement multiple times in this patch:
> > >
> > > WARN_ON_ONCE(q->alloc_srcu != !!(q->tag_set->flags & BLK_MQ_F_BLOCKING));
> > >
> > > Does this mean that the new q->alloc_srcu member variable can be left out
> > > and that it can be replaced with the following test?
> > >
> > > q->tag_set->flags & BLK_MQ_F_BLOCKING
> >
> > q->tag_set can't be used anymore after blk_cleanup_queue() returns,
> > and we need the flag for freeing request_queue instance.
>
> Why not just look at the queue->srcu pointer? it is allocated only
> for BLK_MQ_F_BLOCKING no?

Yeah, we can add one extra srcu pointer to request queue, but this way
needs one extra fetch to q->srcu in fast path compared with current
code base, so io_uring workload may be affected a bit.

Thanks,
Ming