Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT

[Christoph Hellwig]

On Fri, Oct 23, 2020 at 03:52:19PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-10-23 12:21:30 [+0100], Christoph Hellwig wrote:
> > > -	if (!IS_ENABLED(CONFIG_SMP) ||
> > > +	if (!IS_ENABLED(CONFIG_SMP) || IS_ENABLED(CONFIG_PREEMPT_RT) ||
> > >  	    !test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags))
> > 
> > This needs a big fat comment explaining your rationale.  And probably
> > a separate if statement to make it obvious as well.
> 
> Okay.
> How much difference does it make between completing in-softirq vs
> in-IPI?

For normal non-RT builds?  This introduces another context switch, which
for the latencies we are aiming for is noticable.

> I'm asking because acquiring a spinlock_t in an IPI shouldn't be
> done (as per Documentation/locking/locktypes.rst). We don't have
> anything in lockdep that will complain here on !RT and we the above we
> avoid the case on RT.

At least for NVMe we aren't taking locks, but with the number of drivers