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
