Re: [PATCH] blk-mq: make synchronous hw_queue runs RT friendly

[Christoph Hellwig]

On Mon, Dec 13, 2021 at 11:05:29AM -0800, Davidlohr Bueso wrote:
> On Mon, 13 Dec 2021, Christoph Hellwig wrote:
> > But more importantly:  why isn't migrate_disable/enable doing the right
> > thing for !PREEMPT_RT to avoid this mess?
> 
> Please see Peter's description of the situation in af449901b84.

That explains why migrate_disable is a bad idea in PREEMPT_RT, not why it
can't do something sensible for !PREEMPT_RT…

> 
> While I'm not at all a fan of sprinkling migrate_disabling around code,
> I didn't want to add any overhead for the common case. If this, however,
> were not an issue (if most cases are async runs, for example) the ideal
> solution I think would be to just pin current to the hctx->cpumask.

sync running is the performance case.