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

On 2020-10-27 17:23:09 [+0000], Christoph Hellwig wrote:

On Tue, Oct 27, 2020 at 06:05:15PM +0100, Thomas Gleixner wrote:

quoted

quoted

Is there a way to raise a softirq and preferably place it on a given
CPU without our IPI dance?  That should be a win-win situation for
everyone.

Not really. Softirq pending bits are strictly per cpu and we don't have
locking or atomics to set them remotely. Even if we had that, then you'd
still need a mechanism to make sure that the remote CPU actually
processes them. So you'd still need an IPI of some sorts.

Ok.  I was hoping we could hide this in core code somehow, especially
a peterz didn't like the use of smp_call_function_single_async in the
blk-mq completion code very much.

Sebastian, would this solve your preempt-rt and lockdep issues?

second. I'm cooking something.

Sebastian