lkml.org 
[lkml]   [2020]   [Oct]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT
On Tue, Oct 27, 2020 at 06:05:15PM +0100, Thomas Gleixner wrote:
> > 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?


diff --git a/block/blk-mq.c b/block/blk-mq.c
index cdced4aca2e812..5c125fb11b5691 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -626,19 +626,7 @@ static void __blk_mq_complete_request_remote(void *data)
{
struct request *rq = data;

- /*
- * For most of single queue controllers, there is only one irq vector
- * for handling I/O completion, and the only irq's affinity is set
- * to all possible CPUs. On most of ARCHs, this affinity means the irq
- * is handled on one specific CPU.
- *
- * So complete I/O requests in softirq context in case of single queue
- * devices to avoid degrading I/O performance due to irqsoff latency.
- */
- if (rq->q->nr_hw_queues == 1)
- blk_mq_trigger_softirq(rq);
- else
- rq->q->mq_ops->complete(rq);
+ blk_mq_trigger_softirq(rq);
}

static inline bool blk_mq_complete_need_ipi(struct request *rq)
\
 
 \ /
  Last update: 2020-10-27 18:24    [W:0.174 / U:0.456 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site