lkml.org 
[lkml]   [2020]   [Jun]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: blk-softirq vs smp_call_function_single_async()
From
Date
On 6/8/20 10:40 AM, Peter Zijlstra wrote:
> On Mon, Jun 08, 2020 at 09:33:42AM -0700, Christoph Hellwig wrote:
>> On Mon, Jun 08, 2020 at 05:58:33PM +0200, Peter Zijlstra wrote:
>>>> A request can only be completed once.
>>>
>>> Sure, but that doesn't help.
>>>
>>> CPU0 CPU1
>>>
>>> raise_blk_irq() BLOCK_SOFTIRQ
>>> IPI -> CPU1
>>>
>>> // picks up thing from CPU0
>>> req->complete(req);
>>>
>>>
>>> <big hole where CSD is active and request completed>
>>>
>>> <IPI>
>>> trigger_softirq()
>>>
>>>
>>> What happens to a struct request after completion, is it free()d
>>> or reused? If reused, how do we guarantee CSD completion before
>>> free()ing?
>>
>> The request is freed in the block layer sense by __blk_mq_free_request
>> (which doesn't actually free the memory, so it eventually gets reused).
>>
>> __blk_mq_free_request is called from blk_mq_end_request which
>> is called from the drivers ->complete_rq method, which is called
>> from the block softirq.
>>
>> What is the method to guaranteed CSD completion?
>
> There isn't one, it was meant to be used with static allocations.
>
> Frederic proposed changing all these to irq_work, and I think I'll go do
> that. First dinner though.

That sounds good to me, and will also shrink struct request a bit,
which is always nice.

--
Jens Axboe

\
 
 \ /
  Last update: 2020-06-08 23:35    [W:0.050 / U:1.568 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site