linux-kernel - Re: blk-softirq vs smp_call_function_single

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Date:   Mon, 8 Jun 2020 15:34:25 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     frederic@...nel.org, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org
Subject: Re: blk-softirq vs smp_call_function_single_async()

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