From: Jens Axboe <axboe@kernel.dk>
To: Peter Zijlstra <peterz@infradead.org>,
Christoph Hellwig <hch@infradead.org>
Cc: frederic@kernel.org, linux-kernel@vger.kernel.org,
linux-block@vger.kernel.org
Subject: Re: blk-softirq vs smp_call_function_single_async()
Date: Mon, 8 Jun 2020 15:34:25 -0600 [thread overview]
Message-ID: <a77692f6-0c11-9277-6fe7-58c4fefda5fb@kernel.dk> (raw)
In-Reply-To: <20200608164009.GD2531@hirez.programming.kicks-ass.net>
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
prev parent reply other threads:[~2020-06-08 21:34 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-08 11:58 blk-softirq vs smp_call_function_single_async() Peter Zijlstra
2020-06-08 15:45 ` Christoph Hellwig
2020-06-08 15:58 ` Peter Zijlstra
2020-06-08 16:33 ` Christoph Hellwig
2020-06-08 16:40 ` Peter Zijlstra
2020-06-08 16:42 ` Christoph Hellwig
2020-06-09 13:38 ` Peter Zijlstra
2020-06-08 21:34 ` Jens Axboe [this message]
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