linux-kernel - Re: [PATCH v2 1/8] mmc: sdhci: Get rid of finish

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Date:   Fri, 8 Mar 2019 15:36:02 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Faiz Abbas <faiz_abbas@...com>, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-omap@...r.kernel.org
Cc:     ulf.hansson@...aro.org, robh+dt@...nel.org, mark.rutland@....com,
        kishon@...com, zhang.chunyan@...aro.org
Subject: Re: [PATCH v2 1/8] mmc: sdhci: Get rid of finish_tasklet

On 6/03/19 12:00 PM, Faiz Abbas wrote:
> Adrian,
> 
> On 25/02/19 1:47 PM, Adrian Hunter wrote:
>> On 15/02/19 9:20 PM, Faiz Abbas wrote:
>>> sdhci.c has two bottom halves implemented. A threaded_irq for handling
>>> card insert/remove operations and a tasklet for finishing mmc requests.
>>> With the addition of external dma support, dmaengine APIs need to
>>> terminate in non-atomic context before unmapping the dma buffers.
>>>
>>> To facilitate this, remove the finish_tasklet and move the call of
>>> sdhci_request_done() to the threaded_irq() callback.
>>
>> The irq thread has a higher latency than the tasklet.  The performance drop
>> is measurable on the system I tried:
>>
>> Before:
>>
>> # dd if=/dev/mmcblk1 of=/dev/null bs=1G count=1 &
>> 1+0 records in
>> 1+0 records out
>> 1073741824 bytes (1.1 GB) copied, 4.44502 s, 242 MB/s
>>
>> After:
>>
>> # dd if=/dev/mmcblk1 of=/dev/null bs=1G count=1 &
>> 1+0 records in
>> 1+0 records out
>> 1073741824 bytes (1.1 GB) copied, 4.50898 s, 238 MB/s
>>
>> So we only want to resort to the thread for the error case.
>>
> 
> Sorry for the late response here, but this is about 1.6% decrease. I
> tried out the same commands on a dra7xx board here (with about 5
> consecutive dd of 1GB) and the average decrease was 0.3%. I believe you
> will also find a lesser percentage change if you average over multiple
> dd commands.
> 
> Is this really so significant that we have to maintain two different
> bottom halves and keep having difficulty with adding APIs that can sleep?

It is a performance drop that can be avoided, so it might as well be.
Splitting the success path from the failure path is common for I/O drivers
for similar reasons as here: the success path can be optimized whereas the
failure path potentially needs to sleep.

> 
> Also I am not sure how to implement only the error handling part in the
> threaded_irq. We need to enter sdhci_request_done() and get the current
> mrq before we can check for error conditions like I've done in patch 2:
> 
> /* Terminate and synchronize dma in case of an error */
> if (data && (mrq->cmd->error || data->error) &&
>     host->use_external_dma) {
> 	struct dma_chan *chan = sdhci_external_dma_channel(host, data);
> 	dmaengine_terminate_sync(chan);
> }
> 
> On a related note, do we really need to protect everything in
> sdhci_request_done() with spinlocks?
>                                      In patch 2 I have only removed lock
> for the terminate_sync() parts that I added but the whole
> dma_unmap/dma_sync parts should be left unprotected IMO.

As it is written, synchronization is needed to stop the same mrq being
finished twice.

I suggest doing the dmaengine_terminate_sync() before calling
sdhci_request_done().  Perhaps you could record the channel that needs to be
sync'd and then do:

	struct dma_chan *chan = READ_ONCE(host->sync_chan);

	if (chan) {
		dmaengine_terminate_sync(chan);
		host->sync_chan = NULL;
	}