Re: Broadcom WiFi SDIO performance regression after commit "mmc: sdhci: Remove finish_tasklet"

[Adrian Hunter]

On 27/08/20 12:36 pm, Dmitry Osipenko wrote:
> 27.08.2020 09:45, Adrian Hunter пишет:
>> On 27/08/20 9:07 am, Dmitry Osipenko wrote:
>>> Hello!
>>>
>>> I was debugging WiFi performance problems on Acer A500 tablet device
>>> that has BCM4329 WiFi chip which is connected to NVIDIA Terga20 SoC via
>>> SDIO and found that the following commit causes a solid 5-10 Mbit/s of
>>> WiFi throughput regression after 5.2 kernel:
>>
>> What is that in percentage terms?
> 
> That is about 20%.
> 
>>> commit c07a48c2651965e84d35cf193dfc0e5f7892d612
>>> Author: Adrian Hunter <adrian.hunter@xxxxxxxxx>
>>> Date:   Fri Apr 5 15:40:20 2019 +0300
>>>
>>>     mmc: sdhci: Remove finish_tasklet
>>>
>>>     Remove finish_tasklet. Requests that require DMA-unmapping or
>>> sdhci_reset
>>>     are completed either in the IRQ thread or a workqueue if the
>>> completion is
>>>     not initiated by the IRQ.
>>>
>>> Reverting the offending commit on top of recent linux-next resolves the
>>> problem.
>>>
>>> Ulf / Adrian, do you have any ideas what could be done in regards to
>>> restoring the SDIO performance? Should we just revert the offending commit?
>>>
>>
>> Unfortunately I think we are past the point of returning to the tasklet.
>>
>> sdhci can complete requests in the irq handler but only if ->pre_req() and
>> ->post_req() are used, which is not supported by SDIO at present.  pre_req
>> and post_req were introduced to reduce latency for the block driver, so it
>> seems reasonable perhaps to look at using them in SDIO as well.
>>
> 
> I'll try to take a look at pre/post_req(), but I'm not very familiar
> with the MMC code, so it may take quite some time. Will be great if you
> could help with making a patch that I could test!
> 

You could start by seeing if using pre/post_req helps at all, as below.
If that doesn't help, then it might need more analysis.


diff --git a/drivers/mmc/core/sdio_ops.c b/drivers/mmc/core/sdio_ops.c
index 93d346c01110..4c229dd2b6e5 100644
--- a/drivers/mmc/core/sdio_ops.c
+++ b/drivers/mmc/core/sdio_ops.c
@@ -121,6 +121,7 @@ int mmc_io_rw_extended(struct mmc_card *card, int write, unsigned fn,
 	struct sg_table sgtable;
 	unsigned int nents, left_size, i;
 	unsigned int seg_size = card->host->max_seg_size;
+	int err;
 
 	WARN_ON(blksz == 0);
 
@@ -170,28 +171,32 @@ int mmc_io_rw_extended(struct mmc_card *card, int write, unsigned fn,
 
 	mmc_set_data_timeout(&data, card);
 
-	mmc_wait_for_req(card->host, &mrq);
+	mmc_pre_req(card->host, &mrq);
 
-	if (nents > 1)
-		sg_free_table(&sgtable);
+	mmc_wait_for_req(card->host, &mrq);
 
 	if (cmd.error)
-		return cmd.error;
-	if (data.error)
-		return data.error;
-
-	if (mmc_host_is_spi(card->host)) {
+		err = cmd.error;
+	else if (data.error)
+		err = data.error;
+	else if (mmc_host_is_spi(card->host))
 		/* host driver already reported errors */
-	} else {
-		if (cmd.resp[0] & R5_ERROR)
-			return -EIO;
-		if (cmd.resp[0] & R5_FUNCTION_NUMBER)
-			return -EINVAL;
-		if (cmd.resp[0] & R5_OUT_OF_RANGE)
-			return -ERANGE;
-	}
+		err = 0;
+	else if (cmd.resp[0] & R5_ERROR)
+		err = -EIO;
+	else if (cmd.resp[0] & R5_FUNCTION_NUMBER)
+		err = -EINVAL;
+	else if (cmd.resp[0] & R5_OUT_OF_RANGE)
+		err = -ERANGE;
+	else
+		err = 0;
 
-	return 0;
+	mmc_post_req(card->host, &mrq, err);
+
+	if (nents > 1)
+		sg_free_table(&sgtable);
+
+	return err;
 }
 
 int sdio_reset(struct mmc_host *host)