linux-kernel - Re: [PATCH 2/2] block: remove the ioprio field from struct request

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Message-ID: <da02f209-8524-4281-a9d3-1b524bd966da@acm.org>
Date: Fri, 22 Nov 2024 14:18:10 -0800
From: Bart Van Assche <bvanassche@....org>
To: Sam Protsenko <semen.protsenko@...aro.org>, Christoph Hellwig <hch@....de>
Cc: axboe@...nel.dk, linux-block@...r.kernel.org, linux-scsi@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] block: remove the ioprio field from struct request

On 11/22/24 1:55 PM, Sam Protsenko wrote:
> On Fri, Nov 22, 2024 at 6:04 AM Christoph Hellwig <hch@....de> wrote:
>>
>> On Thu, Nov 21, 2024 at 11:04:19PM -0600, Sam Protsenko wrote:
>>> Hi Christoph,
>>>
>>> This patch causes a regression on E850-96 board. Specifically, there are
>>> two noticeable time lags when booting Debian rootfs:
>>
>> What storage driver does this board use?  Anything else interesting
>> about the setup?
>>
> 
> It's an Exynos based board with eMMC, so it uses DW MMC driver, with
> Exynos glue layer on top of it, so:
> 
>      drivers/mmc/host/dw_mmc.c
>      drivers/mmc/host/dw_mmc-exynos.c
> 
> I'm using the regular ARM64 defconfig. Nothing fancy about this setup
> neither, the device tree with eMMC definition (mmc_0) is here:
> 
>      arch/arm64/boot/dts/exynos/exynos850-e850-96.dts
> 
> FWIW, I was able to narrow down the issue to dd_insert_request()
> function. With this hack the freeze is gone:
> 
> 8<-------------------------------------------------------------------->8
> diff --git a/block/mq-deadline.c b/block/mq-deadline.c
> index acdc28756d9d..83d272b66e71 100644
> --- a/block/mq-deadline.c
> +++ b/block/mq-deadline.c
> @@ -676,7 +676,7 @@ static void dd_insert_request(struct blk_mq_hw_ctx
> *hctx, struct request *rq,
>          struct request_queue *q = hctx->queue;
>          struct deadline_data *dd = q->elevator->elevator_data;
>          const enum dd_data_dir data_dir = rq_data_dir(rq);
> -       u16 ioprio = req_get_ioprio(rq);
> +       u16 ioprio = 0; /* the same as old req->ioprio */
>          u8 ioprio_class = IOPRIO_PRIO_CLASS(ioprio);
>          struct dd_per_prio *per_prio;
>          enum dd_prio prio;
> 8<-------------------------------------------------------------------->8
> 
> Does it tell you anything about where the possible issue can be?

It seems like eMMC devices do not tolerate I/O prioritization. How about
disabling I/O prioritization for eMMC setups? Is the ioprio cgroup
controller perhaps activated by the user space software that is running
on this setup?

Thanks,

Bart.