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

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Message-ID: <CAPLW+4==a515TCD93Kp-8zC8iYyYdh92U=j_emnG5sT_d7z64w@mail.gmail.com>
Date: Fri, 22 Nov 2024 15:55:23 -0600
From: Sam Protsenko <semen.protsenko@...aro.org>
To: 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 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?

Thanks!