linux-kernel - Re: [PATCH] io_uring: Use io

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Message-ID: <2023072438-aftermath-fracture-3dff@gregkh>
Date:   Mon, 24 Jul 2023 17:48:22 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Phil Elwell <phil@...pberrypi.com>
Cc:     axboe@...nel.dk, andres@...razel.de, asml.silence@...il.com,
        david@...morbit.com, hch@....de, io-uring@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, linux-xfs@...r.kernel.org,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] io_uring: Use io_schedule* in cqring wait

On Mon, Jul 24, 2023 at 04:35:43PM +0100, Phil Elwell wrote:
> Hi Andres,
> 
> With this commit applied to the 6.1 and later kernels (others not
> tested) the iowait time ("wa" field in top) in an ARM64 build running
> on a 4 core CPU (a Raspberry Pi 4 B) increases to 25%, as if one core
> is permanently blocked on I/O. The change can be observed after
> installing mariadb-server (no configuration or use is required). After
> reverting just this commit, "wa" drops to zero again.

This has been discussed already:
	https://lore.kernel.org/r/12251678.O9o76ZdvQC@natalenko.name

It's not a bug, mariadb does have pending I/O, so the report is correct,
but the CPU isn't blocked at all.

thanks,

greg k-h