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From:   Phil Elwell <phil@raspberrypi.com>
Date:   Mon, 24 Jul 2023 16:35:43 +0100
Message-ID: <CAMEGJJ2RxopfNQ7GNLhr7X9=bHXKo+G5OOe0LUq=+UgLXsv1Xg@mail.gmail.com>
Subject: Re: [PATCH] io_uring: Use io_schedule* in cqring wait
To:     axboe@kernel.dk
Cc:     andres@anarazel.de, asml.silence@gmail.com, david@fromorbit.com,
        hch@lst.de, io-uring@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, linux-xfs@vger.kernel.org,
        stable <stable@vger.kernel.org>
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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.

I can believe that this change hasn't negatively affected performance,
but the result is misleading. I also think it's pushing the boundaries
of what a back-port to stable should do.

Phil