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

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Message-ID: <140065e3-0368-0b5d-8a0d-afe49b741ad2@kernel.dk>
Date:   Mon, 24 Jul 2023 09:50:24 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Phil Elwell <phil@...pberrypi.com>
Cc:     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 7/24/23 9:48?AM, Greg KH wrote:
> 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.

Indeed - only thing I can think of is perhaps mariadb is having a
separate thread waiting on the ring in perpetuity, regardless of whether
or not it currently has IO.

But yes, this is very much ado about nothing...

-- 
Jens Axboe