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From:   Oleksandr Natalenko <oleksandr@natalenko.name>
To:     Jens Axboe <axboe@kernel.dk>, stable@vger.kernel.org,
        Genes Lists <lists@sapience.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andres Freund <andres@anarazel.de>
Subject: Re: [PATCH 6.4 800/800] io_uring: Use io_schedule* in cqring wait
Date:   Sun, 23 Jul 2023 20:06:19 +0200
Message-ID: <2691683.mvXUDI8C0e@natalenko.name>
In-Reply-To: <70e5349a-87af-a2ea-f871-95270f57c6e3@sapience.com>
References: <20230716194949.099592437@linuxfoundation.org>
 <538065ee-4130-6a00-dcc8-f69fbc7d7ba0@kernel.dk>
 <70e5349a-87af-a2ea-f871-95270f57c6e3@sapience.com>
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From: Oleksandr Natalenko <oleksandr@natalenko.name>
Date: Sun, 23 Jul 2023 20:06:19 +0200
Message-ID: <2691683.mvXUDI8C0e@natalenko.name>
In-Reply-To: <70e5349a-87af-a2ea-f871-95270f57c6e3@sapience.com>
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Hello.

On ned=C4=9Ble 23. =C4=8Dervence 2023 19:43:50 CEST Genes Lists wrote:
> On 7/23/23 11:31, Jens Axboe wrote:
> ...
> > Just read the first one, but this is very much expected. It's now just
> > correctly reflecting that one thread is waiting on IO. IO wait being
> > 100% doesn't mean that one core is running 100% of the time, it just
> > means it's WAITING on IO 100% of the time.
> >=20
>=20
> Seems reasonable thank you.
>=20
> Question - do you expect the iowait to stay high for a freshly created=20
> mariadb doing nothing (as far as I can tell anyway) until process=20
> exited? Or Would you think it would drop in this case prior to the=20
> process exiting.
>=20
> For example I tried the following - is the output what you expect?
>=20
> Create a fresh mariab with no databases - monitor the core showing the=20
> iowaits with:
>=20
>     mpstat -P ALL 2 100
>=20
> # rm -f /var/lib/mysql/*
> # mariadb-install-db --user=3Dmysql --basedir=3D/usr --datadir=3D/var/lib=
/mysql
>=20
> # systemctl start mariadb      (iowaits -> 100%)=20
> =20
>=20
> # iotop -bo |grep maria        (shows no output, iowait stays 100%)
>=20
> (this persists until mariadb process exits)
> =20
>=20
> # systemctl stop mariadb       (iowait drops to 0%)=20

This is a visible userspace behaviour change with no changes in the userspa=
ce itself, so we cannot just ignore it. If for some reason this is how it s=
hould be now, how do we explain it to MariaDB devs to get this fixed?

Thanks.

=2D-=20
Oleksandr Natalenko (post-factum)
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