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Wed, 19 Jun 2019 19:40:10 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-116-121.ams2.redhat.com [10.36.116.121]) by smtp.corp.redhat.com (Postfix) with ESMTPS id C1A9E19C71; Wed, 19 Jun 2019 19:40:08 +0000 (UTC) To: Vladimir Sementsov-Ogievskiy References: <20190612220839.1374-1-mreitz@redhat.com> <20190612220839.1374-2-mreitz@redhat.com> <57ae7f82-ae02-a382-74f6-cb96672b2058@virtuozzo.com> <00cce102-a1f1-9205-6eb7-1e48d8991b00@redhat.com> <32adc55c-c71d-59bf-963b-48d2ae5b3e30@virtuozzo.com> From: Max Reitz Openpgp: preference=signencrypt Autocrypt: addr=mreitz@redhat.com; prefer-encrypt=mutual; keydata= mQENBFXOJlcBCADEyyhOTsoa/2ujoTRAJj4MKA21dkxxELVj3cuILpLTmtachWj7QW+TVG8U /PsMCFbpwsQR7oEy8eHHZwuGQsNpEtNC2G/L8Yka0BIBzv7dEgrPzIu+W3anZXQW4702+uES U29G8TP/NGfXRRHGlbBIH9KNUnOSUD2vRtpOLXkWsV5CN6vQFYgQfFvmp5ZpPeUe6xNplu8V mcTw8OSEDW/ZnxJc8TekCKZSpdzYoxfzjm7xGmZqB18VFwgJZlIibt1HE0EB4w5GsD7x5ekh awIe3RwoZgZDLQMdOitJ1tUc8aqaxvgA4tz6J6st8D8pS//m1gAoYJWGwwIVj1DjTYLtABEB AAG0HU1heCBSZWl0eiA8bXJlaXR6QHJlZGhhdC5jb20+iQFTBBMBCAA9AhsDBQkSzAMABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheABQJVzie5FRhoa3A6Ly9rZXlzLmdudXBnLm5ldAAKCRD0 B9sAYdXPQDcIB/9uNkbYEex1rHKz3mr12uxYMwLOOFY9fstP5aoVJQ1nWQVB6m2cfKGdcRe1 2/nFaHSNAzT0NnKz2MjhZVmcrpyd2Gp2QyISCfb1FbT82GMtXFj1wiHmPb3CixYmWGQUUh+I AvUqsevLA+WihgBUyaJq/vuDVM1/K9Un+w+Tz5vpeMidlIsTYhcsMhn0L9wlCjoucljvbDy/ 8C9L2DUdgi3XTa0ORKeflUhdL4gucWoAMrKX2nmPjBMKLgU7WLBc8AtV+84b9OWFML6NEyo4 4cP7cM/07VlJK53pqNg5cHtnWwjHcbpGkQvx6RUx6F1My3y52vM24rNUA3+ligVEgPYBuQEN BFXOJlcBCADAmcVUNTWT6yLWQHvxZ0o47KCP8OcLqD+67T0RCe6d0LP8GsWtrJdeDIQk+T+F xO7DolQPS6iQ6Ak2/lJaPX8L0BkEAiMuLCKFU6Bn3lFOkrQeKp3u05wCSV1iKnhg0UPji9V2 W5eNfy8F4ZQHpeGUGy+liGXlxqkeRVhLyevUqfU0WgNqAJpfhHSGpBgihUupmyUg7lfUPeRM DzAN1pIqoFuxnN+BRHdAecpsLcbR8sQddXmDg9BpSKozO/JyBmaS1RlquI8HERQoe6EynJhd 64aICHDfj61rp+/0jTIcevxIIAzW70IadoS/y3DVIkuhncgDBvGbF3aBtjrJVP+5ABEBAAGJ ASUEGAEIAA8FAlXOJlcCGwwFCRLMAwAACgkQ9AfbAGHVz0CbFwf9F/PXxQR9i4N0iipISYjU sxVdjJOM2TMut+ZZcQ6NSMvhZ0ogQxJ+iEQ5OjnIputKvPVd5U7WRh+4lF1lB/NQGrGZQ1ic alkj6ocscQyFwfib+xIe9w8TG1CVGkII7+TbS5pXHRxZH1niaRpoi/hYtgzkuOPp35jJyqT/ /ELbqQTDAWcqtJhzxKLE/ugcOMK520dJDeb6x2xVES+S5LXby0D4juZlvUj+1fwZu+7Io5+B bkhSVPb/QdOVTpnz7zWNyNw+OONo1aBUKkhq2UIByYXgORPFnbfMY7QWHcjpBVw9MgC4tGeF R4bv+1nAMMxKmb5VvQCExr0eFhJUAHAhVg== Message-ID: <309c0932-a9b4-40a6-8db2-7940171f52e5@redhat.com> Date: Wed, 19 Jun 2019 21:40:07 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <32adc55c-c71d-59bf-963b-48d2ae5b3e30@virtuozzo.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="BYIGlNaa67BThJz2eQtvTxpzr05cFRI9S" X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Wed, 19 Jun 2019 19:40:10 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH 1/2] vl: Drain before (block) job cancel when quitting X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Paolo Bonzini , Denis Plotnikov , "qemu-devel@nongnu.org" , Qemu-block Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --BYIGlNaa67BThJz2eQtvTxpzr05cFRI9S Content-Type: multipart/mixed; boundary="TcI4NUSvh7jQrYDOZx4OOsXlUGO5JLKIZ"; protected-headers="v1" From: Max Reitz To: Vladimir Sementsov-Ogievskiy Cc: Qemu-block , "qemu-devel@nongnu.org" , Kevin Wolf , Paolo Bonzini , Denis Plotnikov Message-ID: <309c0932-a9b4-40a6-8db2-7940171f52e5@redhat.com> Subject: Re: [Qemu-devel] [PATCH 1/2] vl: Drain before (block) job cancel when quitting References: <20190612220839.1374-1-mreitz@redhat.com> <20190612220839.1374-2-mreitz@redhat.com> <57ae7f82-ae02-a382-74f6-cb96672b2058@virtuozzo.com> <00cce102-a1f1-9205-6eb7-1e48d8991b00@redhat.com> <32adc55c-c71d-59bf-963b-48d2ae5b3e30@virtuozzo.com> In-Reply-To: <32adc55c-c71d-59bf-963b-48d2ae5b3e30@virtuozzo.com> --TcI4NUSvh7jQrYDOZx4OOsXlUGO5JLKIZ Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable On 14.06.19 11:22, Vladimir Sementsov-Ogievskiy wrote: > 13.06.2019 19:03, Max Reitz wrote: >> [re-adding the original CCs, why not] >> >> On 13.06.19 16:30, Vladimir Sementsov-Ogievskiy wrote: >>> 13.06.2019 17:21, Max Reitz wrote: >>>> On 13.06.19 16:19, Vladimir Sementsov-Ogievskiy wrote: >>>>> 13.06.2019 1:08, Max Reitz wrote: >>>>>> If the main loop cancels all block jobs while the block layer is n= ot >>>>>> drained, this cancelling may not happen instantaneously. We can s= tart a >>>>>> drained section before vm_shutdown(), which entails another >>>>>> bdrv_drain_all(); this nested bdrv_drain_all() will thus be a no-o= p, >>>>>> basically. >>>>>> >>>>>> We do not have to end the drained section, because we actually do = not >>>>>> want any requests to happen from this point on. >>>>>> >>>>>> Signed-off-by: Max Reitz >>>>>> --- >>>>>> I don't know whether it actually makes sense to never end this dra= ined >>>>>> section. It makes sense to me. Please correct me if I'm wrong. >>>>>> --- >>>>>> vl.c | 11 +++++++++++ >>>>>> 1 file changed, 11 insertions(+) >>>>>> >>>>>> diff --git a/vl.c b/vl.c >>>>>> index cd1fbc4cdc..3f8b3f74f5 100644 >>>>>> --- a/vl.c >>>>>> +++ b/vl.c >>>>>> @@ -4538,6 +4538,17 @@ int main(int argc, char **argv, char **envp= ) >>>>>> */ >>>>>> migration_shutdown(); >>>>>> =20 >>>>>> + /* >>>>>> + * We must cancel all block jobs while the block layer is dra= ined, >>>>>> + * or cancelling will be affected by throttling and thus may = block >>>>>> + * for an extended period of time. >>>>>> + * vm_shutdown() will bdrv_drain_all(), so we may as well inc= lude >>>>>> + * it in the drained section. >>>>>> + * We do not need to end this section, because we do not want= any >>>>>> + * requests happening from here on anyway. >>>>>> + */ >>>>>> + bdrv_drain_all_begin(); >>>>>> + >>>>>> /* No more vcpu or device emulation activity beyond this p= oint */ >>>>>> vm_shutdown(); >>>>>> =20 >>>>>> >>>>> >>>>> So, actually, the problem is that we may wait for job requests twic= e: >>>>> on drain and then on cancel. >>>> >>>> We don=E2=80=99t wait on drain. When the throttle node is drained, = it will >>>> ignore throttling (as noted in the cover letter). >>>> >>>> We do wait when cancelling a job while the throttle node isn=E2=80=99= t drained, >>>> though. That=E2=80=99s the problem. >>> >>> Ah, understand now. >>> >>> Is it safe to drain_begin before stopping cpus? We may finish up then= with some queued >>> somewhere IO requests.. >> >> Hm... Aren=E2=80=99t guest devices prohibited from issuing requests t= o the >> block layer while their respective block device is drained? >=20 > It's at least a buggy place, I remember Denis Plotnikov sent patch to f= ix it and had a huge > discussion with Kevin. > And here it is: > https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg00732.html Ah, I even have that in my inbox... The latest reply I see came in April= : https://lists.nongnu.org/archive/html/qemu-block/2019-04/msg00243.html Where Kevin asked for an RFC patch in the current state. I=E2=80=99m not sure whether I should work around a potential bug here, i= f we can agree that it is a bug, and if it isn=E2=80=99t clear whether this pl= ace would actually be affected. Max --TcI4NUSvh7jQrYDOZx4OOsXlUGO5JLKIZ-- --BYIGlNaa67BThJz2eQtvTxpzr05cFRI9S Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl0Kj5cACgkQ9AfbAGHV z0DZEQf9HtoGDkyrHdM5Ql6zcLNzV58uxCPtUKv4dfKQ+k4fxO8E4NAX8pBtFhu+ rMK59gkcYNLgUKw7SisdEdjig1TePCndljv1GQiOUwwZ5/jwtvVuBCWVCpqaAA0m MxD0Byp8HXCM7JrDozdO6VzC6zI9J3utK+cI7zBen82o4fKqa5q62UDn2K2jNuQB p5C0RiSwG+KJIXgt+YYd86KK6yuRMIVoNKoaZS5LGwaOW26Oe8Ho0j635R26eMo8 wSyvo+2zAEGbDRFV1BgSNmoJGyYk4OqJxjcP/g3uR7mLhgvHmDC2H/SXZG+srTd0 EeCFfB2mq0gLli5EGHTNvWjDOTAUOQ== =+n3J -----END PGP SIGNATURE----- --BYIGlNaa67BThJz2eQtvTxpzr05cFRI9S--