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Thu, 13 Jun 2019 16:03:07 +0000 (UTC) Received: from dresden.str.redhat.com (unknown [10.40.205.187]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 2A6C360A9A; Thu, 13 Jun 2019 16:03:05 +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> 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: <00cce102-a1f1-9205-6eb7-1e48d8991b00@redhat.com> Date: Thu, 13 Jun 2019 18:03:04 +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: <57ae7f82-ae02-a382-74f6-cb96672b2058@virtuozzo.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="M3wl1NTteJ8gK53oiyYBvZsIyRQjRRD9b" X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Thu, 13 Jun 2019 16:03:07 +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 , "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) --M3wl1NTteJ8gK53oiyYBvZsIyRQjRRD9b Content-Type: multipart/mixed; boundary="017Fbqrn0PEJzCqCYB50BGb6u5dJ1R9nv"; protected-headers="v1" From: Max Reitz To: Vladimir Sementsov-Ogievskiy Cc: Qemu-block , "qemu-devel@nongnu.org" , Kevin Wolf , Paolo Bonzini Message-ID: <00cce102-a1f1-9205-6eb7-1e48d8991b00@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> In-Reply-To: <57ae7f82-ae02-a382-74f6-cb96672b2058@virtuozzo.com> --017Fbqrn0PEJzCqCYB50BGb6u5dJ1R9nv Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable [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 not= >>>> drained, this cancelling may not happen instantaneously. We can sta= rt a >>>> drained section before vm_shutdown(), which entails another >>>> bdrv_drain_all(); this nested bdrv_drain_all() will thus be a no-op,= >>>> basically. >>>> >>>> We do not have to end the drained section, because we actually do no= t >>>> 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 drain= ed >>>> 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 drain= ed, >>>> + * or cancelling will be affected by throttling and thus may bl= ock >>>> + * for an extended period of time. >>>> + * vm_shutdown() will bdrv_drain_all(), so we may as well inclu= de >>>> + * it in the drained section. >>>> + * We do not need to end this section, because we do not want a= ny >>>> + * requests happening from here on anyway. >>>> + */ >>>> + bdrv_drain_all_begin(); >>>> + >>>> /* No more vcpu or device emulation activity beyond this poin= t */ >>>> vm_shutdown(); >>>> =20 >>>> >>> >>> So, actually, the problem is that we may wait for job requests twice:= >>> 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=99t= drained, >> though. That=E2=80=99s the problem. >=20 > Ah, understand now. >=20 > Is it safe to drain_begin before stopping cpus? We may finish up then w= ith some queued > somewhere IO requests.. Hm... Aren=E2=80=99t guest devices prohibited from issuing requests to t= he block layer while their respective block device is drained? Otherwise, I suppose I=E2=80=99ll have to move the bdrv_drain_all_begin()= below the vm_shutdown(). That wouldn=E2=80=99t be too big of a problem. Max --017Fbqrn0PEJzCqCYB50BGb6u5dJ1R9nv-- --M3wl1NTteJ8gK53oiyYBvZsIyRQjRRD9b Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQEzBAEBCAAdFiEEkb62CjDbPohX0Rgp9AfbAGHVz0AFAl0Cc7gACgkQ9AfbAGHV z0A5uQf8DX6F12p8sJQ7GftT8/WVVqz0u8csoUw+O7Ralgetd7rFAMMscHGSvWLC VHV6jxXAApnOpKsNMY2IVdwQjrXuVe5k/873Dm/a4aW8wxft29AjgKshdc0ro5dT QfMyLTd8dGTj9+lWuu4GjfctsEZudpl+rxyhl7ig/5IPq3KbEOtFSWdnoEFr+XCX y6RzarcIFckZNjypY6/6S9aUDwKMRQxqI8yyG9V34P4B/BU8z/RCviAgn2FybcHt PeFuu2GUOJd0X8l+5VAlLSd913CYaIjPRcJR7bD04Gj6D2P2E49uEqf/YnhIGrxR 280B6JX9OcOBZF+DQJN2bCBqnIuRlA== =0IQZ -----END PGP SIGNATURE----- --M3wl1NTteJ8gK53oiyYBvZsIyRQjRRD9b--