From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C3F9FC7618F for ; Tue, 16 Jul 2019 16:38:17 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9A2C220449 for ; Tue, 16 Jul 2019 16:38:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9A2C220449 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:51148 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hnQT1-0007RZ-0j for qemu-devel@archiver.kernel.org; Tue, 16 Jul 2019 12:38:15 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:55343) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hnQSn-0006nS-2N for qemu-devel@nongnu.org; Tue, 16 Jul 2019 12:38:02 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hnQSl-0007A4-TW for qemu-devel@nongnu.org; Tue, 16 Jul 2019 12:38:00 -0400 Received: from mx1.redhat.com ([209.132.183.28]:38976) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hnQSe-00070X-Oc; Tue, 16 Jul 2019 12:37:54 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 25955308330C; Tue, 16 Jul 2019 16:37:51 +0000 (UTC) Received: from linux.fritz.box (ovpn-117-24.ams2.redhat.com [10.36.117.24]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CFADB1001B00; Tue, 16 Jul 2019 16:37:47 +0000 (UTC) Date: Tue, 16 Jul 2019 18:37:46 +0200 From: Kevin Wolf To: Max Reitz Message-ID: <20190716163746.GH7297@linux.fritz.box> References: <20190619152603.5937-1-mreitz@redhat.com> <20190716144016.GE7297@linux.fritz.box> <82ce3044-e228-2453-5da2-9f86e4627066@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="7ZAtKRhVyVSsbBD2" Content-Disposition: inline In-Reply-To: <82ce3044-e228-2453-5da2-9f86e4627066@redhat.com> User-Agent: Mutt/1.11.3 (2019-02-01) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Tue, 16 Jul 2019 16:37:51 +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 v2 0/9] block: Delay poll when ending drained sections 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: Stefan Hajnoczi , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --7ZAtKRhVyVSsbBD2 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 16.07.2019 um 18:24 hat Max Reitz geschrieben: > On 16.07.19 16:40, Kevin Wolf wrote: > > Am 19.06.2019 um 17:25 hat Max Reitz geschrieben: > >> Hi, > >> > >> This is v2 to =E2=80=9Cblock: Keep track of parent quiescing=E2=80=9D. > >> > >> Please read this cover letter, because I=E2=80=99m very unsure about t= he design > >> in this series and I=E2=80=99d appreciate some comments. > >> > >> As Kevin wrote in his reply to that series, the actual problem is that > >> bdrv_drain_invoke() polls on every node whenever ending a drain. This > >> may cause graph changes, which is actually forbidden. > >> > >> To solve that problem, this series makes the drain code construct a li= st > >> of undrain operations that have been initiated, and then polls all of > >> them on the root level once graph changes are acceptable. > >> > >> Note that I don=E2=80=99t like this list concept very much, so I=E2=80= =99m open to > >> alternatives. > >=20 > > So drain_end is different from drain_begin in that it wants to wait only > > for all bdrv_drain_invoke() calls to complete, but not for other > > requests that are in flight. Makes sense. > >=20 > > Though instead of managing a whole list, wouldn't a counter suffice? > >=20 > >> Furthermore, all BdrvChildRoles with BDS parents have a broken > >> .drained_end() implementation. The documentation clearly states that > >> this function is not allowed to poll, but it does. So this series > >> changes it to a variant (using the new code) that does not poll. > >> > >> There is a catch, which may actually be a problem, I don=E2=80=99t kno= w: The new > >> variant of that .drained_end() does not poll at all, never. As > >> described above, now every bdrv_drain_invoke() returns an object that > >> describes when it will be done and which can thus be polled for. These > >> objects are just discarded when using BdrvChildRole.drained_end(). Th= at > >> does not feel quite right. It would probably be more correct to let > >> BdrvChildRole.drained_end() return these objects so the top level > >> bdrv_drained_end() can poll for their completion. > >> > >> I decided not to do this, for two reasons: > >> (1) Doing so would spill the =E2=80=9Clist of objects to poll for=E2= =80=9D design to > >> places outside of block/io.c. I don=E2=80=99t like the design ver= y much as > >> it is, but I can live with it as long as it=E2=80=99s constrained = to the > >> core drain code in block/io.c. > >> This is made worse by the fact that currently, those objects are of > >> type BdrvCoDrainData. But it shouldn=E2=80=99t be a problem to ad= d a new > >> type that is externally visible (we only need the AioContext and > >> whether bdrv_drain_invoke_entry() is done). > >> > >> (2) It seems to work as it is. > >> > >> The alternative would be to add the same GSList ** parameter to > >> BdrvChildRole.drained_end() that I added in the core drain code in pat= ch > >> 2, and then let the .drained_end() implementation fill that with objec= ts > >> to poll for. (Which would be accomplished by adding a frontend to > >> bdrv_do_drained_end() that lets bdrv_child_cb_drained_poll() pass the > >> parameter through.) > >> > >> Opinions? > >=20 > > I think I would add an int* to BdrvChildRole.drained_end() so that we > > can just increase the counter whereever we need to. >=20 > So you mean just polling the @bs for which a caller gave poll=3Dtrue until > the counter reaches 0? I=E2=80=99ll try, sounds good (if I can get it to= work). Yes, that's what I have in mind. We expect graph changes to happen during the polling, but I think the caller is responsible for making sure that the top-level @bs stays around, so we don't need to be extra careful here. Also, bdrv_drain_invoke() is always called in the same AioContext as the top-level drain operation, so the whole aio_context_acquire/release stuff from this series should become unnecessary, and we don't need atomics to access the counter either. So I think this should really simplify the series a lot. Kevin --7ZAtKRhVyVSsbBD2 Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIcBAEBAgAGBQJdLf1aAAoJEH8JsnLIjy/WMWAP/jIJmeXmZ9FytP79Hf1ONlRg 7SOP+fDUc/Y9q5Le4JaQpIov/EYpCLvyQRdRMU5/qfW8dfbQt4e4Gmd1VanHTgDI TBOlahc1eezy7f+VopxNWrYk6jjHf0SU2uGkafM04rLhhIRZuvZrDs8gZRUKwE97 XRKjzlmfTxwxWDyqszTcQ4ld39SQJ7O0MZ8CaSxRGmd4c1Px1BGhK0i49Bmazaqr BtMySzEqVDZixWQWAYZmwz/9rLNbonWIhHSse/RmgvuqYwRHmv3ywLT0ZPYI3DV0 aQMZz+Fi7toIy6i5MTSB5KpwxRqqU3vBgYCh3nsFq4R+DnNr0V17p58cMehwCuZ4 kHjkypRplHBwkNGrS3kWnerpa8pjI0KtCjDHr4lVTDzjX2h6rLzh4zRsEzHIqrid IvooBp7Ros4ryQyUF0FXn607nFkc2ku3Qz8BpDfUXHAeuLTfnKLCbL/DKBs86s1j 5YVzKOLc+anSUsJG3jLsmXEmHGnihXOgvdVJnFy4tdqoSjX637AazYgkMM0Pm2up Sqo5iVqg4RcEy0iXmdcenZryCkjpVAvKl6DFyh/EYcScTX5P607iJmBS1DV3tIua WtWt74HzAN6KGQYyeZzGFXsGB671edV51xZ7sWHZT6/DSUCYHlD+u5hFiHSt4Da9 slHhGoXsjqUCwZo2XCPE =OdpM -----END PGP SIGNATURE----- --7ZAtKRhVyVSsbBD2--