From nobody Mon Nov 25 02:49:48 2024 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1717589383785488.77089620486447; Wed, 5 Jun 2024 05:09:43 -0700 (PDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sEpS9-0004sp-P2; Wed, 05 Jun 2024 08:09:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sEpS3-0004sN-Tw for qemu-devel@nongnu.org; Wed, 05 Jun 2024 08:09:11 -0400 Received: from proxmox-new.maurer-it.com ([94.136.29.106]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1sEpS1-00066P-N5 for qemu-devel@nongnu.org; Wed, 05 Jun 2024 08:09:11 -0400 Received: from proxmox-new.maurer-it.com (localhost.localdomain [127.0.0.1]) by proxmox-new.maurer-it.com (Proxmox) with ESMTP id C58C144D7D; Wed, 5 Jun 2024 14:08:58 +0200 (CEST) From: Fiona Ebner To: qemu-devel@nongnu.org Cc: peterx@redhat.com, farosas@suse.de, stefanha@redhat.com, pbonzini@redhat.com Subject: [RFC PATCH] migration/savevm: do not schedule snapshot_save_job_bh in qemu_aio_context Date: Wed, 5 Jun 2024 14:08:48 +0200 Message-Id: <20240605120848.358654-1-f.ebner@proxmox.com> X-Mailer: git-send-email 2.39.2 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=94.136.29.106; envelope-from=f.ebner@proxmox.com; helo=proxmox-new.maurer-it.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZM-MESSAGEID: 1717589386554100007 Content-Type: text/plain; charset="utf-8" The fact that the snapshot_save_job_bh() is scheduled in the main loop's qemu_aio_context AioContext means that it might get executed during a vCPU thread's aio_poll(). But saving of the VM state cannot happen while the guest or devices are active and can lead to assertion failures. See issue #2111 for two examples. Avoid the problem by scheduling the snapshot_save_job_bh() in the iohandler AioContext, which is not polled by vCPU threads. Solves Issue #2111. This change also solves the following issue: Since commit effd60c878 ("monitor: only run coroutine commands in qemu_aio_context"), the 'snapshot-save' QMP call would not respond right after starting the job anymore, but only after the job finished, which can take a long time. The reason is, because after commit effd60c878, do_qmp_dispatch_bh() runs in the iohandler AioContext. When do_qmp_dispatch_bh() wakes the qmp_dispatch() coroutine, the coroutine cannot be entered immediately anymore, but needs to be scheduled to the main loop's qemu_aio_context AioContext. But snapshot_save_job_bh() was scheduled first to the same AioContext and thus gets executed first. Buglink: https://gitlab.com/qemu-project/qemu/-/issues/2111 Signed-off-by: Fiona Ebner --- While initial smoke testing seems fine, I'm not familiar enough with this to rule out any pitfalls with the approach. Any reason why scheduling to the iohandler AioContext could be wrong here? Should the same be done for the snapshot_load_job_bh and snapshot_delete_job_bh to keep it consistent? migration/savevm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/migration/savevm.c b/migration/savevm.c index c621f2359b..0086b76ab0 100644 --- a/migration/savevm.c +++ b/migration/savevm.c @@ -3459,7 +3459,7 @@ static int coroutine_fn snapshot_save_job_run(Job *jo= b, Error **errp) SnapshotJob *s =3D container_of(job, SnapshotJob, common); s->errp =3D errp; s->co =3D qemu_coroutine_self(); - aio_bh_schedule_oneshot(qemu_get_aio_context(), + aio_bh_schedule_oneshot(iohandler_get_aio_context(), snapshot_save_job_bh, job); qemu_coroutine_yield(); return s->ret ? 0 : -1; --=20 2.39.2