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 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 smtp.lore.kernel.org (Postfix) with ESMTPS id C768DC25B76 for ; Tue, 11 Jun 2024 12:10:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1sH0JP-0003QS-3Q; Tue, 11 Jun 2024 08:09:15 -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 1sH0JM-0003Py-Eg for qemu-devel@nongnu.org; Tue, 11 Jun 2024 08:09:12 -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 1sH0JK-0004ZN-1k for qemu-devel@nongnu.org; Tue, 11 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 7226F45A56; Tue, 11 Jun 2024 14:08:57 +0200 (CEST) Message-ID: <6d64f07d-1638-44dc-848b-b307c0ebd0ad@proxmox.com> Date: Tue, 11 Jun 2024 14:08:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] migration/savevm: do not schedule snapshot_save_job_bh in qemu_aio_context To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, peterx@redhat.com, farosas@suse.de, pbonzini@redhat.com References: <20240605120848.358654-1-f.ebner@proxmox.com> <20240606183638.GC198201@fedora.redhat.com> Content-Language: en-US From: Fiona Ebner In-Reply-To: <20240606183638.GC198201@fedora.redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 06.06.24 um 20:36 schrieb Stefan Hajnoczi: > On Wed, Jun 05, 2024 at 02:08:48PM +0200, Fiona Ebner wrote: >> 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? > > If something waits for a BlockJob to finish using aio_poll() from > qemu_aio_context then a deadlock is possible since the iohandler_ctx > won't get a chance to execute. The only suspicious code path I found was > job_completed_txn_abort_locked() -> job_finish_sync_locked() but I'm not > sure whether it triggers this scenario. Please check that code path. > Sorry, I don't understand. Isn't executing the scheduled BH the only additional progress that the iohandler_ctx needs to make compared to before the patch? How exactly would that cause issues when waiting for a BlockJob? Or do you mean something waiting for the SnapshotJob from qemu_aio_context before snapshot_save_job_bh had the chance to run? Best Regards, Fiona