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 AC664C77B7D for ; Mon, 15 May 2023 11:09:16 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pyW3w-0003mX-7M; Mon, 15 May 2023 07:08:20 -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 1pyW3k-0003lX-NZ for qemu-devel@nongnu.org; Mon, 15 May 2023 07:08:11 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pyW3j-0007rl-6r for qemu-devel@nongnu.org; Mon, 15 May 2023 07:08:08 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1684148886; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=V9uunHdkIcrxkaU1Qm32hCojthWVMq6i/lWy7g4lK3U=; b=Dx8AVET2gEWZqWqxEkoyNxBJ6Zw+TIlvYCk2oKW5fDjPLWSbkwyBTfKeijIK+r96ewciGi L/6bInDH1xWOx5uJvrLR6nHNSqkKymIiAA811tw8tOC53UaWK/9R4VvLbIq4RziNzSwh2K 3qmb/k5BAv2Nd+bCsPhjJVlyOqhlpV4= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-259-No5pCQDONd2HGWz0gSxLVQ-1; Mon, 15 May 2023 07:08:02 -0400 X-MC-Unique: No5pCQDONd2HGWz0gSxLVQ-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 7E23C3C0254E; Mon, 15 May 2023 11:08:02 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.91]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3FFE363ABB; Mon, 15 May 2023 11:08:02 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 1EEC621E681C; Mon, 15 May 2023 13:08:01 +0200 (CEST) From: Markus Armbruster To: Fabiano Rosas Cc: qemu-devel@nongnu.org, kwolf@redhat.com, eesposit@redhat.com, vsementsov@yandex-team.ru Subject: Re: Question about QMP and BQL References: <87h6sh2zug.fsf@suse.de> Date: Mon, 15 May 2023 13:08:01 +0200 In-Reply-To: <87h6sh2zug.fsf@suse.de> (Fabiano Rosas's message of "Fri, 12 May 2023 15:01:43 -0300") Message-ID: <87y1lpyhri.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, 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 Kevin, any advice? Fabiano Rosas writes: > Is there a way to execute a long-standing QMP command outside of the > BQL? > > The situation we're seeing is a slow query-block due to a slow system > call (fstat over NFS) causing the main thread to spend too long > holding the global mutex and locking up the vcpu thread when it goes > out of the guest for MMIO. > > The call chain for QMP is: > > qmp_query_block > bdrv_query_info > bdrv_block_device_info > bdrv_query_image_info > bdrv_do_query_node_info > bdrv_get_allocated_file_size > bdrv_poll_co <- Waiting with qemu_global_mutex locked > > [coroutine] bdrv_co_get_allocated_file_size_entry > bdrv_co_get_allocated_file_size > raw_co_get_allocated_file_size > fstat <- SLOW! > > The closest I got was moving the coroutine into a separate iothread, > unlocking the global mutex and releasing the bdrv aio_context around > aio_poll. It feels wrong though because we're technically still > operating on the block state but not holding the context. > > Is there a more standard way if doing this? Is it possible at all?