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 DFDDDC77B7F for ; Tue, 16 May 2023 15:14:11 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pywMw-0005Zz-QG; Tue, 16 May 2023 11:13:42 -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 1pywMt-0005Zg-B4 for qemu-devel@nongnu.org; Tue, 16 May 2023 11:13:39 -0400 Received: from smtp-out2.suse.de ([2001:67c:2178:6::1d]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1pywMr-0004Gk-Ob for qemu-devel@nongnu.org; Tue, 16 May 2023 11:13:39 -0400 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 9E8391FF5E; Tue, 16 May 2023 15:13:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1684250014; h=from:from:reply-to: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=lrNWFt99dXBB4SOQOjaYFirhHHYEebrbMRguni+V0p8=; b=hR/Rgoeh0GDGGPloT5/x78GS8llyhpOXaHQr3fWimKXeBpA0psuZUu6LlfuT5R7PHvmUXW 9W4Ez010SO4m5MzPgAm7E7izj1k92U2zZeqjankO8vhZTwacO8yYKG6zdMu6VRlif50urC nfSjWk02AoGPyX8Ac1AUDdMHyTsG0sE= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1684250014; h=from:from:reply-to: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=lrNWFt99dXBB4SOQOjaYFirhHHYEebrbMRguni+V0p8=; b=5TLZBB8cyQK+QqyWX28OMPh7+Srsk3w3TO+SrUUIByNX31K9iXc1vZ3RURdM+LDBh5RvBR +sjUJYYgFnPlCgCw== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 8767D138F9; Tue, 16 May 2023 15:13:34 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id xzz5Gp6dY2RqfwAAMHmgww (envelope-from ); Tue, 16 May 2023 15:13:34 +0000 From: Fabiano Rosas To: Kevin Wolf Cc: qemu-devel@nongnu.org, armbru@redhat.com, eesposit@redhat.com, vsementsov@yandex-team.ru Subject: Re: Question about QMP and BQL In-Reply-To: References: <87h6sh2zug.fsf@suse.de> Date: Tue, 16 May 2023 12:13:33 -0300 Message-ID: <87r0rg71ia.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain Received-SPF: pass client-ip=2001:67c:2178:6::1d; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, 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 Wolf writes: > Am 12.05.2023 um 20:01 hat Fabiano Rosas geschrieben: >> 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 first part of the right solution there should be moving fstat() to a > worker thread like we do for other requests where we care about not > blocking. See existing raw_thread_pool_submit() callers for examples. > > Note that this isn't the full solution yet. QMP still has to wait for > bdrv_get_allocated_file_size() to return. bdrv_poll_co() runs a nested > event loop, but it doesn't unlock the BQL. > > So the second part would be converting the block-block QMP handler to > a coroutine so that it can actually yield to the main loop, which will > then drop the BQL while waiting. We would have to be careful there to > make sure that we don't break anything because the sets of things > allowed inside and outside coroutines are different. > > Kevin Hi Kevin, Thank you, this is what I was looking for. I was missing the raw_thread_pool_submit right there under my nose! I'll put together an RFC so we can discuss the details.