From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.90_1) id 1jtpWK-0005Um-Vd for mharc-qemu-trivial@gnu.org; Fri, 10 Jul 2020 05:40:40 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:48354) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jtpWJ-0005UT-Jd for qemu-trivial@nongnu.org; Fri, 10 Jul 2020 05:40:39 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:30299 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jtpWH-0005uS-2Z for qemu-trivial@nongnu.org; Fri, 10 Jul 2020 05:40:39 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1594374035; 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=zbsAehH5+P7R9qu9oGq+H9orZQd7S2ufuoDC+RoHI5o=; b=acrQfzizdXqV1A+Vqw1Ua54rW6cmjGTZZO6Sj3169YSdfCwMtLQU7nOvlp8R4pJA7kutmJ GZeVxDVVcd6DL8JO7hVGUCWXaKBh6ijv1pp5vcbhNbLhlq6xtpKzjT4Rkg/99fP4WNUT/x QSDxa6nfFFaV3Hy5rt4zuCXvGmxBVeE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-294-w0BkHcYIPdq5am3E2sN5Sg-1; Fri, 10 Jul 2020 05:40:31 -0400 X-MC-Unique: w0BkHcYIPdq5am3E2sN5Sg-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9EDCF1080; Fri, 10 Jul 2020 09:40:30 +0000 (UTC) Received: from linux.fritz.box (ovpn-114-57.ams2.redhat.com [10.36.114.57]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0D0425BAC3; Fri, 10 Jul 2020 09:40:27 +0000 (UTC) Date: Fri, 10 Jul 2020 11:40:26 +0200 From: Kevin Wolf To: Zhenyu Ye Cc: qemu-devel@nongnu.org, armbru@redhat.com, mreitz@redhat.com, qemu-block@nongnu.org, qemu-trivial@nongnu.org, xiexiangyou@huawei.com Subject: Re: [PATCH v1] qmp: don't hold ctx lock while querying blockstats Message-ID: <20200710094026.GC6641@linux.fritz.box> References: <20200710085400.343-1-yezhenyu2@huawei.com> MIME-Version: 1.0 In-Reply-To: <20200710085400.343-1-yezhenyu2@huawei.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/09 22:08:55 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-trivial@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 10 Jul 2020 09:40:39 -0000 Am 10.07.2020 um 10:54 hat Zhenyu Ye geschrieben: > Because the QMP command runs in the main thread, and changes > to the aio context of iothread will only be executed in the > main thread (they will not be in parallel), so there is no > need a lock protection while querying blockstats. > > If we hold the lock here, while the I/O pressure is high in > vm and the I/O returns slowly, the main thread will be stuck > until the lock is released, which will affect the vcpu operation > and finall cause the vm to be stuck. > > Signed-off-by: Zhenyu Ye Are you saying that you think qmp_query_blockstats() runs in the iothread? This is not true, it runs in the main thread. This makes dropping the lock wrong, and even dropping it shouldn't make a difference for other things that should run in the main thread because the main thread is still busy executing the QMP command. Am I missing something? (Also, a change like this is most definitely *not* trivial.) 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Fri, 10 Jul 2020 09:40:27 +0000 (UTC) Date: Fri, 10 Jul 2020 11:40:26 +0200 From: Kevin Wolf To: Zhenyu Ye Subject: Re: [PATCH v1] qmp: don't hold ctx lock while querying blockstats Message-ID: <20200710094026.GC6641@linux.fritz.box> References: <20200710085400.343-1-yezhenyu2@huawei.com> MIME-Version: 1.0 In-Reply-To: <20200710085400.343-1-yezhenyu2@huawei.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/09 22:08:55 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action 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: qemu-block@nongnu.org, qemu-trivial@nongnu.org, armbru@redhat.com, qemu-devel@nongnu.org, xiexiangyou@huawei.com, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 10.07.2020 um 10:54 hat Zhenyu Ye geschrieben: > Because the QMP command runs in the main thread, and changes > to the aio context of iothread will only be executed in the > main thread (they will not be in parallel), so there is no > need a lock protection while querying blockstats. > > If we hold the lock here, while the I/O pressure is high in > vm and the I/O returns slowly, the main thread will be stuck > until the lock is released, which will affect the vcpu operation > and finall cause the vm to be stuck. > > Signed-off-by: Zhenyu Ye Are you saying that you think qmp_query_blockstats() runs in the iothread? This is not true, it runs in the main thread. This makes dropping the lock wrong, and even dropping it shouldn't make a difference for other things that should run in the main thread because the main thread is still busy executing the QMP command. Am I missing something? (Also, a change like this is most definitely *not* trivial.) Kevin