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Mon, 21 Sep 2020 19:14:41 +0800 (CST) Date: Mon, 21 Sep 2020 11:14:35 +0000 From: Fam Zheng To: Zhenyu Ye Subject: Re: [PATCH v1 0/2] Add timeout mechanism to qmp actions Message-ID: <20200921111435.GA2524022@dev> References: <20200810145246.1049-1-yezhenyu2@huawei.com> <20200810153811.GF14538@linux.fritz.box> <20200914132738.GL579094@stefanha-x1.localdomain> <7ad220bd-7ee5-1f66-b2e5-7dc57d72eb2e@huawei.com> <20200917154415.GB839531@stefanha-x1.localdomain> <20200917160054.GA2453251@dev> <5a5822ec-f3bc-a247-2b5a-f764b941c820@huawei.com> <20200918140628.GA2509473@dev> <1ba06b0d-3fcf-3676-f9e0-52875851ff19@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1ba06b0d-3fcf-3676-f9e0-52875851ff19@huawei.com> X-ZohoCNMailClient: External Received-SPF: pass client-ip=163.53.93.243; envelope-from=fam@euphon.net; helo=sender2-op-o12.zoho.com.cn X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/21 07:15:08 X-ACL-Warn: Detected OS = Linux 3.11 and newer 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, 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 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: Kevin Wolf , qemu-block@nongnu.org, qemu-devel@nongnu.org, xiexiangyou@huawei.com, armbru@redhat.com, Stefan Hajnoczi , pbonzini@redhat.com, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020-09-19 10:22, Zhenyu Ye wrote: > On 2020/9/18 22:06, Fam Zheng wrote: > > > > I can see how blocking in a slow io_submit can cause trouble for main > > thread. I think one way to fix it (until it's made truly async in new > > kernels) is moving the io_submit call to thread pool, and wrapped in a > > coroutine, perhaps. > > > > I'm not sure if any other operation will block the main thread, other > than io_submit(). Then that's a problem with io_submit which should be fixed. Or more precisely, that is a long held lock that we should avoid in QEMU's event loops. > > > I'm not sure qmp timeout is a complete solution because we would still > > suffer from a blocked state for a period, in this exact situation before > > the timeout. > > Anyway, the qmp timeout may be the last measure to prevent the VM > soft lockup. Maybe, but I don't think baking such a workaround into the QMP API is a good idea. No QMP command should be synchronously long running, so having a timeout parameter is just a wrong design. Thanks, Fam