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McKenney" CC: Josh Triplett , "Rafael J. Wysocki" , "jejb@linux.ibm.com" , "martin.petersen@oracle.com" , "linux-scsi@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "hch@lst.de" , "bvanassche@acm.org" , "hare@suse.de" , Michael Kelley , Long Li , "linux-hyperv@vger.kernel.org" , "wei.liu@kernel.org" , Stephen Hemminger , Haiyang Zhang , KY Srinivasan , "linux-pm@vger.kernel.org" Subject: RE: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure Thread-Topic: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure Thread-Index: AQHWGEVb6DFeJK2tFE2eFTHRpRyDH6iEXZcAgAArmvCAAAOn4A== Date: Wed, 22 Apr 2020 04:58:14 +0000 Message-ID: References: <1587514644-47058-1-git-send-email-decui@microsoft.com> <20200422012814.GB299948@T590> <20200422020134.GC299948@T590> <20200422030807.GK17661@paulmck-ThinkPad-P72> <20200422041629.GE299948@T590> In-Reply-To: <20200422041629.GE299948@T590> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Enabled=True; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-Network-Message-Id: 20bcb216-a7f1-4a5d-219e-08d7e679c3b6 X-MS-Exchange-CrossTenant-originalarrivaltime: 22 Apr 2020 04:58:14.0861 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: Ub6P7rs4CXkultXMGS+E5q46F02wtd8+h62s+7EWQK01kJ1z1cIodUL71PCjDKK/BvS4+VrDbQSxikXY/KHlDA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: HK0P153MB0243 Sender: linux-hyperv-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-hyperv@vger.kernel.org > From: Ming Lei > Sent: Tuesday, April 21, 2020 9:16 PM > ... > > > > When we're in storvsc_suspend(), all the userspace processes have b= een > > > > frozen and all the file systems have been flushed, and there should= not > > > > be too much I/O from the kernel space, so IMO scsi_host_block() sho= uld > be > > > > pretty fast here. > > > > > > I guess it depends on RCU's implementation, so CC RCU guys. > > > > > > Hello Paul & Josh, > > > > > > Could you clarify that if sysnchronize_rcu becomes quickly during > > > system suspend? > > > > Once you have all but one CPU offlined, it becomes extremely fast, as > > in roughly a no-op (which is an idea of Josh's from back in the day). > > But if there is more than one CPU online, then synchronize_rcu() still > > takes on the order of several to several tens of jiffies. > > > > So, yes, in some portions of system suspend, synchronize_rcu() becomes > > very fast indeed. >=20 > Hi Paul, >=20 > Thanks for your clarification. >=20 > In system suspend path, device is suspended before > suspend_disable_secondary_cpus(), > so I guess synchronize_rcu() is not quick enough even though user space > processes and some kernel threads are frozen. >=20 > Thanks, > Ming storvsc_suspend() -> scsi_host_block() is only called in the hibernation path, which is not a hot path at all, so IMHO we don't really care if it takes 10ms or 100ms or even 1s. :-) BTW, in my test, typically the scsi_host_block() here takes about 3ms in my 40-vCPU VM. storvsc_suspend() is not called from the runtime PM path, because the runtime_suspend/runtime_resume/runtime_idle ops are not defined=20 at all for the devices on the Hyper-V VMBus bus: these are pure=20 software-emulated devices, so runtime PM is unnecessary for them. Thanks, -- Dexuan