> From: Ming Lei <ming.lei@xxxxxxxxxx> > Sent: Tuesday, April 21, 2020 9:16 PM > ... > > > > When we're in storvsc_suspend(), all the userspace processes have been > > > > 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() should > 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. > > Hi Paul, > > Thanks for your clarification. > > 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. > > 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 at all for the devices on the Hyper-V VMBus bus: these are pure software-emulated devices, so runtime PM is unnecessary for them. Thanks, -- Dexuan
