RE: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure

[Dexuan Cui]

> 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