On Wed, Apr 22, 2020 at 05:23:51PM +0800, Ming Lei wrote: > On Wed, Apr 22, 2020 at 04:58:14AM +0000, Dexuan Cui wrote: > > > 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 > > Are you sure the 'we' can cover all users? > > > scsi_host_block() here takes about 3ms in my 40-vCPU VM. > > If more LUNs are added, the time should be increased proportionallly, > that is why I think scsi_host_block() is bad. If the caller must wait until the grace period ends, then the traditional approach is to use a single synchronize_rcu() to cover all LUNs. This of course can require some reworking of the code. If the caller does not need to wait, then either call_rcu() or kfree_rcu() can work quite well. Thanx, Paul
