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Date:   Wed, 22 Apr 2020 09:19:13 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Dexuan Cui <decui@microsoft.com>,
        Josh Triplett <josh@joshtriplett.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "jejb@linux.ibm.com" <jejb@linux.ibm.com>,
        "martin.petersen@oracle.com" <martin.petersen@oracle.com>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "hch@lst.de" <hch@lst.de>,
        "bvanassche@acm.org" <bvanassche@acm.org>,
        "hare@suse.de" <hare@suse.de>,
        Michael Kelley <mikelley@microsoft.com>,
        Long Li <longli@microsoft.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "wei.liu@kernel.org" <wei.liu@kernel.org>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>
Subject: Re: [PATCH] scsi: storvsc: Fix a panic in the hibernation procedure
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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@redhat.com>
> > > 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