linux-kernel - Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB kthread wakeups

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Message-ID: <20140815150402.GD4752@linux.vnet.ibm.com>
Date:	Fri, 15 Aug 2014 08:04:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Amit Shah <amit.shah@...hat.com>
Cc:	linux-kernel@...r.kernel.org, riel@...hat.com, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 1/2] rcu: Parallelize and economize NOCB
 kthread wakeups

On Fri, Aug 15, 2014 at 10:54:11AM +0530, Amit Shah wrote:
> On (Wed) 13 Aug 2014 [06:00:49], Paul E. McKenney wrote:
> > On Wed, Aug 13, 2014 at 11:14:39AM +0530, Amit Shah wrote:
> > > On (Tue) 12 Aug 2014 [14:41:51], Paul E. McKenney wrote:
> > > > On Tue, Aug 12, 2014 at 02:39:36PM -0700, Paul E. McKenney wrote:
> > > > > On Tue, Aug 12, 2014 at 09:06:21AM -0700, Paul E. McKenney wrote:
> > > > > > On Tue, Aug 12, 2014 at 11:03:21AM +0530, Amit Shah wrote:
> > > > > 
> > > > > [ . . . ]
> > > > > 
> > > > > > > I know of only virtio-console doing this (via userspace only,
> > > > > > > though).
> > > > > > 
> > > > > > As in userspace within the guest?  That would not work.  The userspace
> > > > > > that the qemu is running in might.  There is a way to extract ftrace info
> > > > > > from crash dumps, so one approach would be "sendkey alt-sysrq-c", then
> > > > > > pull the buffer from the resulting dump.  For all I know, there might also
> > > > > > be some script that uses the qemu "x" command to get at the ftrace buffer.
> > > > > > 
> > > > > > Again, I cannot reproduce this, and I have been through the code several
> > > > > > times over the past few days, and am not seeing it.  I could start
> > > > > > sending you random diagnostic patches, but it would be much better if
> > > > > > we could get the trace data from the failure.
> > > 
> > > I think the only recourse I now have is to dump the guest state from
> > > qemu, and attempt to find the ftrace buffers by poking pages and
> > > finding some ftrace-like struct... and then dumping the buffers.
> > 
> > The data exists in the qemu guest state, so it would be good to have
> > it one way or another.  My current (perhaps self-serving) guess is that
> > you have come up with a way to trick qemu into dropping IPIs.
> 
> I didn't get around to doing this yet; will get to it next week.
> 
> In the meantime, I tried this on RHEL6 (with RHEL6 qemu and gcc and
> seabios), and that exhibits the problem similarly with my .config.

And I am running my tests successfully on an x86_64 system running
Ubuntu 12.04.  Some testing on 14.04 seems to require booting with
acpi=off, leading to my perhaps self-serving guess above.

> <snip>
> 
> > > > +
> > > >  	return true;
> > > 
> > > I have return 1; here.
> > > 
> > > I'm on linux.git, c8d6637d0497d62093dbba0694c7b3a80b79bfe1.
> > 
> > I am working on top of my -rcu tree, which contains the fix from "1" to
> > "true" compared to current mainline.  So this will resolve itself, and
> > you should be OK fixing up conflict in either direction.
> 
> Yep, I did do that.  Just noted here that the hunk didn't directly
> apply.

Fair enough, thank you for letting me know.

							Thanx, Paul

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