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

[Amit Shah]

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.

<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.

Thanks,

		Amit
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/