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    SubjectRe: [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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