Re: [uml-devel] [PATCH] um: Switch clocksource to hrtimers

[Richard Weinberger <richard@nod.at>]

Am 31.05.2015 um 21:00 schrieb Thomas Meyer:
>> Ping.
>> Would be nice to have this patch for the 4.2 merge window.
> 
> I can provide you the current version of the patch, but I'm not sure if
> it's ready for inclusion yet.

That's fine. I'll look at it.
Just rebase it against Linus' tree or uml-next.
https://git.kernel.org/cgit/linux/kernel/git/rw/uml.git/log/?h=linux-next

> For example:
> - With this patch I see new zombie processes of UML userspace
> processes. I'm not sure what's going on here.
> - Anton reported some hang he sees with this patch
> - A person from cicso is worried about the potential idle CPU usage
> after the patch, because of the many timers started, i.e. a host with
> hundreds of UMLs.
> 
> Also meanwhile I think is not the correct thing to start a new timer
> for each UML userspace process, because the timer will also trigger the
> userspace process, even the corresponding process isn't scheduled by
> the kernel currently. I think the previous behaviour with the itimer
> was okay, because the virtual timer only did execute when the process
> was executing which is the correct thing to do for the currently active
> task in the UML kernel.
> I see two solutions for above problem: cascade the kernel timer into
> the current active task; there is actually no need to start a timer in
> each userspace process.
> Start/stop each timer when a userspace process becomes active resp.
> becomes inactive again.
> I hope above logic makes some sense at all! What do you think about
> this?

Hm, we definitely don't want a new timer for each userspace proc. The timer
has to work as a regular clock source.
But I'll have to read your/Anton's code in detail first.

Thanks,
//richard

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