Re: live patching design (was: Re: [PATCH 1/3] sched: add sched_task_call())

Jiri Kosina
2015-02-21T18:57:45Z

On Sat, 21 Feb 2015, Ingo Molnar wrote:

> > This means that each and every sleeping task in the system has to be
> > woken up in some way (sending a signal ...) to exit from a syscall it
> > is sleeping in. Same for CPU hogs. All kernel threads need to be
> > parked.
>
> Yes - although I'd not use signals for this, signals have
> side effects - but yes, something functionally equivalent.

This is similar to my proposal I came up with not too long time ago; a
fake signal (analogically to, but not exactly the same, what freezer is
using), that will just make tasks cycle through userspace/kernelspace
boundary without other side-effects.

> > This is exactly what you need to do for kGraft to complete patching.
>
> My understanding of kGraft is that by default you allow tasks to
> continue 'in the new universe' after they are patched. Has this changed
> or have I misunderstood the concept?

What Vojtech meant here, I believe, is that the effort you have to make to
force all tasks to queue themselves to park them on a safe place and then
restart their execution is exactly the same as the effort you have to make
to make kGraft converge and succeed.

But admittedly, if we reserve a special sort-of signal for making the
tasks pass through a safe checkpoint (and make them queue there (your
solution) or make them just pass through it and continue (current
kGraft)), it might reduce the time this effort needs considerably.

--
Jiri Kosina
SUSE Labs