Re: [RFC PATCH 07/86] Revert "livepatch,sched: Add livepatch task switching to cond_resched()"

[Steven Rostedt]

On Thu, 9 Nov 2023 09:26:37 -0800
Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:

> On Tue, Nov 07, 2023 at 06:16:09PM -0500, Steven Rostedt wrote:
> > On Tue,  7 Nov 2023 13:56:53 -0800
> > Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:
> >   
> > > This reverts commit e3ff7c609f39671d1aaff4fb4a8594e14f3e03f8.
> > > 
> > > Note that removing this commit reintroduces "live patches failing to
> > > complete within a reasonable amount of time due to CPU-bound kthreads."
> > > 
> > > Unfortunately this fix depends quite critically on PREEMPT_DYNAMIC and
> > > existence of cond_resched() so this will need an alternate fix.  
> 
> We definitely don't want to introduce a regression, something will need
> to be figured out before removing cond_resched().
> 
> We could hook into preempt_schedule_irq(), but that wouldn't work for
> non-ORC.
> 
> Another option would be to hook into schedule().  Then livepatch could
> set TIF_NEED_RESCHED on remaining unpatched tasks.  But again if they go
> through the preemption path then we have the same problem for non-ORC.
> 
> Worst case we'll need to sprinkle cond_livepatch() everywhere :-/
> 

I guess I'm not fully understanding what the cond rescheds are for. But
would an IPI to all CPUs setting NEED_RESCHED, fix it?

-- Steve