Re: [RFC][PATCH 1/2] sched: Extended scheduler time slice

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-02-12 10:00:01 [-0500], Steven Rostedt wrote:
> On Wed, 12 Feb 2025 13:11:13 +0100
> Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> > > IIUC, today, LAZY causes all SCHED_OTHER tasks to act more like
> > > PREEMPT_NONE. Is that correct?  
> > 
> > Well. First sched-tick will set the LAZY bit, the second sched-tick
> > forces a resched.
> > On PREEMPT_NONE the sched-tick would be set NEED_RESCHED while nothing
> > will force a resched until the task decides to do schedule() on its own.
> > So it is slightly different for kernel threads.
> 
> Except that it should schedule on a cond_resched() and the point of adding
> LAZY was to get rid of all the cond_resched() which in turn gets rid of the
> need for PREEMPT_NONE. Which was what I was getting at. That PREEMPT_LAZY
> is really just NONE without the need to sprinkle cond_resched() all over
> the kernel. Instead of having cond_resched(), we just wait for the next
> tick.

I would argue that we want to get out of the kernel asap and not
schedule() if we stumble upon cond_resched().

> > Unless we talk about userland, here we would have a resched on the
> > return to userland after the sched-tick LAZY or NONE does not matter.
> > 
> > > Now that the PREEMPT_RT is not one of the preemption selections, when you
> > > select PREEMPT_RT, you can pick between CONFIG_PREEMPT and
> > > CONFIG_PREEMPT_LAZY. Where CONFIG_PREEMPT will preempt the kernel at the
> > > scheduler tick if preemption is enabled and CONFIG_PREEMPT_LAZY will
> > > not preempt the kernel on a scheduler tick and wait for exit to user space.  
> > 
> > This is not specific to RT but FULL vs LAZY. But yes. However the second
> 
> Not true. PREEMPT_RT use to enable PREEMPT_FULL as well (it would preempt
> everywhere). The issue we found was that spin_locks which would not have
> been preempted by just FULL alone were being preempted when RT was enabled.
> That caused a lot more contention with spin locks in the kernel.
> 
> That is PREEMPT_RT with PREEMPT_FULL will have a noticeable performance
> degradation compared to just PREEMPT_FULL alone.

okay.

> > sched-tick will force preemption point even without the
> > exit-to-userland.
> > 
> 
> My question still stands. Have you compared PREEMPT_FULL with and without
> PREEMPT_RT?

No I have not.

> -- Steve

Sebastian