On 2025-02-12 10:00:01 [-0500], Steven Rostedt wrote: > On Wed, 12 Feb 2025 13:11:13 +0100 > Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> 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
