Re: [PATCH 0/5] sched: Lazy preemption muck

[Ankur Arora]

Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes:

> On 2024-10-07 09:46:09 [+0200], Peter Zijlstra wrote:
>> Hi!
>>
>> During LPC Thomas reminded me that the lazy preemption stuff was not there yet.
>>
>> So here goes, robot says it builds, and I checked both a regular and PREEMPT_RT
>> build boots and can change the mode.
>>
>> Please have a poke.
>
> While comparing this vs what I have:
> - need_resched()
>   It checked both (tif_need_resched_lazy() || tif_need_resched()) while
>   now it only looks at tif_need_resched().
>   Also ensured that raw_irqentry_exit_cond_resched() does not trigger on
>   lazy.
>   I guess you can argue both ways what makes sense, just noting…

I think we want need_resched() to be only tif_need_resched(). That way
preemption in lazy mode *only* happens at the user mode boundary.

If the scheduler wants to preempt imminently, it just sets (or upgrades to)
TIF_NEED_RESCHED.

> - __account_cfs_rq_runtime() and hrtick_start_fair()
>   Both have a resched_curr() instead of resched_curr_lazy(). Is this on
>   purpose?
>
> This is actually the main difference (ignoring the moving the RT bits
> and dynamic-sched). The lazy-resched is slightly different but it should
> do the same thing.
> I have also tracing and riscv bits which I port tomorrow, test and add
> to your pile.
>
> Sebastian


--
ankur