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Subject: Re: [PATCH 0/5] sched: Lazy preemption muck
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To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
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From: Tianchen Ding <dtcccc@linux.alibaba.com>
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On 2024/10/10 04:43, Steven Rostedt wrote:

[...]

> Hmm, but then again...
> 
> Perhaps these cond_resched() is proper? That is, the need_resched() /
> cond_resched() is not something that is being done for PREEMPT_NONE, but
> for preempt/voluntary kernels too. Maybe these cond_resched() should stay?
> If we spin in the loop for one more tick, that is actually changing the
> behavior of PREEMPT_NONE and PREEMPT_VOLUNTARY, as the need_resched()/cond_resched()
> helps with latency. If we just wait for the next tick, these loops (and
> there's a lot of them) will all now run for one tick longer than if
> PREEMPT_NONE or PREEMPT_VOLUNTARY were set today.
> 

Agree.

And for PREEMPT_LAZIEST, this becomes worse. The fair_class tasks will be 
delayed more than 1 tick. They may be starved until a non-fair class task comes 
to "save" them.

cond_resched() is designed for NONE/VOLUNTARY to avoid spinning in kernel and 
prevent softlockup. However, it is a nop in PREEMPT_LAZIEST, and things may be 
broken...