linux-kernel - Re: sched: Clarify PREEMPT is full preemption model

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Message-ID: <7ce44659-60a6-4647-bb74-fb6017ceb129@linux.ibm.com>
Date: Thu, 16 Jan 2025 14:07:51 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: 余华兵 <yhb@...jie.com.cn>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: Re: sched: Clarify PREEMPT is full preemption model


Please use plaintext for sending mail containing patches.
Not able to see the patch on my mail client.  Also the text is not formatted.

https://docs.kernel.org/process/submitting-patches.html#no-mime-no-links-no-compression-no-attachments-just-plain-text


On 1/15/25 15:24, 余华兵 wrote:
> The configuration file "kernel/Kconfig. preempt" says that PREEMPT_RT is 
> fully preemptible, but from the function preempt_model_full() it can be 
> seen that PREEMPT is full preemption model. -------------------------- 
> kernel/Kconfig.preempt config PREEMPT         bool "Preemptible Kernel 
> (Low-Latency Desktop)" config PREEMPT_RT         bool "Fully Preemptible 
> Kernel (Real-Time)" include/linux/preempt.h static inline bool 
> preempt_model_full(void) {         return IS_ENABLED(CONFIG_PREEMPT); } 
> -------------------------- 

On Preempt_RT kernel, the spinlocks are sleeping locks and irqs can run as threaded.
So even those can be preempted if necessary. That is not true with preempt=full alone.

I suggest that the configuration file
> "kernel/Kconfig. preempt" should be consistent with the function 
> preempt_model_full(). Modify the file "kernel/Kconfig.preempt" as 
> follows. ------------------ config PREEMPT         bool "Fully 
> Preemptible Kernel (Low-Latency Desktop)" config PREEMPT_RT         bool 
> "Real-time Kernel" ------------------