linux-kernel - Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond

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Message-ID: <9FA1822F-76EE-4174-86DD-B20F1F8CE7FC@fb.com>
Date:   Mon, 9 May 2022 08:06:22 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Song Liu <song@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched



> On May 9, 2022, at 12:04 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Sat, May 07, 2022 at 10:46:28AM -0700, Song Liu wrote:
>> Busy kernel threads may block the transition of livepatch. Call
>> klp_try_switch_task from __cond_resched to make the transition easier.
> 
> What will a PREEMPT=y kernel do? How is it not a problem there, and if
> it is, this will not help that.
> 
> That is; I don't think this can be right.

I guess on PREEMPT=y kernel, we can simply preempt the long running 
kernel thread and check the transition? 

In this case (PREEMPT=n), we see a long running kernel thread could not
finish the transition. It calls cond_resched() and gets rescheduled 
(moves among different cores). However, it never finishes the transition,
because live patch doesn’t get a chance to check the stack. 

Does this answer the question?

Thanks,
Song