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

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Date:   Sun, 8 May 2022 22:41:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rik van Riel <riel@...com>
Cc:     Song Liu <songliubraving@...com>,
        "song@...nel.org" <song@...nel.org>,
        "joe.lawrence@...hat.com" <joe.lawrence@...hat.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>
Subject: Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched

On Sat, May 07, 2022 at 07:18:51PM +0000, Rik van Riel wrote:
> Huh, I just looked at that, and the x86 should_resched()
> only seems to check that we _can_ resched, not whether
> we should...
> 
> 
> /*
>  * Returns true when we need to resched and can (barring IRQ state).
>  */
> static __always_inline bool should_resched(int preempt_offset)
> {
>         return unlikely(raw_cpu_read_4(__preempt_count) ==
> preempt_offset);
> }
> 
> I wonder if that was intended, and why, or whether
> the x86 should_resched should also be checking for
> TIF_NEED_RESCHED?

No, it does what you think it should do, you're just getting confused by
the inverted PREEMPT_NEED_RESCHED bit :-)