linux-kernel - Re: [RFC PATCH 40/86] context_tracking: add ct_state

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Message-ID: <20231108091659.GK8262@noisy.programming.kicks-ass.net>
Date:   Wed, 8 Nov 2023 10:16:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 40/86] context_tracking: add ct_state_cpu()

On Tue, Nov 07, 2023 at 01:57:26PM -0800, Ankur Arora wrote:
> While making up its mind about whether to reschedule a target
> runqueue eagerly or lazily, resched_curr() needs to know if the
> target is executing in the kernel or in userspace.
> 
> Add ct_state_cpu().
> 
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> 
> ---
> Using context-tracking for this seems like overkill. Is there a better
> way to achieve this? One problem with depending on user_enter() is that
> it happens much too late for our purposes. From the scheduler's
> point-of-view the exit state has effectively transitioned once the
> task exits the exit_to_user_loop() so we will see stale state
> while the task is done with exit_to_user_loop() but has not yet
> executed user_enter().
> 
> ---
>  include/linux/context_tracking_state.h | 21 +++++++++++++++++++++
>  kernel/Kconfig.preempt                 |  1 +
>  2 files changed, 22 insertions(+)
> 
> diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
> index bbff5f7f8803..6a8f1c7ba105 100644
> --- a/include/linux/context_tracking_state.h
> +++ b/include/linux/context_tracking_state.h
> @@ -53,6 +53,13 @@ static __always_inline int __ct_state(void)
>  {
>  	return raw_atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;
>  }
> +
> +static __always_inline int __ct_state_cpu(int cpu)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> +
> +	return atomic_read(&ct->state) & CT_STATE_MASK;
> +}
>  #endif
>  
>  #ifdef CONFIG_CONTEXT_TRACKING_IDLE
> @@ -139,6 +146,20 @@ static __always_inline int ct_state(void)
>  	return ret;
>  }
>  
> +static __always_inline int ct_state_cpu(int cpu)
> +{
> +	int ret;
> +
> +	if (!context_tracking_enabled_cpu(cpu))
> +		return CONTEXT_DISABLED;
> +
> +	preempt_disable();
> +	ret = __ct_state_cpu(cpu);
> +	preempt_enable();
> +
> +	return ret;
> +}

Those preempt_disable/enable are pointless.

But this patch is problematic, you do *NOT* want to rely on context
tracking. Context tracking adds atomics to the entry path, this is slow
and even with CONFIG_CONTEXT_TRACKING it is disabled until you configure
the NOHZ_FULL nonsense.

This simply cannot be.