linux-kernel - Re: [RFC PATCH 00/86] Make the kernel preemptible

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Message-ID: <20231108101330.GK3818@noisy.programming.kicks-ass.net>
Date:   Wed, 8 Nov 2023 11:13:30 +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,
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        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 00/86] Make the kernel preemptible

On Wed, Nov 08, 2023 at 02:04:02AM -0800, Ankur Arora wrote:

> >>   Revert "riscv: support PREEMPT_DYNAMIC with static keys"
> >>   Revert "livepatch,sched: Add livepatch task switching to
> >>     cond_resched()"
> >>   Revert "arm64: Support PREEMPT_DYNAMIC"
> >>   Revert "sched/preempt: Add PREEMPT_DYNAMIC using static keys"
> 
> What's the best way to handle these? With the lazy bit, cond_resched()
> and might_resched() are gone. So we don't need all of the static
> key inftrastructure for toggling etc.
> 
> The part of preempt_dynamic that makes sense to me is the one that
> switches dynamically between none/voluntary/full. Here it would need
> to be wired onto controls of the lazy bit.
> (Right now the preemption policy is controlled by sched_feat in
> patches 43, and 44 but sched/preempt is a much better interface.)

I'm not understanding, those should stay obviously.

The current preempt_dynamic stuff has 5 toggles:

/*
 * SC:cond_resched
 * SC:might_resched
 * SC:preempt_schedule
 * SC:preempt_schedule_notrace
 * SC:irqentry_exit_cond_resched
 *
 *
 * NONE:
 *   cond_resched               <- __cond_resched
 *   might_resched              <- RET0
 *   preempt_schedule           <- NOP
 *   preempt_schedule_notrace   <- NOP
 *   irqentry_exit_cond_resched <- NOP
 *
 * VOLUNTARY:
 *   cond_resched               <- __cond_resched
 *   might_resched              <- __cond_resched
 *   preempt_schedule           <- NOP
 *   preempt_schedule_notrace   <- NOP
 *   irqentry_exit_cond_resched <- NOP
 *
 * FULL:
 *   cond_resched               <- RET0
 *   might_resched              <- RET0
 *   preempt_schedule           <- preempt_schedule
 *   preempt_schedule_notrace   <- preempt_schedule_notrace
 *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
 */

If you kill voluntary as we know it today, you can remove cond_resched
and might_resched, but the remaining 3 are still needed to switch
between NONE and FULL.

Additionally, you'll get one new state to enable/disable the LAZY stuff.
Neither NONE nor FULL want the LAZY thing on.

You'll then end up with something like:

/*
 * SK:preempt_lazy
 * SC:preempt_schedule
 * SC:preempt_schedule_notrace
 * SC:irqentry_exit_cond_resched
 *
 *
 * NONE:
 *   preempt_lazy		<- OFF
 *   preempt_schedule           <- NOP
 *   preempt_schedule_notrace   <- NOP
 *   irqentry_exit_cond_resched <- NOP
 *
 * VOLUNTARY:
 *   preempt_lazy		<- ON
 *   preempt_schedule           <- preempt_schedule
 *   preempt_schedule_notrace   <- preempt_schedule_notrace
 *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
 *
 * FULL:
 *   preempt_lazy		<- OFF
 *   preempt_schedule           <- preempt_schedule
 *   preempt_schedule_notrace   <- preempt_schedule_notrace
 *   irqentry_exit_cond_resched <- irqentry_exit_cond_resched
 */

For the architectures that do not have static_call but instead use
static_key for everything, the SC's are obviously static_key based
wrappers around the function calls -- like now.