linux-kernel - Re: [PATCH v3 0/7] arm64 / sched/preempt: support PREEMPT

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Message-ID: <20220209195827.GB557593@lothringen>
Date:   Wed, 9 Feb 2022 20:58:27 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, ardb@...nel.org,
        bp@...en8.de, catalin.marinas@....com, dave.hansen@...ux.intel.com,
        james.morse@....com, joey.gouly@....com, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org, luto@...nel.org, mingo@...hat.com,
        peterz@...radead.org, tglx@...utronix.de,
        valentin.schneider@....com, will@...nel.org
Subject: Re: [PATCH v3 0/7] arm64 / sched/preempt: support PREEMPT_DYNAMIC
 with static keys

On Wed, Feb 09, 2022 at 03:35:28PM +0000, Mark Rutland wrote:
> This series enables PREEMPT_DYNAMIC on arm64. To do so, it adds a new
> mechanism allowing the preemption functions to be enabled/disabled using
> static keys rather than static calls, with architectures selecting
> whether they use static calls or static keys.
> 
> With non-inline static calls, each function call results in a call to
> the (out-of-line) trampoline which either tail-calls its associated
> callee or performs an early return.
> 
> The key idea is that where we're only enabling/disabling a single
> callee, we can inline this trampoline into the start of the callee,
> using a static key to decide whether to return early, and leaving the
> remaining codegen to the compiler. The overhead should be similar to
> (and likely lower than) using a static call trampoline. Since most
> codegen is up to the compiler, we sidestep a number of implementation
> pain-points (e.g. things like CFI should "just work" as well as they do
> for any other functions).
> 
> The bulk of the diffstat for kernel/sched/core.c is shuffling the
> PREEMPT_DYNAMIC code later in the file, and the actual additions are
> fairly trivial.
> 
> I've given this very light build+boot testing so far.
> 
> Since v1 [1]:
> * Rework Kconfig text to be clearer
> * Rework arm64 entry code
> * Clarify commit messages.
> 
> Since v2 [2]:
> * Add missing includes
> * Always provide prototype for preempt_schedule()
> * Always provide prototype for preempt_schedule_notrace()
> * Fix __cond_resched() to default to disabled
> * Fix might_resched() to default to disabled
> * Clarify example in commit message

Acked-by: Frederic Weisbecker <frederic@...nel.org>

Thanks!