linux-kernel - Re: [PATCH v4 0/7] arm64 / sched/preempt: support PREEMPT

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Message-ID: <20220217185947.GE23216@worktop.programming.kicks-ass.net>
Date:   Thu, 17 Feb 2022 19:59:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        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,
        tglx@...utronix.de, valentin.schneider@....com, will@...nel.org
Subject: Re: [PATCH v4 0/7] arm64 / sched/preempt: support PREEMPT_DYNAMIC
 with static keys

On Thu, Feb 17, 2022 at 02:52:11PM +0100, Frederic Weisbecker wrote:
> On Mon, Feb 14, 2022 at 04:52:09PM +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.
> 
> Acked-by: Frederic Weisbecker <frederic@...nel.org>

I'll go stick them in sched/core then... :-)