linux-kernel - Re: [RFC PATCH 6/7] preempt/dynamic: Provide irqentry_exit_cond

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Date:   Tue, 10 Nov 2020 11:32:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Mel Gorman <mgorman@...e.de>,
        Michal Hocko <mhocko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH 6/7] preempt/dynamic: Provide
 irqentry_exit_cond_resched() static call

On Tue, Nov 10, 2020 at 01:56:08AM +0100, Frederic Weisbecker wrote:
> [convert from static key to static call, only define static call when
> PREEMPT_DYNAMIC]

>  noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>  {
> @@ -383,8 +386,13 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>  		}
>  
>  		instrumentation_begin();
> -		if (IS_ENABLED(CONFIG_PREEMPTION))
> +		if (IS_ENABLED(CONFIG_PREEMPTION)) {
> +#ifdef CONFIG_PREEMT_DYNAMIC
> +			static_call(irqentry_exit_cond_resched)();
> +#else
>  			irqentry_exit_cond_resched();
> +#endif
> +		}
>  		/* Covers both tracing and lockdep */
>  		trace_hardirqs_on();
>  		instrumentation_end();

The reason I had this a static_branch() is that because if you look at
the code-gen of this function, you'll find it will inline the call.

Not sure it matters much, but it avoids a CALL+RET.