From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Ingo Molnar <mingo@kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
Mark Rutland <mark.rutland@arm.com>,
Julien Thierry <julien.thierry@arm.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: preempt_schedule_irq() loop question
Date: Wed, 16 Jan 2019 14:45:21 +0100 [thread overview]
Message-ID: <20190116134521.GG10803@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <cc989920-a13b-d53b-db83-1584a7f53edc@arm.com>
On Wed, Jan 16, 2019 at 12:50:42PM +0000, Valentin Schneider wrote:
> Hi,
>
> I've been wandering around preempt_schedule_irq() in sched/core.c, and
> got curious regarding how the arch code calls it.
>
> The main part of preempt_schedule_irq() is:
>
> do {
> preempt_disable();
> local_irq_enable();
> __schedule(true);
> local_irq_disable();
> sched_preempt_enable_no_resched();
> } while (need_resched());
>
> Yet all the arch entry.S I looked at (I stopped after arm64, arm, x86_32,
> MIPS, powerpc) wrap the call to preempt_schedule_irq() in another
>
> do { ... } while (need_resched())
>
> For instance, this is what's done in arm64:
>
> 1: bl preempt_schedule_irq // irq en/disable is done inside
> ldr x0, [tsk, #TSK_TI_FLAGS] // get new tasks TI_FLAGS
> tbnz x0, #TIF_NEED_RESCHED, 1b // needs rescheduling?
>
>
> I naively thought this could be attributed to something like
> preempt_schedule_irq() historically not having an inner loop, but it seems
> to have been there since the beginning of time (or at least up to the point
> where the git history stops).
>
> I don't see why we need to have these nested loops - AFAICT the one in
> preempt_schedule_irq() would suffice. What am I missing?
I think you're quite right; but I wasn't doing kernel work back when rml
added the preemptible bits. Ingo, do you have any recollections that far
back?
next prev parent reply other threads:[~2019-01-16 13:47 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-16 12:50 preempt_schedule_irq() loop question Valentin Schneider
2019-01-16 13:45 ` Peter Zijlstra [this message]
2019-01-16 14:57 ` Thomas Gleixner
2019-01-16 15:23 ` Valentin Schneider
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