From: Peter Zijlstra <peterz@infradead.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
rml@tech9.net, Thomas Gleixner <tglx@linutronix.de>,
Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH 2/2] arm64: preempt: Provide our own implementation of asm/preempt.h
Date: Wed, 28 Nov 2018 17:40:58 +0100 [thread overview]
Message-ID: <20181128164058.GB2131@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKv+Gu9s-=M-mU+249-X1jQ7_Z60vWO1cXK1AP0CwKCmaQQSdg@mail.gmail.com>
On Wed, Nov 28, 2018 at 04:35:42PM +0100, Ard Biesheuvel wrote:
> On Tue, 27 Nov 2018 at 20:44, Will Deacon <will.deacon@arm.com> wrote:
> >
> > The asm-generic/preempt.h implementation doesn't make use of the
> > PREEMPT_NEED_RESCHED flag, since this can interact badly with load/store
> > architectures which rely on the preempt_count word being unchanged across
> > an interrupt.
> >
> > However, since we're a 64-bit architecture and the preempt count is
> > only 32 bits wide, we can simply pack it next to the resched flag and
> > load the whole thing in one go, so that a dec-and-test operation doesn't
> > need to load twice.
> >
>
> Since the actual preempt count is a lot narrower than 32 bits, x86
> just uses bit 31.
>
> So what is the reason for using two different words?
See commit:
ba1f14fbe709 ("sched: Remove PREEMPT_NEED_RESCHED from generic code")
WARNING: multiple messages have this Message-ID (diff)
From: peterz@infradead.org (Peter Zijlstra)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 2/2] arm64: preempt: Provide our own implementation of asm/preempt.h
Date: Wed, 28 Nov 2018 17:40:58 +0100 [thread overview]
Message-ID: <20181128164058.GB2131@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAKv+Gu9s-=M-mU+249-X1jQ7_Z60vWO1cXK1AP0CwKCmaQQSdg@mail.gmail.com>
On Wed, Nov 28, 2018 at 04:35:42PM +0100, Ard Biesheuvel wrote:
> On Tue, 27 Nov 2018 at 20:44, Will Deacon <will.deacon@arm.com> wrote:
> >
> > The asm-generic/preempt.h implementation doesn't make use of the
> > PREEMPT_NEED_RESCHED flag, since this can interact badly with load/store
> > architectures which rely on the preempt_count word being unchanged across
> > an interrupt.
> >
> > However, since we're a 64-bit architecture and the preempt count is
> > only 32 bits wide, we can simply pack it next to the resched flag and
> > load the whole thing in one go, so that a dec-and-test operation doesn't
> > need to load twice.
> >
>
> Since the actual preempt count is a lot narrower than 32 bits, x86
> just uses bit 31.
>
> So what is the reason for using two different words?
See commit:
ba1f14fbe709 ("sched: Remove PREEMPT_NEED_RESCHED from generic code")
next prev parent reply other threads:[~2018-11-28 16:41 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-27 19:45 [PATCH 0/2] arm64: Only call into preempt_schedule() if need_resched() Will Deacon
2018-11-27 19:45 ` Will Deacon
2018-11-27 19:45 ` [PATCH 1/2] preempt: Move PREEMPT_NEED_RESCHED definition into arch code Will Deacon
2018-11-27 19:45 ` Will Deacon
2018-11-27 19:45 ` [PATCH 2/2] arm64: preempt: Provide our own implementation of asm/preempt.h Will Deacon
2018-11-27 19:45 ` Will Deacon
2018-11-28 15:35 ` Ard Biesheuvel
2018-11-28 15:35 ` Ard Biesheuvel
2018-11-28 16:40 ` Peter Zijlstra [this message]
2018-11-28 16:40 ` Peter Zijlstra
2018-11-28 16:45 ` Ard Biesheuvel
2018-11-28 16:45 ` Ard Biesheuvel
2018-11-28 8:56 ` [PATCH 0/2] arm64: Only call into preempt_schedule() if need_resched() Peter Zijlstra
2018-11-28 8:56 ` Peter Zijlstra
2018-11-28 9:01 ` Peter Zijlstra
2018-11-28 9:01 ` Peter Zijlstra
2018-11-28 12:04 ` Will Deacon
2018-11-28 12:04 ` Will Deacon
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