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Date: Mon, 15 Sep 2025 12:12:50 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Ankur Arora <ankur.a.arora@oracle.com>
Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, bpf@vger.kernel.org,
	arnd@arndb.de, will@kernel.org, peterz@infradead.org,
	akpm@linux-foundation.org, mark.rutland@arm.com,
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	memxor@gmail.com, zhenglifeng1@huawei.com,
	xueshuai@linux.alibaba.com, joao.m.martins@oracle.com,
	boris.ostrovsky@oracle.com, konrad.wilk@oracle.com
Subject: Re: [PATCH v5 0/5] barrier: Add smp_cond_load_*_timeout()
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References: <20250911034655.3916002-1-ankur.a.arora@oracle.com>
 <aMLd2QOFrZnaKcWf@arm.com>
 <87tt18k5y7.fsf@oracle.com>
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On Thu, Sep 11, 2025 at 02:57:52PM -0700, Ankur Arora wrote:
> Catalin Marinas <catalin.marinas@arm.com> writes:
> > On Wed, Sep 10, 2025 at 08:46:50PM -0700, Ankur Arora wrote:
> >> This series adds waited variants of the smp_cond_load() primitives:
> >> smp_cond_load_relaxed_timeout(), and smp_cond_load_acquire_timeout().
> >>
> >> As the name suggests, the new interfaces are meant for contexts where
> >> you want to wait on a condition variable for a finite duration. This
> >> is easy enough to do with a loop around cpu_relax() and a periodic
> >> timeout check (pretty much what we do in poll_idle(). However, some
> >> architectures (ex. arm64) also allow waiting on a cacheline. So,
> >>
> >>   smp_cond_load_relaxed_timeout(ptr, cond_expr, time_check_expr)
> >>   smp_cond_load_acquire_timeout(ptr, cond_expr, time_check_expr)
> >>
> >> do a mixture of spin/wait with a smp_cond_load() thrown in.
> >>
> >> The added parameter, time_check_expr, determines the bail out condition.
> >>
> >> There are two current users for these interfaces. poll_idle() with
> >> the change:
> >>
> >>   poll_idle() {
> >>       ...
> >>       time_end = local_clock_noinstr() + cpuidle_poll_time(drv, dev);
> >>
> >>       raw_local_irq_enable();
> >>       if (!current_set_polling_and_test())
> >>       	 flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
> >>       					(VAL & _TIF_NEED_RESCHED),
> >>       					((local_clock_noinstr() >= time_end)));
> >>       dev->poll_time_limit = !(flags & _TIF_NEED_RESCHED);
> >>       raw_local_irq_disable();
> >>       ...
> >>   }
> >
> > You should have added this as a patch in the series than include the
> > implementation in the cover letter.
> 
> This was probably an overkill but I wanted to not add another subsystem
> to this series.

If you include it, it's easier to poke the cpuidle maintainers and ask
if they are ok with the proposed API as I want to avoid changing it
afterwards. It doesn't mean they'll have to be merged together, they can
go upstream via separate routes.

> Will take care of the cpuidle changes in the arm64 polling in idle series.

Thanks. We also need Will, Peter Z and Arnd to ack the API and the
generic changes (probably once you added the linux/atomic.h changes).

-- 
Catalin