linux-kernel - Re: [PATCH v5 0/5] barrier: Add smp_cond_load_*

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Message-ID: <87ikhjhsn5.fsf@oracle.com>
Date: Mon, 15 Sep 2025 22:29:34 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        bpf@...r.kernel.org, arnd@...db.de, will@...nel.org,
        peterz@...radead.org, akpm@...ux-foundation.org, mark.rutland@....com,
        harisokn@...zon.com, cl@...two.org, ast@...nel.org, memxor@...il.com,
        zhenglifeng1@...wei.com, xueshuai@...ux.alibaba.com,
        joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v5 0/5] barrier: Add smp_cond_load_*_timeout()


Catalin Marinas <catalin.marinas@....com> writes:

> On Thu, Sep 11, 2025 at 02:57:52PM -0700, Ankur Arora wrote:
>> Catalin Marinas <catalin.marinas@....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.

That makes a lot of sense. Will include this patch as well.

>> 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).

Makes sense.


Thanks

--
ankur