linux-kernel - Re: Unfair qspinlocks on ARM64 without LSE atomics => 3ms delay in interrupt handling

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Date:   Thu, 27 Apr 2023 15:45:09 +0200
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Jan Kiszka <jan.kiszka@...mens.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Bouska, Zdenek" <zdenek.bouska@...mens.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>,
        Nishanth Menon <nm@...com>, Puranjay Mohan <p-mohan@...com>
Subject: Re: Unfair qspinlocks on ARM64 without LSE atomics => 3ms delay in
 interrupt handling

On Thu Apr 27 2023, Jan Kiszka wrote:
> On 26.04.23 23:29, Thomas Gleixner wrote:
>> On Wed, Apr 26 2023 at 12:03, Zdenek Bouska wrote:
>>> following patch is my current approach for fixing this issue. I introduced
>>> big_cpu_relax(), which uses Will's implementation [1] on ARM64 without
>>> LSE atomics and original cpu_relax() on any other CPU.
>> 
>> Why is this interrupt handling specific? Just because it's the place
>> where you observed it?
>> 
>> That's a general issue for any code which uses atomics for forward
>> progress. LL/SC simply does not guarantee that.
>> 
>> So if that helps, then this needs to be addressed globaly and not with
>> some crude hack in the interrupt handling code.
>
> My impression is that the retry loop of irq_finalize_oneshot is
> particularly susceptible to that issue due to the high acquire/relax
> pressure and inter-dependency between holder and waiter it generates -
> which does not mean it cannot occur in other places.
>
> Are we aware of other concrete case where it bites? Even with just
> "normal" contented spin_lock usage?

Well, some years ago I've observed a similar problem with ARM64
spinlocks, cpu_relax() and retry loops (in the futex code). It also
generated latency spikes up to 2-3ms. Back then, it was easily
reproducible using stress-ng --ptrace 4.

Thanks,
Kurt

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