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From:   Kurt Kanzenbach <kurt@linutronix.de>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Jan Kiszka <jan.kiszka@siemens.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Bouska, Zdenek" <zdenek.bouska@siemens.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-rt-users@vger.kernel.org" <linux-rt-users@vger.kernel.org>,
        Nishanth Menon <nm@ti.com>, Puranjay Mohan <p-mohan@ti.com>
Subject: Re: Unfair qspinlocks on ARM64 without LSE atomics => 3ms delay in
 interrupt handling
In-Reply-To: <20230428073016.YfrIGGoN@linutronix.de>
References: <AS1PR10MB567534190B05A4493674173BEB659@AS1PR10MB5675.EURPRD10.PROD.OUTLOOK.COM>
 <87pm7qxrg6.ffs@tglx> <19641ab0-ab6a-9af7-8c64-34030e187848@siemens.com>
 <871qk5782i.fsf@kurt> <20230428073016.YfrIGGoN@linutronix.de>
Date:   Fri, 28 Apr 2023 09:37:03 +0200
Message-ID: <87wn1wv4o0.fsf@kurt>
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On Fri Apr 28 2023, Sebastian Andrzej Siewior wrote:
> On 2023-04-27 15:45:09 [+0200], Kurt Kanzenbach wrote:
>> > Are we aware of other concrete case where it bites? Even with just
>> > "normal" contented spin_lock usage?
>>=20
>> 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.
>
> That was fixed by
> 	https://patchwork.kernel.org/project/linux-arm-kernel/patch/1399528508-2=
806-1-git-send-email-arjun.kv@samsung.com
>
> if my memory serves me well.

We've had a different processor, but yes, we configured CPUACTLR_EL1[31]
to 1 in U-Boot to avoid any kernel modifications. Maybe it helps here
too?

Thanks,
Kurt

[1] - https://developer.arm.com/documentation/100095/0003/way1382037666700

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