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

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
Date: Fri, 28 Apr 2023 09:37:03 +0200	[thread overview]
Message-ID: <87wn1wv4o0.fsf@kurt> (raw)
In-Reply-To: <20230428073016.YfrIGGoN@linutronix.de>

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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?
>> 
>> 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/1399528508-2806-1-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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