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

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Message-ID: <20230427100656.GA10855@willie-the-truck>
Date:   Thu, 27 Apr 2023 11:06:57 +0100
From:   Will Deacon <will@...nel.org>
To:     "Bouska, Zdenek" <zdenek.bouska@...mens.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Kiszka, Jan" <jan.kiszka@...mens.com>,
        "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 at 09:38:59AM +0000, Bouska, Zdenek wrote:
> > Why is this interrupt handling specific? Just because it's the place
> > where you observed it?
> Yes.
> 
> > So if that helps, then this needs to be addressed globaly and not with
> > some crude hack in the interrupt handling code.
> I just wrote, what helps for me. I didn't mean it as a proposal for merge.
> Sorry for confusion.
> 
> I tried using Will's cpu_relax() implementation [1] everywhere but I was not 
> successful with that yet. ARM64's VDSO makes it complicating and even
> if I left original cpu_relax() just in VDSO, Linux did not boot for me.

It definitely seemed to work when I posted it all those years ago, but I'm
not surprised if it needs some TLC to revive it on more recent kernels.

Will