linux-kernel - Re: [PATCH RFC 09/13] x86/irq: Install posted MSI notification handler

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Date:   Wed, 15 Nov 2023 21:25:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 Kernel <x86@...nel.org>,
        iommu@...ts.linux.dev, Thomas Gleixner <tglx@...utronix.de>,
        Lu Baolu <baolu.lu@...ux.intel.com>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, maz@...nel.org,
        seanjc@...gle.com, Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH RFC 09/13] x86/irq: Install posted MSI notification
 handler

On Wed, Nov 15, 2023 at 12:04:01PM -0800, Jacob Pan wrote:

> we are interleaving cacheline read and xchg. So made it to

Hmm, I wasn't expecting that to be a problem, but sure.

> 	for (i = 0; i < 4; i++) {
> 		pir_copy[i] = pid->pir_l[i];
> 	}
> 
> 	for (i = 0; i < 4; i++) {
> 		if (pir_copy[i]) {
> 			pir_copy[i] = arch_xchg(&pid->pir_l[i], 0);
> 			handled = true;
> 		}
> 	}
> 
> With DSA MEMFILL test just one queue one MSI, we are saving 3 xchg per loop.
> Here is the performance comparison in IRQ rate:
> 
> Original RFC 9.29 m/sec, 
> Optimized in your email 8.82m/sec,
> Tweaked above: 9.54m/s
> 
> I need to test with more MSI vectors spreading out to all 4 u64. I suspect
> the benefit will decrease since we need to do both read and xchg for
> non-zero entries.

Ah, but performance was not the reason I suggested this. Code
compactness and clarity was.

Possibly using less xchg is just a bonus :-)