linux-kernel - Re: [PATCH 0/2] SMMU v3 CMDQ fix and improvement

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Message-ID: <20251017095031.00000b84@linux.microsoft.com>
Date: Fri, 17 Oct 2025 09:50:31 -0700
From: Jacob Pan <jacob.pan@...ux.microsoft.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Mostafa Saleh <smostafa@...gle.com>, linux-kernel@...r.kernel.org,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, Will Deacon
 <will@...nel.org>, Robin Murphy <robin.murphy@....com>, Nicolin Chen
 <nicolinc@...dia.com>, Zhang Yu <zhangyu1@...ux.microsoft.com>, Jean
 Philippe-Brucker <jean-philippe@...aro.org>, Alexander Grest
 <Alexander.Grest@...rosoft.com>
Subject: Re: [PATCH 0/2] SMMU v3 CMDQ fix and improvement

On Fri, 17 Oct 2025 10:51:45 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Fri, Oct 17, 2025 at 10:57:52AM +0000, Mostafa Saleh wrote:
> > On Wed, Sep 24, 2025 at 10:54:36AM -0700, Jacob Pan wrote:  
> > > Hi Will et al,
> > > 
> > > These two patches are derived from testing SMMU driver with
> > > smaller CMDQ sizes where we see soft lockups.
> > > 
> > > This happens on HyperV emulated SMMU v3 as well as baremetal ARM
> > > servers with artificially reduced queue size and microbenchmark
> > > to stress test concurrency.  
> > 
> > Is it possible to share what are the artificial sizes and does the
> > HW/emulation support range invalidation (IRD3.RIL)?
> > 
> > I'd expect it would be really hard to overwhelm the command queue,
> > unless the HW doesn't support range invalidation and/or the queue
> > entries are close to the number of CPUs.  
> 
> At least on Jacob's system there is no RIL and there are 72/144 CPU
> cores potentially banging on this.
> 
> I think it is combination of lots of required invalidation commands,
> low queue depth and slow retirement of commands that make it easier to
> create a queue full condition.
> 
> Without RIL one SVA invalidation may take out the entire small queue,
> for example.
Right, no range invalidation and queue depth is 256 in this case.