linux-kernel - Re: [PATCH 1/3] iommu/io-pgtable-arm: Add nents_per_pgtable in struct io_pgtable

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Date: Thu, 25 Jan 2024 13:47:28 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: "will@...nel.org" <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	"joro@...tes.org" <joro@...tes.org>,
	"jean-philippe@...aro.org" <jean-philippe@...aro.org>,
	Alistair Popple <apopple@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: Re: [PATCH 1/3] iommu/io-pgtable-arm: Add nents_per_pgtable in
 struct io_pgtable_cfg

On Thu, Jan 25, 2024 at 09:23:00AM -0800, Nicolin Chen wrote:
> > When the soft lockup issue is solved you can consider if a tunable is
> > still interesting..
> 
> Yea, it would be on top of the soft lockup fix. I assume we are
> still going with your change: arm_smmu_inv_range_too_big, though
> I wonder if we should apply before your rework series to make it
> a bug fix..

It depends what change you settle on..

> > > > Maybe it is really just a simple thing - compute how many invalidation
> > > > commands are needed, if they don't all fit in the current queue space,
> > > > then do an invalidate all instead?
> > > 
> > > The queue could actually have a large space. But one large-size
> > > invalidation would be divided into batches that have to execute
> > > back-to-back. And the batch size is 64 commands in 64-bit case,
> > > which might be too small as a cap.
> > 
> > Yes, some notable code reorganizing would be needed to implement
> > something like this
> > 
> > Broadly I'd sketch sort of:
> > 
> >  - Figure out how fast the HW can execute a lot of commands
> >  - The above should drive some XX maximum number of commands, maybe we
> >    need to measure at boot, IDK
> >  - Strongly time bound SVA invalidation:
> >    * No more than XX commands, if more needed then push invalidate
> >      all
> >    * All commands must fit in the available queue space, if more
> >      needed then push invalidate all
> >  - The total queue depth must not be larger than YY based on the
> >    retire rate so that even a full queue will complete invalidation
> >    below the target time.
> > 
> > A tunable indicating what the SVA time bound target should be might be
> > appropriate..
> 
> Thanks for listing it out. I will draft something with that, and
> should we just confine it to SVA or non DMA callers in general?

Also, how much of this SVA issue is multithreaded? Will multiple
command queues improve anything?

Jason