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

[Nicolin Chen]

On Thu, Jan 25, 2024 at 01:47:28PM -0400, Jason Gunthorpe wrote:
> 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..

I mean your arm_smmu_inv_range_too_big patch. Should it be a bug
fix CCing the stable tree? My previous SVA fix was, by the way.

> > > > > 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?

The bottleneck from measurement is mostly at SMMU consuming the
commands with a single CMDQ HW, so multithreading unlikely helps.
And VCMDQ only provides a multi-queue interface/wrapper for VM
isolations.

Thanks
Nic