linux-kernel - Re: [PATCH 1/2] mm: export find_extend_vma and handle_mm

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Message-ID: <20141106130122.GJ8354@suse.de>
Date:	Thu, 6 Nov 2014 14:01:22 +0100
From:	Joerg Roedel <jroedel@...e.de>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Oded Gabbay <oded.gabbay@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: export find_extend_vma and handle_mm_fault for
 driver use

On Wed, Nov 05, 2014 at 01:51:09PM -0800, Jesse Barnes wrote:
> On Wed, 5 Nov 2014 13:03:51 +0100
> Joerg Roedel <jroedel@...e.de> wrote:
> > Thanks for testing Oded. Jesse, the patch looks good to me, except the
> > task accounting for the page-faults. I'd like to get rid of using
> > task_struct in the IOMMUv2 driver entirely if possible. Also it is not
> > really the CPU task causing the faults, but some non-CPU process.
> 
> Hm, but the CPU task initiates the activity on the GPU, so we should
> account for it somewhere, right?  I guess I had been thinking of the
> "task" as spanning the CPUs and GPUs and other devices in the system,
> rather than just representing the CPU activity.

One problem is that the task that called amd_iommu_bind_pasid() isn't
necessarily the same task (thread) that queues the jobs to the device.
The thread that called amd_iommu_bind_pasid() might even exit while
other threads still use the mappings.

Besides that, from an abstract point of view, what is running on the
device (GPU) is a logically seperate 'thread' of the process which we
should account for seperatly. If we want accounting for these off-CPU
threads we should probably introduce some concept of a non-CPU task to
the kernel and do the accounting there?

	Joerg

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