gpu-paging: Allow for more than one buffer to be swapped at a timertss22-aegpu-paging

This uses a very primitive linear disk sector allocation scheme.
Sectors are only reused when userspace resets assignment to 0 with
an NVGPU_AS_IOCTL_SWAP_RESET ioctl (which invalidates all current
swap buffers).

This sector assignment scheme is sufficient for use in a TimeWall-
like system, where all allocations are assumed to be static after
after task system release. This is not suitable for a system with
dynamic allocations, unless userspace manually resets swap state
regularly (benchmarks run a reset at start for example).

Support for dynamic allocations is on the backlog.

No significant speed impact.

Benchmarks, 100 iters, after:
gpu_paging_speed, write: 186.0ms +/- 3.51
gpu_paging_speed, read: 162.7ms +/- 2.58
gpu_paging_overhead_speed, write start: 35.4ms +/- 4.47
gpu_paging_overhead_speed, write finish: 3.3ms +/- 0.18
gpu_paging_overhead_speed, read start: 69.8ms +/- 6.42
gpu_paging_overhead_speed, read finish: 43.2ms +/- 0.91

1 files changed, 8 insertions, 0 deletions

diff --git a/drivers/gpu/nvgpu/os/linux/ioctl_as.c b/drivers/gpu/nvgpu/os/linux/ioctl_as.c
index 6348bb2a..2bf8363a 100644
--- a/drivers/gpu/nvgpu/os/linux/ioctl_as.c
+++ b/drivers/gpu/nvgpu/os/linux/ioctl_as.c
@@ -672,6 +672,14 @@ long gk20a_as_dev_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
                 err = nvgpu_as_dev_ioctl_swap(cmd, as_share,
                         (struct nvgpu_as_swap_buffer_args *)buf);
                 break;
+        case NVGPU_AS_IOCTL_SWAP_RESET:
+                // On-disk sector assignment is linear currently, and needs to
+                // be reset to the start between task systems to avoid disk
+                // space exhaustion.
+                // TODO: Support garbage-collection- or callback-driven sector
+                //       reclaiming rather than requiring manual reset.
+                atomic64_set(&nvgpu_swap_next_sector, 4);
+                break;
         default:
                 err = -ENOTTY;
                 break;