Speed up nvtaskset by skipping CUDA context creation if possible

The GPU needs to be on before the GPC-to-TPC mapping registers can
be read. The easiest way to power on the GPU is to create a CUDA
context, but this is fairly expensive.

In nvtaskset, some GPU-using task will likely already be running,
and so we can skip CUDA context creation in the common case.

Bug fixes:
- Delete the temporary CUDA context created by nvtaskset after
  it is done with it. Fixes bug where nvtaskset would leak this
  context into any program it launches.

Additional changes:
- Style fixes in libsmctrl.c
- Remove superfluous newlines from error() calls in nvtaskset.c

1 files changed, 27 insertions, 2 deletions

diff --git a/nvtaskset.c b/nvtaskset.c
index 4901cbe..5cf3a85 100644
--- a/nvtaskset.c
+++ b/nvtaskset.c
@@ -68,6 +68,25 @@ void libsmctrl_get_gpc_info_ext_easy(uint32_t* num_gpcs, uint128_t** masks, int
         int res;
         CUcontext ctx;
         char *old_order = NULL;
+        int old_stderr, dev_null_fd;
+
+        // Attempt to read the configuration, assuming the GPU is on, and fall
+        // back to creating a context if this fails.
+        // (Creating a CUDA context is very expensive and best avoided)
+        // (Redirect stderr while doing this to mute libsmctrl error messages)
+        if ((dev_null_fd = open("/dev/null", O_WRONLY)) == -1)
+                error(1, errno, "Unable to open /dev/null");
+        if (old_stderr = dup(STDERR_FILENO) == -1)
+                error(1, errno, "Unable to duplicate stderr file descriptor");
+        if (dup2(dev_null_fd, STDERR_FILENO) == -1)
+                error(1, errno, "Unable to overwrite stderr file descriptor");
+        res = libsmctrl_get_gpc_info_ext(num_gpcs, masks, gpu_id);
+        if (dup2(old_stderr, STDERR_FILENO) == -1)
+                error(1, errno, "Unable to restore stderr file descriptor");
+        // End if we were successful, otherwise fallback
+        if (res == 0)
+                return;
+
         // Tell CUDA to use PCI device id ordering (to match nvdebug)
         putenv((char*)"CUDA_DEVICE_ORDER=PCI_BUS_ID");
         // Allow CUDA to see all devices (to better match nvdebug)
@@ -80,12 +99,12 @@ void libsmctrl_get_gpc_info_ext_easy(uint32_t* num_gpcs, uint128_t** masks, int
         if ((res = cuInit(0))) {
                 const char* name;
                 cuGetErrorName(res, &name);
-                error(1, 0, "Unable to create a initialize CUDA, error %s\n", name);
+                error(1, 0, "Unable to create a initialize CUDA, error %s", name);
         }
         if ((res = cuCtxCreate(&ctx, 0, gpu_id))) {
                 const char* name;
                 cuGetErrorName(res, &name);
-                error(1, 0, "Unable to create a CUDA context, error %s\n", name);
+                error(1, 0, "Unable to create a CUDA context, error %s", name);
         }
         // Pull topology information from libsmctrl
         if ((res = libsmctrl_get_gpc_info_ext(num_gpcs, masks, gpu_id)) != 0) {
@@ -96,6 +115,12 @@ void libsmctrl_get_gpc_info_ext_easy(uint32_t* num_gpcs, uint128_t** masks, int
                         fprintf(stderr, "%s: Is the GPU powered on, i.e., is there an active context?\n", program_invocation_name);
                 exit(1);
         }
+        // Delete the CUDA context
+        if (res = cuCtxDestroy(ctx)) {
+                const char* name;
+                cuGetErrorName(res, &name);
+                error(1, 0, "Unable to destroy CUDA context, error %s", name);
+        }
         // Restore the environment (in case we exec() later)
         unsetenv("CUDA_DEVICE_ORDER");
         if (old_order) {