On 07/03/2024 11:29, Thomas Schwinge wrote:
Hi!
On 2019-11-12T13:29:16+0000, Andrew Stubbs <a...@codesourcery.com> wrote:
This patch contributes the GCN libgomp plugin, with the various
configure and make bits to go with it.
An issue with libgomp GCN plugin 'GCN_SUPPRESS_HOST_FALLBACK' (which is
different from the libgomp-level host-fallback execution):
--- /dev/null
+++ b/libgomp/plugin/plugin-gcn.c
+/* Flag to decide if the runtime should suppress a possible fallback to host
+ execution. */
+
+static bool suppress_host_fallback;
+static void
+init_environment_variables (void)
+{
+ [...]
+ if (secure_getenv ("GCN_SUPPRESS_HOST_FALLBACK"))
+ suppress_host_fallback = true;
+ else
+ suppress_host_fallback = false;
+/* Return true if the HSA runtime can run function FN_PTR. */
+
+bool
+GOMP_OFFLOAD_can_run (void *fn_ptr)
+{
+ struct kernel_info *kernel = (struct kernel_info *) fn_ptr;
+
+ init_kernel (kernel);
+ if (kernel->initialization_failed)
+ goto failure;
+
+ return true;
+
+failure:
+ if (suppress_host_fallback)
+ GOMP_PLUGIN_fatal ("GCN host fallback has been suppressed");
+ GCN_WARNING ("GCN target cannot be launched, doing a host fallback\n");
+ return false;
+}
This originates in the libgomp HSA plugin, where the idea was -- in my
understanding -- that you wouldn't have device code available for all
'fn_ptr's, and in that case transparently (shared-memory system!) do
host-fallback execution. Or, with 'GCN_SUPPRESS_HOST_FALLBACK' set,
you'd get those diagnosed.
This has then been copied into the libgomp GCN plugin (see above).
However, is it really still applicable there; don't we assume that we're
generating device code for all relevant functions? (I suppose everyone
really is testing with 'GCN_SUPPRESS_HOST_FALLBACK' set?) Should we thus
actually remove 'suppress_host_fallback' (that is, make it
always-'true'), including removal of the 'can_run' hook? (I suppose that
even in a future shared-memory "GCN" configuration, we're not expecting
to use this again; expecting always-'true' for 'can_run'?)
Now my actual issue: the libgomp GCN plugin then invented an additional
use of 'GCN_SUPPRESS_HOST_FALLBACK':
+/* Initialize hsa_context if it has not already been done.
+ Return TRUE on success. */
+
+static bool
+init_hsa_context (void)
+{
+ hsa_status_t status;
+ int agent_index = 0;
+
+ if (hsa_context.initialized)
+ return true;
+ init_environment_variables ();
+ if (!init_hsa_runtime_functions ())
+ {
+ GCN_WARNING ("Run-time could not be dynamically opened\n");
+ if (suppress_host_fallback)
+ GOMP_PLUGIN_fatal ("GCN host fallback has been suppressed");
+ return false;
+ }
That is, if 'GCN_SUPPRESS_HOST_FALLBACK' is (routinely) set (for its
original purpose), and you have the libgomp GCN plugin configured, but
don't have 'libhsa-runtime64.so.1' available, you run into a fatal error.
The libgomp nvptx plugin in such cases silently disables the
plugin/device (and thus lets libgomp proper do its thing), and I propose
we do the same here. OK to push the attached
"GCN: Even with 'GCN_SUPPRESS_HOST_FALLBACK' set, failure to
'init_hsa_runtime_functions' is not fatal"?
If you try to run the offload testsuite on a device that is not properly
configured then we want FAIL, not pass-via-fallback. You're breaking that.
Andrew
