sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 196 insertions, 0 deletions

diff --git a/tools/perf/util/llvm-c-helpers.cpp b/tools/perf/util/llvm-c-helpers.cpp
new file mode 100644
index 000000000..004081bd1
--- /dev/null
+++ b/tools/perf/util/llvm-c-helpers.cpp
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Must come before the linux/compiler.h include, which defines several
+ * macros (e.g. noinline) that conflict with compiler builtins used
+ * by LLVM.
+ */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-parameter"  /* Needed for LLVM <= 15 */
+#include <llvm/DebugInfo/Symbolize/Symbolize.h>
+#include <llvm/Support/TargetSelect.h>
+#pragma GCC diagnostic pop
+
+#include <inttypes.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <linux/compiler.h>
+extern "C" {
+#include <linux/zalloc.h>
+}
+#include "llvm-c-helpers.h"
+
+extern "C"
+char *dso__demangle_sym(struct dso *dso, int kmodule, const char *elf_name);
+
+using namespace llvm;
+using llvm::symbolize::LLVMSymbolizer;
+
+/*
+ * Allocate a static LLVMSymbolizer, which will live to the end of the program.
+ * Unlike the bfd paths, LLVMSymbolizer has its own cache, so we do not need
+ * to store anything in the dso struct.
+ */
+static LLVMSymbolizer *get_symbolizer()
+{
+	static LLVMSymbolizer *instance = nullptr;
+	if (instance == nullptr) {
+		LLVMSymbolizer::Options opts;
+		/*
+		 * LLVM sometimes demangles slightly different from the rest
+		 * of the code, and this mismatch can cause new_inline_sym()
+		 * to get confused and mark non-inline symbol as inlined
+		 * (since the name does not properly match up with base_sym).
+		 * Thus, disable the demangling and let the rest of the code
+		 * handle it.
+		 */
+		opts.Demangle = false;
+		instance = new LLVMSymbolizer(opts);
+	}
+	return instance;
+}
+
+/* Returns 0 on error, 1 on success. */
+static int extract_file_and_line(const DILineInfo &line_info, char **file,
+				 unsigned int *line)
+{
+	if (file) {
+		if (line_info.FileName == "<invalid>") {
+			/* Match the convention of libbfd. */
+			*file = nullptr;
+		} else {
+			/* The caller expects to get something it can free(). */
+			*file = strdup(line_info.FileName.c_str());
+			if (*file == nullptr)
+				return 0;
+		}
+	}
+	if (line)
+		*line = line_info.Line;
+	return 1;
+}
+
+extern "C"
+int llvm_addr2line(const char *dso_name, u64 addr,
+		   char **file, unsigned int *line,
+		   bool unwind_inlines,
+		   llvm_a2l_frame **inline_frames)
+{
+	LLVMSymbolizer *symbolizer = get_symbolizer();
+	object::SectionedAddress sectioned_addr = {
+		addr,
+		object::SectionedAddress::UndefSection
+	};
+
+	if (unwind_inlines) {
+		Expected<DIInliningInfo> res_or_err =
+			symbolizer->symbolizeInlinedCode(dso_name,
+							 sectioned_addr);
+		if (!res_or_err)
+			return 0;
+		unsigned num_frames = res_or_err->getNumberOfFrames();
+		if (num_frames == 0)
+			return 0;
+
+		if (extract_file_and_line(res_or_err->getFrame(0),
+					  file, line) == 0)
+			return 0;
+
+		*inline_frames = (llvm_a2l_frame *)calloc(
+			num_frames, sizeof(**inline_frames));
+		if (*inline_frames == nullptr)
+			return 0;
+
+		for (unsigned i = 0; i < num_frames; ++i) {
+			const DILineInfo &src = res_or_err->getFrame(i);
+
+			llvm_a2l_frame &dst = (*inline_frames)[i];
+			if (src.FileName == "<invalid>")
+				/* Match the convention of libbfd. */
+				dst.filename = nullptr;
+			else
+				dst.filename = strdup(src.FileName.c_str());
+			dst.funcname = strdup(src.FunctionName.c_str());
+			dst.line = src.Line;
+
+			if (dst.filename == nullptr ||
+			    dst.funcname == nullptr) {
+				for (unsigned j = 0; j <= i; ++j) {
+					zfree(&(*inline_frames)[j].filename);
+					zfree(&(*inline_frames)[j].funcname);
+				}
+				zfree(inline_frames);
+				return 0;
+			}
+		}
+
+		return num_frames;
+	} else {
+		if (inline_frames)
+			*inline_frames = nullptr;
+
+		Expected<DILineInfo> res_or_err =
+			symbolizer->symbolizeCode(dso_name, sectioned_addr);
+		if (!res_or_err)
+			return 0;
+		return extract_file_and_line(*res_or_err, file, line);
+	}
+}
+
+static char *
+make_symbol_relative_string(struct dso *dso, const char *sym_name,
+			    u64 addr, u64 base_addr)
+{
+	if (!strcmp(sym_name, "<invalid>"))
+		return NULL;
+
+	char *demangled = dso__demangle_sym(dso, 0, sym_name);
+	if (base_addr && base_addr != addr) {
+		char buf[256];
+		snprintf(buf, sizeof(buf), "%s+0x%" PRIx64,
+			 demangled ? demangled : sym_name, addr - base_addr);
+		free(demangled);
+		return strdup(buf);
+	} else {
+		if (demangled)
+			return demangled;
+		else
+			return strdup(sym_name);
+	}
+}
+
+extern "C"
+char *llvm_name_for_code(struct dso *dso, const char *dso_name, u64 addr)
+{
+	LLVMSymbolizer *symbolizer = get_symbolizer();
+	object::SectionedAddress sectioned_addr = {
+		addr,
+		object::SectionedAddress::UndefSection
+	};
+	Expected<DILineInfo> res_or_err =
+		symbolizer->symbolizeCode(dso_name, sectioned_addr);
+	if (!res_or_err) {
+		return NULL;
+	}
+	return make_symbol_relative_string(
+		dso, res_or_err->FunctionName.c_str(),
+		addr, res_or_err->StartAddress ? *res_or_err->StartAddress : 0);
+}
+
+extern "C"
+char *llvm_name_for_data(struct dso *dso, const char *dso_name, u64 addr)
+{
+	LLVMSymbolizer *symbolizer = get_symbolizer();
+	object::SectionedAddress sectioned_addr = {
+		addr,
+		object::SectionedAddress::UndefSection
+	};
+	Expected<DIGlobal> res_or_err =
+		symbolizer->symbolizeData(dso_name, sectioned_addr);
+	if (!res_or_err) {
+		return NULL;
+	}
+	return make_symbol_relative_string(
+		dso, res_or_err->Name.c_str(),
+		addr, res_or_err->Start);
+}