[Disco] Set worker CPU affinity with env variable

This PR enables setting the CPU affinity of disco workers in
MLC, following the support in apache/tvm#16807. The purpose is
to try reduce the CPU core switch overhead brought to disco workers
which may cause extra bubble times in disco workers before/during
tasks.

We use a macro `MLC_DISCO_WORKER_CPU_BINDING` to specify the CPU
affinities of workers. This is by default not used. To enable it,
you can run the command like

```shell
MLC_DISCO_WORKER_CPU_BINDING=64,65,66,67 python some_mlc_app.py
```

to specify the four CPU core ids for the four workers.

cpp/serve/function_table.cc

@@ -13,17 +13,44 @@
 #include <tvm/runtime/packed_func.h>
 #include <tvm/runtime/registry.h>
 
+#include <cstdlib>
 #include <filesystem>
 #include <string>
 #include <vector>
 
 #include "../support/load_bytes_from_file.h"
+#include "../support/utils.h"
 #include "sampler/sampler.h"
 
 namespace mlc {
 namespace llm {
 namespace serve {
 
+Optional<IntTuple> GetDiscoWorkerCPUBinding(int num_workers) {
+  const char* raw_cpu_binding = std::getenv("MLC_DISCO_WORKER_CPU_BINDING");
+  if (raw_cpu_binding == nullptr) {
+    return NullOpt;
+  }
+
+  std::string cpu_binding_str(raw_cpu_binding);
+  std::vector<std::string> cpu_ids_str = Split(cpu_binding_str, ',');
+  std::vector<int64_t> cpu_ids;
+  for (const std::string& cpu_id_str : cpu_ids_str) {
+    try {
+      cpu_ids.push_back(std::stol(cpu_id_str));
+    } catch (std::invalid_argument const& ex) {
+      LOG(FATAL) << "Invalid MLC_DISCO_WORKER_CPU_BINDING \"" << cpu_binding_str << "\"";
+    }
+  }
+  if (static_cast<int>(cpu_ids.size()) < num_workers) {
+    LOG(FATAL) << "Insufficient number of specified CPU workers in MLC_DISCO_WORKER_CPU_BINDING, "
+                  "expecting at least "
+               << num_workers << "CPU ids but only " << cpu_ids.size() << " are given.";
+  }
+
+  return IntTuple{cpu_ids};
+}
+
 PackedFunc FunctionTable::SessionFuncAsPackedFunc(Session sess, DRef sess_func, String name) {
   return PackedFunc([sess, func = std::move(sess_func), name = std::move(name)](
                         TVMArgs args, TVMRetValue* rv) -> void {
@@ -100,6 +127,10 @@ void FunctionTable::Init(TVMArgValue reload_lib, Device device, picojson::object
       }
       return SessionFuncAsPackedFunc(sess, func, name);
     };
+    if (Optional<IntTuple> cpu_ids = GetDiscoWorkerCPUBinding(/*num_workers=*/num_shards)) {
+      IntTuple cpu_ids_value = cpu_ids.value();
+      sess->CallPacked(sess->GetGlobalFunc("runtime.disco.bind_worker_to_cpu_core"), cpu_ids_value);
+    }
     this->get_global_func = [this](const std::string& name) -> PackedFunc {
       return SessionFuncAsPackedFunc(sess, sess->GetGlobalFunc(name), name);
     };

cpp/support/utils.h

@@ -0,0 +1,24 @@
+/*!
+ * Copyright (c) 2023 by Contributors
+ * \file utils.h
+ * \brief Utility functions.
+ */
+#include <sstream>
+#include <string>
+#include <vector>
+
+namespace mlc {
+namespace llm {
+
+inline std::vector<std::string> Split(const std::string& str, char delim) {
+  std::string item;
+  std::istringstream is(str);
+  std::vector<std::string> ret;
+  while (std::getline(is, item, delim)) {
+    ret.push_back(item);
+  }
+  return ret;
+}
+
+}  // namespace llm
+}  // namespace mlc