Allow creation of a new sched::thread pinned to a specific CPU.

Previously, we had the option to create a pinned thread, but it always
runs on the same CPU as the current thread, which is kind of odd. Changed
the boolean attribute "pinned" to a cpu* attribute specifying the cpu to
pin to.

Example code to run a start a new thread pinned on cpu 1:
new sched::thread([&]{...}, sched::thread::attr(sched::cpus[1]));

I need this feature to test the cross-CPU TLB flushing feature - I need
to be able to run two threads on two different CPUs.

arch/x64/smp.cc

@@ -100,15 +100,14 @@ void smp_launch()
         if (c == boot_cpu) {
             sched::thread::current()->_cpu = c;
             c->init_on_cpu();
-            sched::thread::attr attr;
-            attr.pinned = true;
-            (new sched::thread([c] { c->load_balance(); }, attr))->start();
+            (new sched::thread([c] { c->load_balance(); },
+                    sched::thread::attr(c)))->start();
             c->idle_thread.start();
             continue;
         }
         sched::thread::attr attr;
         attr.stack = { new char[81920], 81920 };
-        attr.pinned = true;
+        attr.pinned_cpu = c;
         c->bringup_thread = new sched::thread([=] { ap_bringup(c); }, attr, true);
     }
     apic->write(apicreg::ICR, 0xc4500); // INIT

core/sched.cc

@@ -38,14 +38,9 @@ constexpr u64 max_slice = 10_ms;
 
 namespace sched {
 
-void schedule_force();
-
-const thread::attr idle_thread_attr(true);
-
 cpu::cpu()
-    : idle_thread([this] { idle(); }, idle_thread_attr)
+    : idle_thread([this] { idle(); }, thread::attr(this))
 {
-    idle_thread._cpu = this;
 }
 
 void cpu::schedule(bool yield)
@@ -191,7 +186,7 @@ void cpu::load_balance()
         }
         with_lock(irq_lock, [this, min] {
             auto i = std::find_if(runqueue.rbegin(), runqueue.rend(),
-                    [](thread& t) { return !t._attr.pinned; });
+                    [](thread& t) { return !t._attr.pinned_cpu; });
             if (i == runqueue.rend()) {
                 return;
             }
@@ -293,7 +288,7 @@ thread::~thread()
 
 void thread::start()
 {
-    _cpu = current()->tcpu();
+    _cpu = _attr.pinned_cpu ? _attr.pinned_cpu : current()->tcpu();
     assert(_status == status::unstarted);
     _status.store(status::waiting);
     wake();

include/sched.hh

@@ -142,8 +142,8 @@ public:
     };
     struct attr {
         stack_info stack;
-        bool pinned;
-        attr(bool pinned = false) : pinned(pinned) { }
+        cpu *pinned_cpu;
+        attr(cpu *pinned_cpu = nullptr) : pinned_cpu(pinned_cpu) { }
     };
 
 public: