argdist: Fix -p behavior to filter tgid and not pid

argdist remained one of the last holdouts to use the `-p` switch
inconsistently with other tools, filtering for kernel pid (thread
id from user space perspective) and not kernel tgid (process id
from user space perspective). This is now fixed.

Additionally, minor nits around generating pid filters were fixed,
and a potential collision with user-provided argument names was
fixed too (in general, script-generated arguments/locals should
probably stick to reserved identifiers, such as `__whatever` rather
than `whatever`).

tools/argdist.py

@@ -20,7 +20,7 @@ import sys
 class Probe(object):
         next_probe_index = 0
         streq_index = 0
-        aliases = {"$PID": "bpf_get_current_pid_tgid()"}
+        aliases = {"$PID": "(bpf_get_current_pid_tgid() >> 32)"}
 
         def _substitute_aliases(self, expr):
                 if expr is None:
@@ -47,7 +47,9 @@ class Probe(object):
                 text = """
 int PROBENAME(struct pt_regs *ctx SIGNATURE)
 {
-        u32 pid = bpf_get_current_pid_tgid();
+        u64 __pid_tgid = bpf_get_current_pid_tgid();
+        u32 __pid      = __pid_tgid;        // lower 32 bits
+        u32 __tgid     = __pid_tgid >> 32;  // upper 32 bits
         PID_FILTER
         COLLECT
         return 0;
@@ -56,19 +58,17 @@ int PROBENAME(struct pt_regs *ctx SIGNATURE)
                 text = text.replace("PROBENAME", self.entry_probe_func)
                 text = text.replace("SIGNATURE",
                      "" if len(self.signature) == 0 else ", " + self.signature)
-                pid_filter = "" if self.is_user or self.pid is None \
-                                else "if (pid != %d) { return 0; }" % self.pid
-                text = text.replace("PID_FILTER", pid_filter)
+                text = text.replace("PID_FILTER", self._generate_pid_filter())
                 collect = ""
                 for pname in self.args_to_probe:
                         param_hash = self.hashname_prefix + pname
                         if pname == "__latency":
                                 collect += """
 u64 __time = bpf_ktime_get_ns();
-%s.update(&pid, &__time);
+%s.update(&__pid, &__time);
                         """ % param_hash
                         else:
-                                collect += "%s.update(&pid, &%s);\n" % \
+                                collect += "%s.update(&__pid, &%s);\n" % \
                                            (param_hash, pname)
                 text = text.replace("COLLECT", collect)
                 return text
@@ -108,7 +108,7 @@ u64 __time = bpf_ktime_get_ns();
                 # argument we needed to probe using $entry(name), and they all
                 # have values (which isn't necessarily the case if we missed
                 # the method entry probe).
-                text = "u32 __pid = bpf_get_current_pid_tgid();\n"
+                text = ""
                 self.param_val_names = {}
                 for pname in self.args_to_probe:
                         val_name = "__%s_val" % pname
@@ -345,8 +345,7 @@ static inline bool %s(char const *ignored, char const *str) {
                 # Kernel probes need to explicitly filter pid, because the
                 # attach interface doesn't support pid filtering
                 if self.pid is not None and not self.is_user:
-                        return "u32 pid = bpf_get_current_pid_tgid();\n" + \
-                               "if (pid != %d) { return 0; }" % self.pid
+                        return "if (__tgid != %d) { return 0; }" % self.pid
                 else:
                         return ""
 
@@ -360,6 +359,9 @@ DATA_DECL
                     if self.probe_type == "t"
                     else "int PROBENAME(struct pt_regs *ctx SIGNATURE)") + """
 {
+        u64 __pid_tgid = bpf_get_current_pid_tgid();
+        u32 __pid      = __pid_tgid;        // lower 32 bits
+        u32 __tgid     = __pid_tgid >> 32;  // upper 32 bits
         PID_FILTER
         PREFIX
         if (!(FILTER)) return 0;