[llvm] [llvm-exegesis] Add support for pinning benchmarking process to a CPU (PR #85168)

[Clement Courbet via llvm-commits]

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@@ -310,6 +321,29 @@ class SubProcessFunctionExecutorImpl
     }
 
     if (ParentOrChildPID == 0) {
+      if (BenchmarkProcessCPU) {
+        // Set the CPU affinity for the child process, so that we ensure that if
+        // the user specified a CPU the process should run on, the benchmarking
+        // process is running on that CPU.
+        cpu_set_t CPUMask;
+        CPU_ZERO(&CPUMask);
+        CPU_SET(*BenchmarkProcessCPU, &CPUMask);
+        // TODO(boomanaiden154): Rewrite this to use LLVM primitives once they
+        // are available.
+        int SetAffinityReturn = sched_setaffinity(0, sizeof(CPUMask), &CPUMask);
+        if (SetAffinityReturn == -1) {
+          exit(ChildProcessExitCodeE::SetCPUAffinityFailed);
+        }
+
+        // Check (if assertions are enabled) that we are actually running on the
+        // CPU that was specified by the user.
+        unsigned int CurrentCPU;
+        assert(getcpu(&CurrentCPU, nullptr) == 0 &&
+               "Expected getcpu call to succeed.");
+        assert(static_cast<int>(CurrentCPU) == *BenchmarkProcessCPU &&
+               "Expected current CPU to equal the CPU requested by the user");
+      }
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legrosbuffle wrote:

Please move this to a separate function.

https://github.com/llvm/llvm-project/pull/85168