Set SCHED_BATCH priority on the loadblk thread.

While reading another PR I saw a mention of #6358. The use case for
SCHED_BATCH is to hint to the kernel that the thread is running a
non-interactive workload that consumes a lot of CPU time. This is
helpful on desktop machines where the loadblk thread can interfere with
interactive applications. More details can be found in the sched(7) man
page.

src/init.cpp

@@ -625,6 +625,7 @@ void ThreadImport(std::vector<fs::path> vImportFiles)
 {
     const CChainParams& chainparams = Params();
     RenameThread("bitcoin-loadblk");
+    ScheduleBatchPriority();
 
     {
     CImportingNow imp;

src/util.cpp

@@ -31,6 +31,7 @@
 
 #include <algorithm>
 #include <fcntl.h>
+#include <sched.h>
 #include <sys/resource.h>
 #include <sys/stat.h>
 
@@ -966,3 +967,17 @@ fs::path AbsPathForConfigVal(const fs::path& path, bool net_specific)
 {
     return fs::absolute(path, GetDataDir(net_specific));
 }
+
+int ScheduleBatchPriority(void)
+{
+#ifdef SCHED_BATCH
+    const static sched_param param{.sched_priority = 0};
+    if (int ret = pthread_setschedparam(0, SCHED_BATCH, &param)) {
+        LogPrintf("Failed to pthread_setschedparam: %s\n", strerror(errno));
+        return ret;
+    }
+    return 0;
+#else
+    return 1;
+#endif
+}

src/util.h

@@ -357,4 +357,13 @@ std::unique_ptr<T> MakeUnique(Args&&... args)
     return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
 }
 
+/**
+ * On platforms that support it, tell the kernel the calling thread is
+ * CPU-intensive and non-interactive. See SCHED_BATCH in sched(7) for details.
+ *
+ * @return The return value of sched_setschedule(), or 1 on systems without
+ * sched_setchedule().
+ */
+int ScheduleBatchPriority(void);
+
 #endif // BITCOIN_UTIL_H