diff --git a/processcore/processes_linux_p.cpp b/processcore/processes_linux_p.cpp
--- a/processcore/processes_linux_p.cpp
+++ b/processcore/processes_linux_p.cpp
@@ -29,6 +29,7 @@
 #include <QSet>
 #include <QByteArray>
 #include <QTextStream>
+#include <QDir>
 
 //for sysconf
 #include <unistd.h>
@@ -630,9 +631,11 @@
         errorCode = Processes::InvalidPid;
         return false;
     }
-    if (setpriority( PRIO_PROCESS, pid, priority )) {
-        switch (errno) {
+    auto error =
+        [this] {
+            switch (errno) {
             case ESRCH:
+            case ENOENT:
                 errorCode = Processes::ProcessDoesNotExistOrZombie;
                 break;
             case EINVAL:
@@ -644,10 +647,25 @@
                 break;
             default:
                 break;
-        }
-        //set niceness failed
+            }
+        };
+    auto threadList { QDir(QString::fromLatin1("/proc/%1/task").arg(pid)).
+                     entryList( QDir::NoDotAndDotDot | QDir::Dirs ) };
+    if (threadList.isEmpty()) {
+        error();
         return false;
     }
+    for (auto entry : threadList) {
+        int threadId = entry.toInt();
+        if (!threadId) {
+            errorCode = Processes::InvalidParameter;
+            return false;
+        }
+        if (setpriority( PRIO_PROCESS, threadId, priority )) {
+            error();
+            return false;
+        }
+    }
     return true;
 }
 
@@ -687,22 +705,41 @@
           return false;
     }
 
-    if (sched_setscheduler( pid, policy, &params) != 0) {
-        switch (errno) {
+    auto error =
+        [this] {
+            switch (errno) {
             case ESRCH:
+            case ENOENT:
                 errorCode = Processes::ProcessDoesNotExistOrZombie;
                 break;
             case EINVAL:
                 errorCode = Processes::InvalidParameter;
                 break;
+            case EACCES:
             case EPERM:
                 errorCode = Processes::InsufficientPermissions;
                 break;
             default:
                 break;
-        }
+            }
+        };
+    auto threadList { QDir(QString::fromLatin1("/proc/%1/task").arg(pid)).
+                     entryList( QDir::NoDotAndDotDot | QDir::Dirs ) };
+    if (threadList.isEmpty()) {
+        error();
         return false;
     }
+    for (auto entry : threadList) {
+        int threadId = entry.toInt();
+        if (!threadId) {
+            errorCode = Processes::InvalidParameter;
+            return false;
+        }
+        if (sched_setscheduler( pid, policy, &params) != 0) {
+            error();
+            return false;
+        }
+    }
     return true;
 }