Hello Rajesh,
Please verify whether the SM21 entry being raised by the Dispatcher (DP) or by a work process (DIA XX, where XX is the work process number).
Then, access the corresponding trace file (dev_disp or dev_wXX) and locate the timestamp of the SM21 entry.
Paste the trace entries from that timestamp at this thread, so we can try to assist you.
Regards,
Isaías