s390/smp: save timestamp on external calls

This is supposed to make debugging easier: if within a dump we can see
that an external call or emergency signal IPI is pending but all cpus
are idle, we have no idea for how long the interrupt is outstanding.

Therefore save a timestamp into the per cpu pcpu array of the target
cpu whenever such an IPI is sent.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/smp.c

@@ -66,6 +66,7 @@ static DEFINE_PER_CPU(struct cpu *, cpu_device);
 struct pcpu {
 	struct _lowcore *lowcore;	/* lowcore page(s) for the cpu */
 	unsigned long ec_mask;		/* bit mask for ec_xxx functions */
+	unsigned long ec_clk;		/* sigp timestamp for ec_xxx */
 	signed char state;		/* physical cpu state */
 	signed char polarization;	/* physical polarization */
 	u16 address;			/* physical cpu address */
@@ -174,6 +175,7 @@ static void pcpu_ec_call(struct pcpu *pcpu, int ec_bit)
 	if (test_and_set_bit(ec_bit, &pcpu->ec_mask))
 		return;
 	order = pcpu_running(pcpu) ? SIGP_EXTERNAL_CALL : SIGP_EMERGENCY_SIGNAL;
+	pcpu->ec_clk = get_tod_clock_fast();
 	pcpu_sigp_retry(pcpu, order, 0);
 }