Commit 38f2c691 authored by Martin Schwidefsky's avatar Martin Schwidefsky Committed by Heiko Carstens

s390: improve wait logic of stop_machine

The stop_machine loop to advance the state machine and to wait for all
affected CPUs to check-in calls cpu_relax_yield in a tight loop until
the last missing CPUs acknowledged the state transition.

On a virtual system where not all logical CPUs are backed by real CPUs
all the time it can take a while for all CPUs to check-in. With the
current definition of cpu_relax_yield a diagnose 0x44 is done which
tells the hypervisor to schedule *some* other CPU. That can be any
CPU and not necessarily one of the CPUs that need to run in order to
advance the state machine. This can lead to a pretty bad diagnose 0x44
storm until the last missing CPU finally checked-in.

Replace the undirected cpu_relax_yield based on diagnose 0x44 with a
directed yield. Each CPU in the wait loop will pick up the next CPU
in the cpumask of stop_machine. The diagnose 0x9c is used to tell the
hypervisor to run this next CPU instead of the current one. If there
is only a limited number of real CPUs backing the virtual CPUs we
end up with the real CPUs passed around in a round-robin fashion.

[heiko.carstens@de.ibm.com]:
    Use cpumask_next_wrap as suggested by Peter Zijlstra.
Signed-off-by: default avatarMartin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
parent 79282605
......@@ -36,6 +36,7 @@
#ifndef __ASSEMBLY__
#include <linux/cpumask.h>
#include <linux/linkage.h>
#include <linux/irqflags.h>
#include <asm/cpu.h>
......@@ -225,7 +226,7 @@ static __no_kasan_or_inline unsigned short stap(void)
* Give up the time slice of the virtual PU.
*/
#define cpu_relax_yield cpu_relax_yield
void cpu_relax_yield(void);
void cpu_relax_yield(const struct cpumask *cpumask);
#define cpu_relax() barrier()
......
......@@ -31,6 +31,7 @@ struct cpu_info {
};
static DEFINE_PER_CPU(struct cpu_info, cpu_info);
static DEFINE_PER_CPU(int, cpu_relax_retry);
static bool machine_has_cpu_mhz;
......@@ -58,13 +59,19 @@ void s390_update_cpu_mhz(void)
on_each_cpu(update_cpu_mhz, NULL, 0);
}
void notrace cpu_relax_yield(void)
void notrace cpu_relax_yield(const struct cpumask *cpumask)
{
if (!smp_cpu_mtid && MACHINE_HAS_DIAG44) {
diag_stat_inc(DIAG_STAT_X044);
asm volatile("diag 0,0,0x44");
int cpu, this_cpu;
this_cpu = smp_processor_id();
if (__this_cpu_inc_return(cpu_relax_retry) >= spin_retry) {
__this_cpu_write(cpu_relax_retry, 0);
cpu = cpumask_next_wrap(this_cpu, cpumask, this_cpu, false);
if (cpu >= nr_cpu_ids)
return;
if (arch_vcpu_is_preempted(cpu))
smp_yield_cpu(cpu);
}
barrier();
}
EXPORT_SYMBOL(cpu_relax_yield);
......
......@@ -414,7 +414,7 @@ void smp_yield_cpu(int cpu)
diag_stat_inc_norecursion(DIAG_STAT_X09C);
asm volatile("diag %0,0,0x9c"
: : "d" (pcpu_devices[cpu].address));
} else if (MACHINE_HAS_DIAG44) {
} else if (MACHINE_HAS_DIAG44 && !smp_cpu_mtid) {
diag_stat_inc_norecursion(DIAG_STAT_X044);
asm volatile("diag 0,0,0x44");
}
......
......@@ -1519,7 +1519,7 @@ static inline int set_cpus_allowed_ptr(struct task_struct *p, const struct cpuma
#endif
#ifndef cpu_relax_yield
#define cpu_relax_yield() cpu_relax()
#define cpu_relax_yield(cpumask) cpu_relax()
#endif
extern int yield_to(struct task_struct *p, bool preempt);
......
......@@ -183,6 +183,7 @@ static int multi_cpu_stop(void *data)
struct multi_stop_data *msdata = data;
enum multi_stop_state curstate = MULTI_STOP_NONE;
int cpu = smp_processor_id(), err = 0;
const struct cpumask *cpumask;
unsigned long flags;
bool is_active;
......@@ -192,15 +193,18 @@ static int multi_cpu_stop(void *data)
*/
local_save_flags(flags);
if (!msdata->active_cpus)
is_active = cpu == cpumask_first(cpu_online_mask);
else
is_active = cpumask_test_cpu(cpu, msdata->active_cpus);
if (!msdata->active_cpus) {
cpumask = cpu_online_mask;
is_active = cpu == cpumask_first(cpumask);
} else {
cpumask = msdata->active_cpus;
is_active = cpumask_test_cpu(cpu, cpumask);
}
/* Simple state machine */
do {
/* Chill out and ensure we re-read multi_stop_state. */
cpu_relax_yield();
cpu_relax_yield(cpumask);
if (msdata->state != curstate) {
curstate = msdata->state;
switch (curstate) {
......
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