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Subject[PATCH/RFC 2/3] s390: improve wait logic of stop_machine
Date
From: Martin Schwidefsky <schwidefsky@de.ibm.com>

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.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---
arch/s390/include/asm/processor.h | 3 ++-
arch/s390/kernel/processor.c | 19 ++++++++++++++-----
arch/s390/kernel/smp.c | 2 +-
include/linux/sched.h | 2 +-
kernel/stop_machine.c | 14 +++++++++-----
5 files changed, 27 insertions(+), 13 deletions(-)

diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
index b0fcbc37b637..445ce9ee4404 100644
--- a/arch/s390/include/asm/processor.h
+++ b/arch/s390/include/asm/processor.h
@@ -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()

diff --git a/arch/s390/kernel/processor.c b/arch/s390/kernel/processor.c
index 5de13307b703..2c781e2b0078 100644
--- a/arch/s390/kernel/processor.c
+++ b/arch/s390/kernel/processor.c
@@ -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 +59,21 @@ 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;
+
+ if (__this_cpu_inc_return(cpu_relax_retry) >= spin_retry) {
+ __this_cpu_write(cpu_relax_retry, 0);
+ cpu = cpumask_next(smp_processor_id(), cpumask);
+ if (cpu >= nr_cpu_ids) {
+ cpu = cpumask_first(cpumask);
+ if (cpu == smp_processor_id())
+ return;
+ }
+ if (arch_vcpu_is_preempted(cpu))
+ smp_yield_cpu(cpu);
}
- barrier();
}
EXPORT_SYMBOL(cpu_relax_yield);

diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
index 35fafa2b91a8..a8eef7b7770a 100644
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -418,7 +418,7 @@ 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");
}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 11837410690f..1f9f3160da7e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -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);
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 2b5a6754646f..b8b0c5ff8da9 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -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) {
--
2.17.1
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