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s390: improve wait logic of stop_machine

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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: Martin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
This commit is contained in:
Martin Schwidefsky
2019-05-17 12:50:42 +02:00
committed by Heiko Carstens
parent 7928260539
commit 38f2c691a4
5 changed files with 25 additions and 13 deletions
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14
kernel/stop_machine.c
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@@ -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) {