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Subject[RFC PATCH v3 01/12] smp: add generic SMP-stop support to common code
Date
There was a lot of code duplication across architectures regarding the
SMP stop procedures' logic; moreover some of this duplicated code logic
happened to be similarly faulty across a few architectures: while fixing
such logic, move such generic logic as much as possible inside common
code.

Collect all the common logic related to SMP stop operations into the
common SMP code; any architecture willing to use such centralized logic
can select CONFIG_ARCH_USE_COMMON_STOP=y and provide the related
arch-specific helpers: in such a scenario, those architectures will
transparently start using the common code provided by smp_send_stop()
common function.

On the other side, Architectures not willing to use common code SMP stop
logic will simply leave CONFIG_ARCH_USE_COMMON_STOP undefined and carry
on executing their local arch-specific smp_send_stop() as before.

Suggested-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v2 --> v3
- reviewed wait/timeout helpers avoiding broken shared static globals
v1 --> v2
- moved related Kconfig to common code inside arch/Kconfig
- introduced additional CONFIG_USE_COMMON_STOP selected by
CONFIG_ARCH_USE_COMMON_STOP
- introduced helpers to let architectures optionally alter
the default common code behaviour while waiting for CPUs:
change timeout or wait for ever. (will be needed by x86)
---
arch/Kconfig | 7 ++++
include/linux/smp.h | 39 +++++++++++++++++++
kernel/smp.c | 91 +++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 137 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 48b5e103bdb0..99550754e2ea 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -169,6 +169,13 @@ config ARCH_USE_BUILTIN_BSWAP
instructions should set this. And it shouldn't hurt to set it
on architectures that don't have such instructions.

+config ARCH_USE_COMMON_SMP_STOP
+ def_bool n
+
+config USE_COMMON_SMP_STOP
+ depends on SMP && ARCH_USE_COMMON_SMP_STOP
+ def_bool y
+
config KRETPROBES
def_bool y
depends on KPROBES && HAVE_KRETPROBES
diff --git a/include/linux/smp.h b/include/linux/smp.h
index 6fc856c9eda5..22a0adeb7350 100644
--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -77,6 +77,45 @@ int smp_call_function_single_async(int cpu, call_single_data_t *csd);
*/
extern void smp_send_stop(void);

+#ifdef CONFIG_USE_COMMON_SMP_STOP
+/*
+ * An Architecture can optionally use this helper to change the default
+ * waiting behaviour of common STOP logic.
+ */
+void smp_stop_set_wait_forever(bool wait_forever);
+
+/*
+ * An Architecture can optionally use this helper to change the default
+ * waiting timeout of common STOP logic.
+ * A ZERO timeout means no waiting at all as long as waiting forever was
+ * not also previously set.
+ *
+ * Note that, in this way, waiting forever and timeout period settings
+ * remain disjoint.
+ */
+void smp_stop_set_wait_timeout_us(unsigned long timeout);
+
+/*
+ * Retrieve the current SMP STOP wait settings.
+ * Returns true if waiting forever is set.
+ */
+bool smp_stop_get_wait_settings(unsigned long *timeout);
+
+/*
+ * Any Architecture willing to use STOP common logic implementation
+ * MUST at least provide the arch_smp_stop_call() helper which implements
+ * the arch-specific CPU-stop mechanism.
+ */
+extern void arch_smp_stop_call(cpumask_t *cpus);
+
+/*
+ * An Architecture CAN also provide the arch_smp_cpus_stop_complete()
+ * dedicated helper, to perform any final arch-specific operation on
+ * the local CPU once the other CPUs have been successfully stopped.
+ */
+void arch_smp_cpus_stop_complete(void);
+#endif
+
/*
* sends a 'reschedule' event to another CPU:
*/
diff --git a/kernel/smp.c b/kernel/smp.c
index 7dbcb402c2fc..de29cd94a948 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -20,6 +20,7 @@
#include <linux/sched.h>
#include <linux/sched/idle.h>
#include <linux/hypervisor.h>
+#include <linux/delay.h>

#include "smpboot.h"

@@ -817,3 +818,93 @@ int smp_call_on_cpu(unsigned int cpu, int (*func)(void *), void *par, bool phys)
return sscs.ret;
}
EXPORT_SYMBOL_GPL(smp_call_on_cpu);
+
+#ifdef CONFIG_USE_COMMON_SMP_STOP
+static atomic_t smp_stop_wait_forever;
+static atomic_t smp_stop_wait_timeout = ATOMIC_INIT(USEC_PER_SEC);
+
+void smp_stop_set_wait_forever(bool wait_forever)
+{
+ atomic_set(&smp_stop_wait_forever, wait_forever);
+ /* ensure wait atomic-op is visible */
+ smp_mb__after_atomic();
+}
+
+void smp_stop_set_wait_timeout_us(unsigned long timeout)
+{
+ atomic_set(&smp_stop_wait_timeout, timeout);
+ /* ensure timeout atomic-op is visible */
+ smp_mb__after_atomic();
+}
+
+bool smp_stop_get_wait_settings(unsigned long *timeout)
+{
+ if (timeout)
+ *timeout = atomic_read(&smp_stop_wait_timeout);
+ return atomic_read(&smp_stop_wait_forever);
+}
+
+void __weak arch_smp_cpus_stop_complete(void) { }
+
+static inline bool any_other_cpus_online(cpumask_t *mask,
+ unsigned int this_cpu_id)
+{
+ cpumask_copy(mask, cpu_online_mask);
+ cpumask_clear_cpu(this_cpu_id, mask);
+
+ return !cpumask_empty(mask);
+}
+
+/*
+ * This centralizes the common logic to:
+ *
+ * - evaluate which CPUs are online and needs to be notified for stop,
+ * while considering properly the status of the calling CPU
+ *
+ * - call the arch-specific helpers to request the effective stop
+ *
+ * - wait for the stop operation to be completed across all involved CPUs
+ * monitoring the cpu_online_mask
+ */
+void smp_send_stop(void)
+{
+ unsigned int this_cpu_id;
+ cpumask_t mask;
+
+ this_cpu_id = smp_processor_id();
+ if (any_other_cpus_online(&mask, this_cpu_id)) {
+ bool wait_forever;
+ unsigned long timeout;
+ unsigned int this_cpu_online = cpu_online(this_cpu_id);
+
+ if (system_state <= SYSTEM_RUNNING)
+ pr_crit("stopping secondary CPUs\n");
+ arch_smp_stop_call(&mask);
+
+ /*
+ * Defaults to wait up to one second for other CPUs to stop;
+ * architectures can modify the default timeout or request
+ * to wait forever.
+ *
+ * Here we rely simply on cpu_online_mask to sync with
+ * arch-specific stop code without bloating the code with an
+ * additional atomic_t ad-hoc counter.
+ *
+ * As a consequence we'll need proper explicit memory barriers
+ * in case the other CPUs running the arch-specific stop-code
+ * would need to commit to memory some data (like saved_regs).
+ */
+ wait_forever = smp_stop_get_wait_settings(&timeout);
+ while (num_online_cpus() > this_cpu_online &&
+ (wait_forever || timeout--))
+ udelay(1);
+ /* ensure any stopping-CPUs memory access is made visible */
+ smp_rmb();
+ if (num_online_cpus() > this_cpu_online)
+ pr_warn("failed to stop secondary CPUs %*pbl\n",
+ cpumask_pr_args(cpu_online_mask));
+ }
+ /* Perform final (possibly arch-specific) work on this CPU */
+ arch_smp_cpus_stop_complete();
+}
+#endif
--
2.17.1
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