[RFC PATCH 5/8] shazptr: Allow skip self scan in synchronize_shaptr()

Boqun Feng
2025-04-14T06:01:27Z

Add a module parameter for shazptr to allow skip the self scan in
synchronize_shaptr(). This can force every synchronize_shaptr() to use
shazptr scan kthread, and help testing the shazptr scan kthread.

Another reason users may want to set this paramter is to reduce the self
scan CPU cost in synchronize_shaptr().

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
kernel/locking/shazptr.c | 28 +++++++++++++++++++++-------
1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/kernel/locking/shazptr.c b/kernel/locking/shazptr.c
index a8559cb559f8..b3f7e8390eb2 100644
--- a/kernel/locking/shazptr.c
+++ b/kernel/locking/shazptr.c
@@ -14,11 +14,17 @@
#include <linux/completion.h>
#include <linux/kthread.h>
#include <linux/list.h>
+#include <linux/moduleparam.h>
#include <linux/mutex.h>
#include <linux/shazptr.h>
#include <linux/slab.h>
#include <linux/sort.h>

+#ifdef MODULE_PARAM_PREFIX
+#undef MODULE_PARAM_PREFIX
+#endif
+#define MODULE_PARAM_PREFIX "shazptr."
+
DEFINE_PER_CPU_SHARED_ALIGNED(void *, shazptr_slots);
EXPORT_PER_CPU_SYMBOL_GPL(shazptr_slots);

@@ -252,6 +258,10 @@ static void synchronize_shazptr_busywait(void *ptr)
}
}

+/* Disabled by default. */
+static int skip_synchronize_self_scan;
+module_param(skip_synchronize_self_scan, int, 0644);
+
static void synchronize_shazptr_normal(void *ptr)
{
int cpu;
@@ -259,15 +269,19 @@ static void synchronize_shazptr_normal(void *ptr)

smp_mb(); /* Synchronize with the smp_mb() in shazptr_acquire(). */

- for_each_possible_cpu(cpu) {
- void **slot = per_cpu_ptr(&shazptr_slots, cpu);
- void *val;
+ if (unlikely(skip_synchronize_self_scan)) {
+ blocking_grp_mask = ~0UL;
+ } else {
+ for_each_possible_cpu(cpu) {
+ void **slot = per_cpu_ptr(&shazptr_slots, cpu);
+ void *val;

- /* Pair with smp_store_release() in shazptr_clear(). */
- val = smp_load_acquire(slot);
+ /* Pair with smp_store_release() in shazptr_clear(). */
+ val = smp_load_acquire(slot);

- if (val == ptr || val == SHAZPTR_WILDCARD)
- blocking_grp_mask |= 1UL << (cpu / shazptr_scan.cpu_grp_size);
+ if (val == ptr || val == SHAZPTR_WILDCARD)
+ blocking_grp_mask |= 1UL << (cpu / shazptr_scan.cpu_grp_size);
+ }
}

/* Found blocking slots, prepare to wait. */
--
2.47.1