Re: [PATCH v1 2/2] locking/rwsem: Make reader optimistic spinning optional

From: Waiman Long
Date: Thu Aug 31 2023 - 11:24:29 EST



On 8/30/23 23:00, Bongkyu Kim wrote:
Disable reader optimistic spinning by default. And, can enable it
by "rwsem.opt_rspin" cmdline.

Also, fix compile error without CONFIG_RWSEM_SPIN_ON_OWNER (reported by
kernel test robot)

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202306010043.VJHcuCnb-lkp@xxxxxxxxx/
Signed-off-by: Bongkyu Kim <bongkyu7.kim@xxxxxxxxxxx>
---
.../admin-guide/kernel-parameters.txt | 9 +++++++++
kernel/locking/rwsem.c | 18 +++++++++++++++++-
2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 772b54df084b..adf16a07fe4d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5605,6 +5605,15 @@
rw [KNL] Mount root device read-write on boot
+ rwsem.opt_rspin [KNL]
+ Use rwsem reader optimistic spinning. Reader optimistic
+ spinning is helpful when the reader critical section is
+ short and there aren't that many readers around.
+ For example, enable this option may improve performance
+ in mobile workload that there're not many readers, but
+ may reduce performance in server workload that there're
+ many readers.
+
S [KNL] Run init in single mode
s390_iommu= [HW,S390]
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 9c0462d515c1..de3f173d94d4 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -117,6 +117,19 @@
# define DEBUG_RWSEMS_WARN_ON(c, sem)
#endif
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+static bool rwsem_opt_rspin;

You need to move this definition out of the "ifdef CONFIG_RWSEM_SPIN_ON_OWNER" block or a compilation error in rwsem_down_read_slowpath() may happen if CONFIG_RWSEM_SPIN_ON_OWNER isn't defined. You may also add __ro_after_init to rwsem_opt_rspin.

Cheers,
Longman


+
+static int __init opt_rspin(char *str)
+{
+ rwsem_opt_rspin = true;
+
+ return 0;
+}
+
+early_param("rwsem.opt_rspin", opt_rspin);
+#endif
+
/*
* On 64-bit architectures, the bit definitions of the count are:
*
@@ -1083,7 +1096,7 @@ static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
return false;
}
-static inline bool rwsem_no_spinners(sem)
+static inline bool rwsem_no_spinners(struct rw_semaphore *sem)
{
return false;
}
@@ -1157,6 +1170,9 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
return sem;
}
+ if (!IS_ENABLED(CONFIG_RWSEM_SPIN_ON_OWNER) || !rwsem_opt_rspin)
+ goto queue;
+
/*
* Save the current read-owner of rwsem, if available, and the
* reader nonspinnable bit.