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@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306010043.VJHcuCnb-lkp@intel.com/
Signed-off-by: Bongkyu Kim <bongkyu7.kim@samsung.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  9 +++++++++
 kernel/locking/rwsem.c                          | 16 +++++++++++++++-
 2 files changed, 24 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..47c467880af5 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -117,6 +117,17 @@
 # define DEBUG_RWSEMS_WARN_ON(c, sem)
 #endif
 
+static bool __ro_after_init rwsem_opt_rspin;
+
+static int __init opt_rspin(char *str)
+{
+        rwsem_opt_rspin = true;
+
+        return 0;
+}
+
+early_param("rwsem.opt_rspin", opt_rspin);
+
 /*
  * On 64-bit architectures, the bit definitions of the count are:
  *
@@ -1083,7 +1094,7 @@ 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 +1168,9 @@ 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.
-- 
2.36.1

On Fri, Sep 01, 2023 at 10:07:04AM +0900, Bongkyu Kim wrote:

> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index 9c0462d515c1..47c467880af5 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -117,6 +117,17 @@
>  # define DEBUG_RWSEMS_WARN_ON(c, sem)
>  #endif
>  
> +static bool __ro_after_init rwsem_opt_rspin;
> +
> +static int __init opt_rspin(char *str)
> +{
> +        rwsem_opt_rspin = true;
> +
> +        return 0;
> +}
> +
> +early_param("rwsem.opt_rspin", opt_rspin);
> +
>  /*
>   * On 64-bit architectures, the bit definitions of the count are:
>   *
> @@ -1083,7 +1094,7 @@ 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 +1168,9 @@ 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;
> +

At the very least this should be a static_branch(), but I still very
much want an answer on how all this interacts with the handoff stuff.