From: Waiman Long <Waiman.Long@hpe.com>
To: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org,
linux-s390@vger.kernel.org, linux-arch@vger.kernel.org,
linux-doc@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
Jonathan Corbet <corbet@lwn.net>,
Scott J Norton <scott.norton@hpe.com>,
Douglas Hatch <doug.hatch@hpe.com>,
Waiman Long <Waiman.Long@hpe.com>
Subject: [RFC PATCH-tip/locking/core v3 09/10] locking/rwsem: Enable reactivation of reader spinning
Date: Fri, 17 Jun 2016 11:41:35 -0400 [thread overview]
Message-ID: <1466178096-5623-10-git-send-email-Waiman.Long@hpe.com> (raw)
In-Reply-To: <1466178096-5623-1-git-send-email-Waiman.Long@hpe.com>
Reader optimistic spinning will be disabled once the rspin_enabled
count reaches 0. After that, it cannot be re-enabled. This may cause
an eligible rwsem locked out of reader spinning because of a series
of unfortunate events.
This patch looks at the regular writer-on-writer spinning history. If
there are sufficient more successful spin attempts than failed ones,
it will try to reactivate reader spinning.
Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
---
include/linux/rwsem.h | 12 ++++++++----
kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++--
2 files changed, 33 insertions(+), 6 deletions(-)
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 8978f87..98284b4 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -32,7 +32,11 @@ struct rw_semaphore {
raw_spinlock_t wait_lock;
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
struct optimistic_spin_queue osq; /* spinner MCS lock */
- int rspin_enabled; /* protected by osq lock */
+ /*
+ * Reader optimistic spinning fields protected by osq lock
+ */
+ uint16_t rspin_enabled;
+ int16_t wspin_cnt;
/*
* Write owner. Used as a speculative check to see
@@ -74,10 +78,10 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
/*
* Each successful reader spin will increment the rspin_enabled by 1.
* Each unsuccessful spin, on the other hand, will decrement it by 2.
- * Reader spinning will be permanently disabled when it reaches 0.
+ * Reader spinning will be disabled when it reaches 0.
*/
#ifndef RWSEM_RSPIN_ENABLED_DEFAULT
-# define RWSEM_RSPIN_ENABLED_DEFAULT 40
+# define RWSEM_RSPIN_ENABLED_DEFAULT 30
#endif
#define RWSEM_RSPIN_ENABLED_MAX 1024
@@ -87,7 +91,7 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
#define __RWSEM_OPT_INIT(lockname) , .osq = OSQ_LOCK_UNLOCKED, .owner = NULL, \
- .rspin_enabled = RWSEM_RSPIN_ENABLED_DEFAULT
+ .rspin_enabled = RWSEM_RSPIN_ENABLED_DEFAULT, .wspin_cnt = 0
#else
#define __RWSEM_OPT_INIT(lockname)
#endif
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 42c8dda..c6b6105 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -108,6 +108,7 @@ void __init_rwsem(struct rw_semaphore *sem, const char *name,
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
sem->owner = NULL;
sem->rspin_enabled = RWSEM_RSPIN_ENABLED_DEFAULT;
+ sem->wspin_cnt = 0;
osq_lock_init(&sem->osq);
#endif
}
@@ -466,10 +467,32 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
if (taken && (sem->rspin_enabled < RWSEM_RSPIN_ENABLED_MAX)) {
sem->rspin_enabled++;
} else if (!taken) {
- if (sem->rspin_enabled > 2)
+ if (sem->rspin_enabled > 2) {
sem->rspin_enabled -= 2;
- else
+ } else if (sem->rspin_enabled) {
sem->rspin_enabled = 0;
+ /*
+ * Reset wspin_cnt so that it won't get
+ * re-enabled too soon.
+ */
+ if (sem->wspin_cnt > -30)
+ sem->wspin_cnt = -30;
+ }
+ }
+ } else if (type == RWSEM_WAITING_FOR_WRITE) {
+ /*
+ * Every 10 successful writer-on-writer spins more than failed
+ * spins will increment rspin_enabled to encourage more
+ * writer-on-reader spinning attempts.
+ */
+ if (taken) {
+ if ((++sem->wspin_cnt >= 10) &&
+ (sem->rspin_enabled < RWSEM_RSPIN_ENABLED_MAX)) {
+ sem->wspin_cnt = 0;
+ sem->rspin_enabled++;
+ }
+ } else if (sem->wspin_cnt > -100) {
+ sem->wspin_cnt--;
}
}
osq_unlock(&sem->osq);
--
1.7.1
next prev parent reply other threads:[~2016-06-17 15:41 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20160617154126.71Vh8a1JOORZu3aHkOcixTGNFjmD5alLAem_RcYrOKU@z>
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 00/10] locking/rwsem: Enable reader optimistic spinning Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 01/10] locking/osq: Make lock/unlock proper acquire/release barrier Waiman Long
[not found] ` <20160617154128.dZjcIpfk8ojrKoAz90paHLdLQe4gogrYQfQRsy6J9G0@z>
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 02/10] locking/rwsem: Stop active read lock ASAP Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 03/10] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 04/10] locking/rwsem: Enable count-based spinning on reader Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 05/10] locking/rwsem: move down rwsem_down_read_failed function Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 06/10] locking/rwsem: Move common rwsem macros to asm-generic/rwsem_types.h Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 07/10] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation Waiman Long
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 08/10] locking/rwsem: Enable spinning readers Waiman Long
2016-06-17 15:41 ` Waiman Long [this message]
2016-06-17 15:41 ` [RFC PATCH-tip/locking/core v3 10/10] locking/rwsem: Add a boot parameter to reader spinning threshold Waiman Long
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1466178096-5623-10-git-send-email-Waiman.Long@hpe.com \
--to=waiman.long@hpe.com \
--cc=corbet@lwn.net \
--cc=dave@stgolabs.net \
--cc=david@fromorbit.com \
--cc=doug.hatch@hpe.com \
--cc=jason.low2@hp.com \
--cc=linux-alpha@vger.kernel.org \
--cc=linux-arch@vger.kernel.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-ia64@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-s390@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=scott.norton@hpe.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).