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Message-Id: <20230427111937.2745231-4-bigeasy@linutronix.de>
Date: Thu, 27 Apr 2023 13:19:36 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: Ben Segall <bsegall@...gle.com>, Boqun Feng <boqun.feng@...il.com>,
Crystal Wood <swood@...hat.com>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Ingo Molnar <mingo@...hat.com>,
John Stultz <jstultz@...gle.com>,
Juri Lelli <juri.lelli@...hat.com>,
Mel Gorman <mgorman@...e.de>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
Valentin Schneider <vschneid@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Waiman Long <longman@...hat.com>,
Will Deacon <will@...nel.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v2 3/4] locking/rtmutex: Avoid pointless blk_flush_plug() invocations
With DEBUG_RT_MUTEXES enabled the fast-path rt_mutex_cmpxchg_acquire()
always fails and all lock operations take the slow path, which leads to the
invocation of blk_flush_plug() even if the lock is not contended which is
unnecessary and avoids batch processing of requests.
Provide a new helper inline rt_mutex_try_acquire() which maps to
rt_mutex_cmpxchg_acquire() in the non-debug case. For the debug case it
invokes rt_mutex_slowtrylock() which can acquire a non-contended rtmutex
under full debug coverage.
Replace the rt_mutex_cmpxchg_acquire() invocations in __rt_mutex_lock() and
__ww_rt_mutex_lock() with the new helper function, which avoid the
blk_flush_plug() for the non-contended case and preserves the debug
mechanism.
[ tglx: Created a new helper and massaged changelog ]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
kernel/locking/rtmutex.c | 25 ++++++++++++++++++++++++-
kernel/locking/ww_rt_mutex.c | 2 +-
2 files changed, 25 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index aa66a3c5950a7..dd76c1b9b7d21 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -218,6 +218,11 @@ static __always_inline bool rt_mutex_cmpxchg_acquire(struct rt_mutex_base *lock,
return try_cmpxchg_acquire(&lock->owner, &old, new);
}
+static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)
+{
+ return rt_mutex_cmpxchg_acquire(lock, NULL, current);
+}
+
static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
struct task_struct *old,
struct task_struct *new)
@@ -297,6 +302,24 @@ static __always_inline bool rt_mutex_cmpxchg_acquire(struct rt_mutex_base *lock,
}
+static int __sched rt_mutex_slowtrylock(struct rt_mutex_base *lock);
+
+static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)
+{
+ /*
+ * With debug enabled rt_mutex_cmpxchg trylock() will always fail,
+ * which will unconditionally invoke sched_submit/resume_work() in
+ * the slow path of __rt_mutex_lock() and __ww_rt_mutex_lock() even
+ * in the non-contended case.
+ *
+ * Avoid that by using rt_mutex_slow_trylock() which is covered by
+ * the debug code and can acquire a non-contended rtmutex. On
+ * success the callsite avoids the sched_submit/resume_work()
+ * dance.
+ */
+ return rt_mutex_slowtrylock(lock);
+}
+
static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
struct task_struct *old,
struct task_struct *new)
@@ -1704,7 +1727,7 @@ static int __sched rt_mutex_slowlock(struct rt_mutex_base *lock,
static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock,
unsigned int state)
{
- if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
+ if (likely(rt_mutex_try_acquire(lock)))
return 0;
return rt_mutex_slowlock(lock, NULL, state);
diff --git a/kernel/locking/ww_rt_mutex.c b/kernel/locking/ww_rt_mutex.c
index d1473c624105c..c7196de838edc 100644
--- a/kernel/locking/ww_rt_mutex.c
+++ b/kernel/locking/ww_rt_mutex.c
@@ -62,7 +62,7 @@ __ww_rt_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ww_ctx,
}
mutex_acquire_nest(&rtm->dep_map, 0, 0, nest_lock, ip);
- if (likely(rt_mutex_cmpxchg_acquire(&rtm->rtmutex, NULL, current))) {
+ if (likely(rt_mutex_try_acquire(&rtm->rtmutex))) {
if (ww_ctx)
ww_mutex_set_context_fastpath(lock, ww_ctx);
return 0;
--
2.40.1
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