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Date:   Tue, 27 Oct 2020 09:49:49 -0700
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Subject: [PATCH 2/3] sched: Add needbreak for rwlocks
From:   Ben Gardon <bgardon@google.com>
To:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Peter Shier <pshier@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Ben Gardon <bgardon@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Contention awareness while holding a spin lock is essential for reducing
latency when long running kernel operations can hold that lock. Add the
same contention detection interface for read/write spin locks.

Signed-off-by: Ben Gardon <bgardon@google.com>
---
 include/linux/sched.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 063cd120b4593..77179160ec3ab 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1870,6 +1870,23 @@ static inline int spin_needbreak(spinlock_t *lock)
 #endif
 }
 
+/*
+ * Check if a rwlock is contended.
+ * Returns non-zero if there is another task waiting on the rwlock.
+ * Returns zero if the lock is not contended or the system / underlying
+ * rwlock implementation does not support contention detection.
+ * Technically does not depend on CONFIG_PREEMPTION, but a general need
+ * for low latency.
+ */
+static inline int rwlock_needbreak(rwlock_t *lock)
+{
+#ifdef CONFIG_PREEMPTION
+	return rwlock_is_contended(lock);
+#else
+	return 0;
+#endif
+}
+
 static __always_inline bool need_resched(void)
 {
 	return unlikely(tif_need_resched());
-- 
2.29.0.rc2.309.g374f81d7ae-goog