From: Alexander A Sverdlin <alexander.sverdlin@nokia.com>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org
Cc: Alexander Sverdlin <alexander.sverdlin@nokia.com>,
Russell King <linux@armlinux.org.uk>,
linux-arm-kernel@lists.infradead.org
Subject: [PATCH 1/2] qspinlock: Ensure writes are pushed out of core write buffer
Date: Wed, 27 Jan 2021 21:01:08 +0100 [thread overview]
Message-ID: <20210127200109.16412-1-alexander.sverdlin@nokia.com> (raw)
From: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Ensure writes are pushed out of core write buffer to prevent waiting code
on another cores from spinning longer than necessary.
6 threads running tight spinlock loop competing for the same lock
on 6 cores on MIPS/Octeon do 1000000 iterations...
before the patch in: 4.3 sec
after the patch in: 1.2 sec
Same 6-core Octeon machine:
sysbench --test=mutex --num-threads=64 --memory-scope=local run
w/o patch: 1.53s
with patch: 1.28s
This will also allow to remove the smp_wmb() in
arch/arm/include/asm/mcs_spinlock.h (was it actually addressing the same
issue?).
Finally our internal quite diverse test suite of different IPC/network
aspects didn't detect any regressions on ARM/ARM64/x86_64.
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
---
kernel/locking/mcs_spinlock.h | 5 +++++
kernel/locking/qspinlock.c | 6 ++++++
2 files changed, 11 insertions(+)
diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index 5e10153..10e497a 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -89,6 +89,11 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
return;
}
WRITE_ONCE(prev->next, node);
+ /*
+ * This is necessary to make sure that the corresponding "while" in the
+ * mcs_spin_unlock() doesn't loop forever
+ */
+ smp_wmb();
/* Wait until the lock holder passes the lock down. */
arch_mcs_spin_lock_contended(&node->locked);
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index cbff6ba..577fe01 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -469,6 +469,12 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
/* Link @node into the waitqueue. */
WRITE_ONCE(prev->next, node);
+ /*
+ * This is necessary to make sure that the corresponding
+ * smp_cond_load_relaxed() below (running on another core)
+ * doesn't spin forever.
+ */
+ smp_wmb();
pv_wait_node(node, prev);
arch_mcs_spin_lock_contended(&node->locked);
--
2.10.2
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next reply other threads:[~2021-01-27 20:02 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-27 20:01 Alexander A Sverdlin [this message]
2021-01-27 20:01 ` [PATCH 2/2] ARM: mcs_spinlock: Drop smp_wmb in arch_mcs_spin_lock_contended() Alexander A Sverdlin
2021-01-27 22:22 ` Will Deacon
2021-01-27 22:21 ` [PATCH 1/2] qspinlock: Ensure writes are pushed out of core write buffer Will Deacon
2021-01-28 7:36 ` Alexander Sverdlin
2021-01-28 11:24 ` Peter Zijlstra
2021-01-27 22:43 ` Peter Zijlstra
2021-01-28 7:42 ` Alexander Sverdlin
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