From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
Thomas Gleixner <tglx@linutronix.de>
Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
linux-mips@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
sparclinux@vger.kernel.org, linux-riscv@lists.infradead.org,
linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
Davidlohr Bueso <dave@stgolabs.net>,
linux-c6x-dev@linux-c6x.org, linux-hexagon@vger.kernel.org,
x86@kernel.org, Waiman Long <longman@redhat.com>,
uclinux-h8-devel@lists.sourceforge.jp,
linux-xtensa@linux-xtensa.org, Arnd Bergmann <arnd@arndb.de>,
linux-um@lists.infradead.org, linux-m68k@lists.linux-m68k.org,
openrisc@lists.librecores.org, Borislav Petkov <bp@alien8.de>,
linux-arm-kernel@lists.infradead.org,
Tim Chen <tim.c.chen@linux.intel.com>,
linux-parisc@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org,
nios2-dev@lists.rocketboards.org,
Andrew Morton <akpm@linux-foundation.org>,
linuxppc-dev@lists.ozlabs.org
Subject: [PATCH v4 3/3] locking/rwsem: Optimize down_read_trylock()
Date: Wed, 13 Feb 2019 17:00:17 -0500 [thread overview]
Message-ID: <1550095217-12047-4-git-send-email-longman@redhat.com> (raw)
In-Reply-To: <1550095217-12047-1-git-send-email-longman@redhat.com>
Modify __down_read_trylock() to optimize for an unlocked rwsem and make
it generate slightly better code.
Before this patch, down_read_trylock:
0x0000000000000000 <+0>: callq 0x5 <down_read_trylock+5>
0x0000000000000005 <+5>: jmp 0x18 <down_read_trylock+24>
0x0000000000000007 <+7>: lea 0x1(%rdx),%rcx
0x000000000000000b <+11>: mov %rdx,%rax
0x000000000000000e <+14>: lock cmpxchg %rcx,(%rdi)
0x0000000000000013 <+19>: cmp %rax,%rdx
0x0000000000000016 <+22>: je 0x23 <down_read_trylock+35>
0x0000000000000018 <+24>: mov (%rdi),%rdx
0x000000000000001b <+27>: test %rdx,%rdx
0x000000000000001e <+30>: jns 0x7 <down_read_trylock+7>
0x0000000000000020 <+32>: xor %eax,%eax
0x0000000000000022 <+34>: retq
0x0000000000000023 <+35>: mov %gs:0x0,%rax
0x000000000000002c <+44>: or $0x3,%rax
0x0000000000000030 <+48>: mov %rax,0x20(%rdi)
0x0000000000000034 <+52>: mov $0x1,%eax
0x0000000000000039 <+57>: retq
After patch, down_read_trylock:
0x0000000000000000 <+0>: callq 0x5 <down_read_trylock+5>
0x0000000000000005 <+5>: xor %eax,%eax
0x0000000000000007 <+7>: lea 0x1(%rax),%rdx
0x000000000000000b <+11>: lock cmpxchg %rdx,(%rdi)
0x0000000000000010 <+16>: jne 0x29 <down_read_trylock+41>
0x0000000000000012 <+18>: mov %gs:0x0,%rax
0x000000000000001b <+27>: or $0x3,%rax
0x000000000000001f <+31>: mov %rax,0x20(%rdi)
0x0000000000000023 <+35>: mov $0x1,%eax
0x0000000000000028 <+40>: retq
0x0000000000000029 <+41>: test %rax,%rax
0x000000000000002c <+44>: jns 0x7 <down_read_trylock+7>
0x000000000000002e <+46>: xor %eax,%eax
0x0000000000000030 <+48>: retq
By using a rwsem microbenchmark, the down_read_trylock() rate (with a
load of 10 to lengthen the lock critical section) on a x86-64 system
before and after the patch were:
Before Patch After Patch
# of Threads rlock rlock
------------ ----- -----
1 14,496 14,716
2 8,644 8,453
4 6,799 6,983
8 5,664 7,190
On a ARM64 system, the performance results were:
Before Patch After Patch
# of Threads rlock rlock
------------ ----- -----
1 23,676 24,488
2 7,697 9,502
4 4,945 3,440
8 2,641 1,603
For the uncontended case (1 thread), the new down_read_trylock() is a
little bit faster. For the contended cases, the new down_read_trylock()
perform pretty well in x86-64, but performance degrades at high
contention level on ARM64.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/locking/rwsem.h | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index 45ee002..1f5775a 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -174,14 +174,17 @@ static inline int __down_read_killable(struct rw_semaphore *sem)
static inline int __down_read_trylock(struct rw_semaphore *sem)
{
- long tmp;
+ /*
+ * Optimize for the case when the rwsem is not locked at all.
+ */
+ long tmp = RWSEM_UNLOCKED_VALUE;
- while ((tmp = atomic_long_read(&sem->count)) >= 0) {
- if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp,
- tmp + RWSEM_ACTIVE_READ_BIAS)) {
+ do {
+ if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
+ tmp + RWSEM_ACTIVE_READ_BIAS)) {
return 1;
}
- }
+ } while (tmp >= 0);
return 0;
}
--
1.8.3.1
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-02-13 22:01 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-13 22:00 [PATCH v4 0/3] locking/rwsem: Rwsem rearchitecture part 0 Waiman Long
2019-02-13 22:00 ` [PATCH v4 1/3] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-20 14:47 ` Will Deacon
2019-02-13 22:00 ` [PATCH v4 2/3] locking/rwsem: Remove rwsem-spinlock.c & use rwsem-xadd.c for all archs Waiman Long
2019-02-14 10:54 ` Geert Uytterhoeven
2019-02-14 11:12 ` Peter Zijlstra
2019-02-20 14:47 ` Will Deacon
2019-02-13 22:00 ` Waiman Long [this message]
2019-02-21 14:14 ` [PATCH v4 3/3] locking/rwsem: Optimize down_read_trylock() Will Deacon
2019-02-22 3:28 ` Waiman Long
2019-02-14 10:37 ` [PATCH v4 0/3] locking/rwsem: Rwsem rearchitecture part 0 Peter Zijlstra
2019-02-14 22:05 ` Waiman Long
2019-02-15 18:40 ` Will Deacon
2019-02-15 18:58 ` Waiman Long
2019-02-18 14:58 ` Will Deacon
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