| Message ID | 1549913486-16799-3-git-send-email-longman@redhat.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | locking/rwsem: Remove arch specific rwsem files | expand |
| Context | Check | Description |
|---|---|---|
| snowpatch_ozlabs/apply_patch | success | next/apply_patch Successfully applied |
| snowpatch_ozlabs/build-ppc64le | success | build succeeded & removed 0 sparse warning(s) |
| snowpatch_ozlabs/build-ppc64be | success | build succeeded & removed 0 sparse warning(s) |
| snowpatch_ozlabs/build-ppc64e | success | build succeeded & removed 0 sparse warning(s) |
| snowpatch_ozlabs/build-pmac32 | success | build succeeded & removed 0 sparse warning(s) |
| snowpatch_ozlabs/checkpatch | warning | total: 0 errors, 0 warnings, 1 checks, 15 lines checked |
On Mon, Feb 11, 2019 at 02:31:26PM -0500, Waiman Long wrote: > Modify __down_read_trylock() to make it generate slightly better code > (smaller and maybe a tiny bit faster). > > 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>: mov (%rdi),%rax > 0x0000000000000008 <+8>: test %rax,%rax > 0x000000000000000b <+11>: js 0x2f <down_read_trylock+47> > 0x000000000000000d <+13>: lea 0x1(%rax),%rdx > 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi) > 0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8> > 0x0000000000000018 <+24>: mov %gs:0x0,%rax > 0x0000000000000021 <+33>: or $0x3,%rax > 0x0000000000000025 <+37>: mov %rax,0x20(%rdi) > 0x0000000000000029 <+41>: mov $0x1,%eax > 0x000000000000002e <+46>: retq > 0x000000000000002f <+47>: xor %eax,%eax > 0x0000000000000031 <+49>: retq > > By using a rwsem microbenchmark, the down_read_trylock() rate on a > x86-64 system before and after the patch were: > > Before Patch After Patch > # of Threads rlock rlock > ------------ ----- ----- > 1 27,787 28,259 > 2 8,359 9,234 From 1/2: 1 29,201 30,143 29,458 28,615 30,172 29,201 2 6,807 13,299 1,171 7,725 15,025 1,804 > > On a ARM64 system, the performance results were: > > Before Patch After Patch > # of Threads rlock rlock > ------------ ----- ----- > 1 24,155 25,000 > 2 6,820 8,699 > > Suggested-by: Peter Zijlstra <peterz@infradead.org> > Signed-off-by: Waiman Long <longman@redhat.com> > --- > kernel/locking/rwsem.h | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h > index 067e265..028bc33 100644 > --- a/kernel/locking/rwsem.h > +++ b/kernel/locking/rwsem.h > @@ -175,11 +175,11 @@ static inline int __down_read_killable(struct rw_semaphore *sem) > > static inline int __down_read_trylock(struct rw_semaphore *sem) > { > - long tmp; > + long tmp = atomic_long_read(&sem->count); > > - while ((tmp = atomic_long_read(&sem->count)) >= 0) { > - if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp, > - tmp + RWSEM_ACTIVE_READ_BIAS)) { > + while (tmp >= 0) { > + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, > + tmp + RWSEM_ACTIVE_READ_BIAS)) { > return 1; > } > } > -- > 1.8.3.1 >
On Tue, Feb 12, 2019 at 02:24:04PM +0100, Peter Zijlstra wrote: > On Mon, Feb 11, 2019 at 02:31:26PM -0500, Waiman Long wrote: > > Modify __down_read_trylock() to make it generate slightly better code > > (smaller and maybe a tiny bit faster). > > > > 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>: mov (%rdi),%rax > > 0x0000000000000008 <+8>: test %rax,%rax > > 0x000000000000000b <+11>: js 0x2f <down_read_trylock+47> > > 0x000000000000000d <+13>: lea 0x1(%rax),%rdx > > 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi) > > 0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8> > > 0x0000000000000018 <+24>: mov %gs:0x0,%rax > > 0x0000000000000021 <+33>: or $0x3,%rax > > 0x0000000000000025 <+37>: mov %rax,0x20(%rdi) > > 0x0000000000000029 <+41>: mov $0x1,%eax > > 0x000000000000002e <+46>: retq > > 0x000000000000002f <+47>: xor %eax,%eax > > 0x0000000000000031 <+49>: retq > > > > By using a rwsem microbenchmark, the down_read_trylock() rate on a > > x86-64 system before and after the patch were: > > > > Before Patch After Patch > > # of Threads rlock rlock > > ------------ ----- ----- > > 1 27,787 28,259 > > 2 8,359 9,234 > > From 1/2: > > 1 29,201 30,143 29,458 28,615 30,172 29,201 > 2 6,807 13,299 1,171 7,725 15,025 1,804 Argh, fat fingered and send before I was done typing. What I wanted to say was; those rlock numbers don't match up. What gives? The before _this_ patch number of 27k787 should be the same as the after first patch number of 30k172.
On 02/12/2019 08:25 AM, Peter Zijlstra wrote: > On Tue, Feb 12, 2019 at 02:24:04PM +0100, Peter Zijlstra wrote: >> On Mon, Feb 11, 2019 at 02:31:26PM -0500, Waiman Long wrote: >>> Modify __down_read_trylock() to make it generate slightly better code >>> (smaller and maybe a tiny bit faster). >>> >>> 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>: mov (%rdi),%rax >>> 0x0000000000000008 <+8>: test %rax,%rax >>> 0x000000000000000b <+11>: js 0x2f <down_read_trylock+47> >>> 0x000000000000000d <+13>: lea 0x1(%rax),%rdx >>> 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi) >>> 0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8> >>> 0x0000000000000018 <+24>: mov %gs:0x0,%rax >>> 0x0000000000000021 <+33>: or $0x3,%rax >>> 0x0000000000000025 <+37>: mov %rax,0x20(%rdi) >>> 0x0000000000000029 <+41>: mov $0x1,%eax >>> 0x000000000000002e <+46>: retq >>> 0x000000000000002f <+47>: xor %eax,%eax >>> 0x0000000000000031 <+49>: retq >>> >>> By using a rwsem microbenchmark, the down_read_trylock() rate on a >>> x86-64 system before and after the patch were: >>> >>> Before Patch After Patch >>> # of Threads rlock rlock >>> ------------ ----- ----- >>> 1 27,787 28,259 >>> 2 8,359 9,234 >> From 1/2: >> >> 1 29,201 30,143 29,458 28,615 30,172 29,201 >> 2 6,807 13,299 1,171 7,725 15,025 1,804 > Argh, fat fingered and send before I was done typing. > > What I wanted to say was; those rlock numbers don't match up. What > gives? > > The before _this_ patch number of 27k787 should be the same as the after > first patch number of 30k172. The rlock number in patch 1 refers to down_read() which uses xadd. The number here in patch 2 refers specifically to down_read_trylock() which uses cmpxchg() as this patch changes only __down_read_tryulock(). So the performance data differ. Cheers, Longman
On 02/12/2019 01:36 PM, Waiman Long wrote: > On 02/12/2019 08:25 AM, Peter Zijlstra wrote: >> On Tue, Feb 12, 2019 at 02:24:04PM +0100, Peter Zijlstra wrote: >>> On Mon, Feb 11, 2019 at 02:31:26PM -0500, Waiman Long wrote: >>>> Modify __down_read_trylock() to make it generate slightly better code >>>> (smaller and maybe a tiny bit faster). >>>> >>>> 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>: mov (%rdi),%rax >>>> 0x0000000000000008 <+8>: test %rax,%rax >>>> 0x000000000000000b <+11>: js 0x2f <down_read_trylock+47> >>>> 0x000000000000000d <+13>: lea 0x1(%rax),%rdx >>>> 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi) >>>> 0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8> >>>> 0x0000000000000018 <+24>: mov %gs:0x0,%rax >>>> 0x0000000000000021 <+33>: or $0x3,%rax >>>> 0x0000000000000025 <+37>: mov %rax,0x20(%rdi) >>>> 0x0000000000000029 <+41>: mov $0x1,%eax >>>> 0x000000000000002e <+46>: retq >>>> 0x000000000000002f <+47>: xor %eax,%eax >>>> 0x0000000000000031 <+49>: retq >>>> >>>> By using a rwsem microbenchmark, the down_read_trylock() rate on a >>>> x86-64 system before and after the patch were: >>>> >>>> Before Patch After Patch >>>> # of Threads rlock rlock >>>> ------------ ----- ----- >>>> 1 27,787 28,259 >>>> 2 8,359 9,234 >>> From 1/2: >>> >>> 1 29,201 30,143 29,458 28,615 30,172 29,201 >>> 2 6,807 13,299 1,171 7,725 15,025 1,804 >> Argh, fat fingered and send before I was done typing. >> >> What I wanted to say was; those rlock numbers don't match up. What >> gives? >> >> The before _this_ patch number of 27k787 should be the same as the after >> first patch number of 30k172. > The rlock number in patch 1 refers to down_read() which uses xadd. The > number here in patch 2 refers specifically to down_read_trylock() which > uses cmpxchg() as this patch changes only __down_read_tryulock(). So the > performance data differ. You can see that the performance is worse if we use cmpxchg for down_read instead of using xadd. Cheers, Longman
On Mon, Feb 11, 2019 at 11:31 AM Waiman Long <longman@redhat.com> wrote: > > Modify __down_read_trylock() to make it generate slightly better code > (smaller and maybe a tiny bit faster). This looks good, but I would ask you to try one slightly different approach. Instead of this: > long tmp = atomic_long_read(&sem->count); > > while (tmp >= 0) { > if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, > tmp + RWSEM_ACTIVE_READ_BIAS)) { > return 1; > } > } try doing this instead: long tmp = 0; do { if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, tmp + RWSEM_ACTIVE_READ_BIAS)) { return 1; } while (tmp >= 0); return 0; because especially when it comes to locking, it's usually better to just *guess* that the lock is unlocked, than it is to actually read from the line to see what the state is. Often - but certainly not always - the lock is the first access to the target cacheline, and assuming the trylock is successful (which I think is the case we want to optimize for), we're much better off causing that first access to be a read-for-ownership, rather than a read-for-sharing. Because if you first read from the line, and then do a cmpxchg, and if the line was not in the cache, your cache coherency protocol will generally go through two states: first shared (for the initial read) and then exclusive-dirty (for the cmpxchg). Now, this is obviously very micro-architecture dependent, and in fact the microarchitecture could even see the "predict fallthrough to a cmpxchg with the same address" and turn the first read into a read-for-ownership, but we've done this at some point before, and the "guess unlocked" was actually the one that performed better. Of course, the downside is that it might be worse when the guess is incorrect - either because of a nested read lock or due to an actual conflict with a write, but on the whole those *should* be the rare cases, and not the cases where we necessarily optimize for latency of the operation. Hmm? Linus
On 02/12/2019 02:58 PM, Linus Torvalds wrote: > On Mon, Feb 11, 2019 at 11:31 AM Waiman Long <longman@redhat.com> wrote: >> Modify __down_read_trylock() to make it generate slightly better code >> (smaller and maybe a tiny bit faster). > This looks good, but I would ask you to try one slightly different approach. > > Instead of this: > >> long tmp = atomic_long_read(&sem->count); >> >> while (tmp >= 0) { >> if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, >> tmp + RWSEM_ACTIVE_READ_BIAS)) { >> return 1; >> } >> } > try doing this instead: > > long tmp = 0; > > do { > if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, > tmp + RWSEM_ACTIVE_READ_BIAS)) { > return 1; > } while (tmp >= 0); > return 0; > > because especially when it comes to locking, it's usually better to > just *guess* that the lock is unlocked, than it is to actually read > from the line to see what the state is. > > Often - but certainly not always - the lock is the first access to the > target cacheline, and assuming the trylock is successful (which I > think is the case we want to optimize for), we're much better off > causing that first access to be a read-for-ownership, rather than a > read-for-sharing. > > Because if you first read from the line, and then do a cmpxchg, and if > the line was not in the cache, your cache coherency protocol will > generally go through two states: first shared (for the initial read) > and then exclusive-dirty (for the cmpxchg). > > Now, this is obviously very micro-architecture dependent, and in fact > the microarchitecture could even see the "predict fallthrough to a > cmpxchg with the same address" and turn the first read into a > read-for-ownership, but we've done this at some point before, and the > "guess unlocked" was actually the one that performed better. > > Of course, the downside is that it might be worse when the guess is > incorrect - either because of a nested read lock or due to an actual > conflict with a write, but on the whole those *should* be the rare > cases, and not the cases where we necessarily optimize for latency of > the operation. > > Hmm? I looked at the assembly code in arch/x86/include/asm/rwsem.h. For both trylocks (read & write), the count is read first before attempting to lock it. We did the same for all trylock functions in other locks. Depending on how the trylock is used and how contended the lock is, it may help or hurt performance. Changing down_read_trylock to do an unconditional cmpxchg will change the performance profile of existing code. So I would prefer keeping the current code. I do notice now that the generic down_write_trylock() code is doing an unconditional compxchg. So I wonder if we should change it to read the lock first like other trylocks or just leave it as it is. Cheers, Longman
* Waiman Long <longman@redhat.com> wrote: > I looked at the assembly code in arch/x86/include/asm/rwsem.h. For both > trylocks (read & write), the count is read first before attempting to > lock it. We did the same for all trylock functions in other locks. > Depending on how the trylock is used and how contended the lock is, it > may help or hurt performance. Changing down_read_trylock to do an > unconditional cmpxchg will change the performance profile of existing > code. So I would prefer keeping the current code. > > I do notice now that the generic down_write_trylock() code is doing an > unconditional compxchg. So I wonder if we should change it to read the > lock first like other trylocks or just leave it as it is. No, I think we should instead move the other trylocks to the try-for-ownership model as well, like Linus suggested. That's the general assumption we make in locking primitives, that we optimize for the common, expected case - which would be that the trylock succeeds, and I don't see why trylock primitives should be different. In fact I can see more ways for read-for-sharing to perform suboptimally on larger systems. Thanks, Ingo
On 02/13/2019 02:45 AM, Ingo Molnar wrote: > * Waiman Long <longman@redhat.com> wrote: > >> I looked at the assembly code in arch/x86/include/asm/rwsem.h. For both >> trylocks (read & write), the count is read first before attempting to >> lock it. We did the same for all trylock functions in other locks. >> Depending on how the trylock is used and how contended the lock is, it >> may help or hurt performance. Changing down_read_trylock to do an >> unconditional cmpxchg will change the performance profile of existing >> code. So I would prefer keeping the current code. >> >> I do notice now that the generic down_write_trylock() code is doing an >> unconditional compxchg. So I wonder if we should change it to read the >> lock first like other trylocks or just leave it as it is. > No, I think we should instead move the other trylocks to the > try-for-ownership model as well, like Linus suggested. > > That's the general assumption we make in locking primitives, that we > optimize for the common, expected case - which would be that the trylock > succeeds, and I don't see why trylock primitives should be different. > > In fact I can see more ways for read-for-sharing to perform suboptimally > on larger systems. I don't mind changing to the try-for-ownership model for rwsem and mutex. I do have some concern to do that for spinlock. Some of the lock slowpath code do optimistic trylock. Making them unconditional cmpxchg will impact lock contention performance. I will update this rwsem patch to make the change while I am working on it. For other locks, I will suggest we go slow and carefully evaluate the performance implication before we make the changes. Cheers, Longman
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h index 067e265..028bc33 100644 --- a/kernel/locking/rwsem.h +++ b/kernel/locking/rwsem.h @@ -175,11 +175,11 @@ static inline int __down_read_killable(struct rw_semaphore *sem) static inline int __down_read_trylock(struct rw_semaphore *sem) { - long tmp; + long tmp = atomic_long_read(&sem->count); - while ((tmp = atomic_long_read(&sem->count)) >= 0) { - if (tmp == atomic_long_cmpxchg_acquire(&sem->count, tmp, - tmp + RWSEM_ACTIVE_READ_BIAS)) { + while (tmp >= 0) { + if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, + tmp + RWSEM_ACTIVE_READ_BIAS)) { return 1; } }
Modify __down_read_trylock() to make it generate slightly better code (smaller and maybe a tiny bit faster). 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>: mov (%rdi),%rax 0x0000000000000008 <+8>: test %rax,%rax 0x000000000000000b <+11>: js 0x2f <down_read_trylock+47> 0x000000000000000d <+13>: lea 0x1(%rax),%rdx 0x0000000000000011 <+17>: lock cmpxchg %rdx,(%rdi) 0x0000000000000016 <+22>: jne 0x8 <down_read_trylock+8> 0x0000000000000018 <+24>: mov %gs:0x0,%rax 0x0000000000000021 <+33>: or $0x3,%rax 0x0000000000000025 <+37>: mov %rax,0x20(%rdi) 0x0000000000000029 <+41>: mov $0x1,%eax 0x000000000000002e <+46>: retq 0x000000000000002f <+47>: xor %eax,%eax 0x0000000000000031 <+49>: retq By using a rwsem microbenchmark, the down_read_trylock() rate on a x86-64 system before and after the patch were: Before Patch After Patch # of Threads rlock rlock ------------ ----- ----- 1 27,787 28,259 2 8,359 9,234 On a ARM64 system, the performance results were: Before Patch After Patch # of Threads rlock rlock ------------ ----- ----- 1 24,155 25,000 2 6,820 8,699 Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Waiman Long <longman@redhat.com> --- kernel/locking/rwsem.h | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)