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Groups > linux.kernel > #1684517
| From | Manfred Spraul <manfred@colorfullife.com> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [PATCH v2 0/9] Remove spin_unlock_wait() |
| Date | 2017-07-10 19:30 +0200 |
| Message-ID | <u1KFu-7Ua-67@gated-at.bofh.it> (permalink) |
| References | <u10Mh-4rS-11@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
Hi Alan,
On 07/08/2017 06:21 PM, Alan Stern wrote:
> Pardon me for barging in, but I found this whole interchange extremely
> confusing...
>
> On Sat, 8 Jul 2017, Ingo Molnar wrote:
>
>> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
>>
>>> On Sat, Jul 08, 2017 at 10:35:43AM +0200, Ingo Molnar wrote:
>>>> * Manfred Spraul <manfred@colorfullife.com> wrote:
>>>>
>>>>> Hi Ingo,
>>>>>
>>>>> On 07/07/2017 10:31 AM, Ingo Molnar wrote:
>>>>>> There's another, probably just as significant advantage: queued_spin_unlock_wait()
>>>>>> is 'read-only', while spin_lock()+spin_unlock() dirties the lock cache line. On
>>>>>> any bigger system this should make a very measurable difference - if
>>>>>> spin_unlock_wait() is ever used in a performance critical code path.
>>>>> At least for ipc/sem:
>>>>> Dirtying the cacheline (in the slow path) allows to remove a smp_mb() in the
>>>>> hot path.
>>>>> So for sem_lock(), I either need a primitive that dirties the cacheline or
>>>>> sem_lock() must continue to use spin_lock()/spin_unlock().
> This statement doesn't seem to make sense. Did Manfred mean to write
> "smp_mb()" instead of "spin_lock()/spin_unlock()"?
Option 1:
fastpath:
spin_lock(local_lock)
smp_mb(); [[1]]
smp_load_acquire(global_flag);
slow path:
global_flag = 1;
smp_mb();
<spin_unlock_wait_without_cacheline_dirtying>
Option 2:
fastpath:
spin_lock(local_lock);
smp_load_acquire(global_flag)
slow path:
global_flag = 1;
spin_lock(local_lock);spin_unlock(local_lock).
Rational:
The ACQUIRE from spin_lock is at the read of local_lock, not at the write.
i.e.: Without the smp_mb() at [[1]], the CPU can do:
read local_lock;
read global_flag;
write local_lock;
For Option 2, the smp_mb() is not required, because fast path and slow
path acquire the same lock.
>>>> Technically you could use spin_trylock()+spin_unlock() and avoid the lock acquire
>>>> spinning on spin_unlock() and get very close to the slow path performance of a
>>>> pure cacheline-dirtying behavior.
> This is even more confusing. Did Ingo mean to suggest using
> "spin_trylock()+spin_unlock()" in place of "spin_lock()+spin_unlock()"
> could provide the desired ordering guarantee without delaying other
> CPUs that may try to acquire the lock? That seems highly questionable.
I agree :-)
--
Manfred
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[PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 01:40 +0200
[PATCH v2 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 01:40 +0200
RE: [PATCH v2 0/9] Remove spin_unlock_wait() David Laight <David.Laight@ACULAB.COM> - 2017-07-06 16:20 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 17:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:20 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 18:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:10 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-06 18:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 19:00 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-06 21:40 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 18:10 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 18:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-06 19:00 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Will Deacon <will.deacon@arm.com> - 2017-07-06 19:10 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-06 19:20 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 10:40 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-07 10:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 12:40 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Peter Zijlstra <peterz@infradead.org> - 2017-07-07 13:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 16:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 10:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 13:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Manfred Spraul <manfred@colorfullife.com> - 2017-07-07 19:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 10:40 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 13:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-08 14:40 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-08 16:50 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Alan Stern <stern@rowland.harvard.edu> - 2017-07-08 18:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Manfred Spraul <manfred@colorfullife.com> - 2017-07-10 19:30 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 10:10 +0200
Re: [PATCH v2 0/9] Remove spin_unlock_wait() Ingo Molnar <mingo@kernel.org> - 2017-07-07 11:40 +0200
[PATCH v3 0/9] Remove spin_unlock_wait() "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:30 +0200
[PATCH v3 9/9] arch: Remove spin_unlock_wait() arch-specific definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
[PATCH v3 5/9] exit: Replace spin_unlock_wait() with lock/unlock pair "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
[PATCH v3 8/9] locking: Remove spin_unlock_wait() generic definitions "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-07-07 21:40 +0200
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