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SubjectRe: [PATCH v3 2/2] locking/qrwlock: Don't contend with readers when setting _QW_WAITING
On 06/18/2015 08:40 AM, Will Deacon wrote:
> On Thu, Jun 18, 2015 at 02:33:56AM +0100, Waiman Long wrote:
>> On 06/16/2015 02:02 PM, Will Deacon wrote:
>>> On Mon, Jun 15, 2015 at 11:24:03PM +0100, Waiman Long wrote:
>>>> The current cmpxchg() loop in setting the _QW_WAITING flag for writers
>>>> in queue_write_lock_slowpath() will contend with incoming readers
>>>> causing possibly extra cmpxchg() operations that are wasteful. This
>>>> patch changes the code to do a byte cmpxchg() to eliminate contention
>>>> with new readers.
>>>>
>>>> A multithreaded microbenchmark running 5M read_lock/write_lock loop
>>>> on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel
>>>> with the qspinlock patch have the following execution times (in ms)
>>>> with and without the patch:
>>>>
>>>> With R:W ratio = 5:1
>>>>
>>>> Threads w/o patch with patch % change
>>>> ------- --------- ---------- --------
>>>> 2 990 895 -9.6%
>>>> 3 2136 1912 -10.5%
>>>> 4 3166 2830 -10.6%
>>>> 5 3953 3629 -8.2%
>>>> 6 4628 4405 -4.8%
>>>> 7 5344 5197 -2.8%
>>>> 8 6065 6004 -1.0%
>>>> 9 6826 6811 -0.2%
>>>> 10 7599 7599 0.0%
>>>> 15 9757 9766 +0.1%
>>>> 20 13767 13817 +0.4%
>>>>
>>>> With small number of contending threads, this patch can improve
>>>> locking performance by up to 10%. With more contending threads,
>>>> however, the gain diminishes.
>>>>
>>>> Signed-off-by: Waiman Long<Waiman.Long@hp.com>
>>>> ---
>>>> kernel/locking/qrwlock.c | 28 ++++++++++++++++++++++++----
>>>> 1 files changed, 24 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
>>>> index d7d7557..559198a 100644
>>>> --- a/kernel/locking/qrwlock.c
>>>> +++ b/kernel/locking/qrwlock.c
>>>> @@ -22,6 +22,26 @@
>>>> #include<linux/hardirq.h>
>>>> #include<asm/qrwlock.h>
>>>>
>>>> +/*
>>>> + * This internal data structure is used for optimizing access to some of
>>>> + * the subfields within the atomic_t cnts.
>>>> + */
>>>> +struct __qrwlock {
>>>> + union {
>>>> + atomic_t cnts;
>>>> + struct {
>>>> +#ifdef __LITTLE_ENDIAN
>>>> + u8 wmode; /* Writer mode */
>>>> + u8 rcnts[3]; /* Reader counts */
>>>> +#else
>>>> + u8 rcnts[3]; /* Reader counts */
>>>> + u8 wmode; /* Writer mode */
>>>> +#endif
>>>> + };
>>>> + };
>>>> + arch_spinlock_t lock;
>>>> +};
>>>> +
>>>> /**
>>>> * rspin_until_writer_unlock - inc reader count& spin until writer is gone
>>>> * @lock : Pointer to queue rwlock structure
>>>> @@ -109,10 +129,10 @@ void queue_write_lock_slowpath(struct qrwlock *lock)
>>>> * or wait for a previous writer to go away.
>>>> */
>>>> for (;;) {
>>>> - cnts = atomic_read(&lock->cnts);
>>>> - if (!(cnts& _QW_WMASK)&&
>>>> - (atomic_cmpxchg(&lock->cnts, cnts,
>>>> - cnts | _QW_WAITING) == cnts))
>>>> + struct __qrwlock *l = (struct __qrwlock *)lock;
>>>> +
>>>> + if (!READ_ONCE(l->wmode)&&
>>>> + (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
>>>> break;
>>> Maybe you could also update the x86 implementation of queue_write_unlock
>>> to write the wmode field instead of casting to u8 *?
>>>
>> The queue_write_unlock() function is in the header file. I don't want to
>> expose the internal structure to other files.
> Then I don't see the value in the new data structure -- why not just cast
> to u8 * instead? In my mind, the structure has the advantage of supporting
> both big and little endian systems, but to be useful it would need to be
> available in the header file for architectures that chose to override
> queue_write_unlock.

Casting to (u8 *) directly will require ugly endian conditional
compilation code in the function. It is much easier to look at and
understand to do that in the data structure instead.

> As an aside, I have some patches to get this up and running on arm64
> which would need something like this structure for the big-endian case.

If there is going to be other consumer of the internal structure, I
think it will be worthwhile to put that into the header file directly. I
will update the patch to make that changes.

Cheers,
Longman


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