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SubjectRe: [PATCH RFC v3 1/3] sbitmap: fix that same waitqueue can be woken up continuously
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Hi!

在 2022/07/11 22:20, Jan Kara 写道:
> On Sun 10-07-22 12:21:58, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> __sbq_wake_up __sbq_wake_up
>> sbq_wake_ptr -> assume 0
>> sbq_wake_ptr -> 0
>> atomic_dec_return
>> atomic_dec_return
>> atomic_cmpxchg -> succeed
>> atomic_cmpxchg -> failed
>> return true
>>
>> __sbq_wake_up
>> sbq_wake_ptr
>> atomic_read(&sbq->wake_index) -> still 0
>> sbq_index_atomic_inc -> inc to 1
>> if (waitqueue_active(&ws->wait))
>> if (wake_index != atomic_read(&sbq->wake_index))
>> atomic_set -> reset from 1 to 0
>> wake_up_nr -> wake up first waitqueue
>> // continue to wake up in first waitqueue
>>
>> Fix the problem by using atomic_cmpxchg() instead of atomic_set()
>> to update 'wake_index'.
>>
>> Fixes: 417232880c8a ("sbitmap: Replace cmpxchg with xchg")
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>
> I don't think this patch is really needed after the following patches. As
> I see it, wake_index is just a performance optimization (plus a fairness
> improvement) but in principle the code in sbq_wake_ptr() is always prone to
> races as the waitqueue it returns needn't have any waiters by the time we
> return. So for correctness the check-and-retry loop needs to happen at
> higher level than inside sbq_wake_ptr() and occasional wrong setting of
> wake_index will result only in a bit of unfairness and more scanning
> looking for suitable waitqueue but I don't think that really justifies the
> cost of atomic operations in cmpxchg loop...

It's right this patch just improve fairness. However, in hevyload tests
I found that the 'wrong setting of wake_index' can happen frequently,
for consequence, some waitqueue can be empty while some waitqueue have
a lot of waiters.

There shoud be lots of work to fix unfairness throughly, I can remove
this patch for now.

Thanks,
Kuai
>
> Honza
>> ---
>> lib/sbitmap.c | 15 ++++++++++-----
>> 1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
>> index 29eb0484215a..b46fce1beb3a 100644
>> --- a/lib/sbitmap.c
>> +++ b/lib/sbitmap.c
>> @@ -579,19 +579,24 @@ EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>>
>> static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
>> {
>> - int i, wake_index;
>> + int i, wake_index, old_wake_index;
>>
>> +again:
>> if (!atomic_read(&sbq->ws_active))
>> return NULL;
>>
>> - wake_index = atomic_read(&sbq->wake_index);
>> + old_wake_index = wake_index = atomic_read(&sbq->wake_index);
>> for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
>> struct sbq_wait_state *ws = &sbq->ws[wake_index];
>>
>> if (waitqueue_active(&ws->wait)) {
>> - if (wake_index != atomic_read(&sbq->wake_index))
>> - atomic_set(&sbq->wake_index, wake_index);
>> - return ws;
>> + if (wake_index == old_wake_index)
>> + return ws;
>> +
>> + if (atomic_cmpxchg(&sbq->wake_index, old_wake_index,
>> + wake_index) == old_wake_index)
>> + return ws;
>> + goto again;
>> }
>>
>> wake_index = sbq_index_inc(wake_index);
>> --
>> 2.31.1
>>

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