Re: [Linuxarm] Re: [PATCH net-next v2 2/2] ptr_ring: make __ptr_ring_empty() checking more reliable

[Yunsheng Lin]

On 2021/6/27 14:07, Michael S. Tsirkin wrote:
> On Fri, Jun 25, 2021 at 05:20:10PM +0800, Yunsheng Lin wrote:
>> On 2021/6/25 14:39, Michael S. Tsirkin wrote:
>>> On Fri, Jun 25, 2021 at 11:18:56AM +0800, Yunsheng Lin wrote:
>>>> Currently r->queue[] is cleared after r->consumer_head is moved
>>>> forward, which makes the __ptr_ring_empty() checking called in
>>>> page_pool_refill_alloc_cache() unreliable if the checking is done
>>>> after the r->queue clearing and before the consumer_head moving
>>>> forward.
>>>>
>>>> Move the r->queue[] clearing after consumer_head moving forward
>>>> to make __ptr_ring_empty() checking more reliable.
>>>>
>>>> As a side effect of above change, a consumer_head checking is
>>>> avoided for the likely case, and it has noticeable performance
>>>> improvement when it is tested using the ptr_ring_test selftest
>>>> added in the previous patch.
>>>>
>>>> Using "taskset -c 1 ./ptr_ring_test -s 1000 -m 0 -N 100000000"
>>>> to test the case of single thread doing both the enqueuing and
>>>> dequeuing:
>>>>
>>>>  arch     unpatched           patched       delta
>>>> arm64      4648 ms            4464 ms       +3.9%
>>>>  X86       2562 ms            2401 ms       +6.2%
>>>>
>>>> Using "taskset -c 1-2 ./ptr_ring_test -s 1000 -m 1 -N 100000000"
>>>> to test the case of one thread doing enqueuing and another thread
>>>> doing dequeuing concurrently, also known as single-producer/single-
>>>> consumer:
>>>>
>>>>  arch      unpatched             patched         delta
>>>> arm64   3624 ms + 3624 ms   3462 ms + 3462 ms    +4.4%
>>>>  x86    2758 ms + 2758 ms   2547 ms + 2547 ms    +7.6%
>>>
>>> Nice but it's small - could be a fluke.
>>> How many tests did you run? What is the variance?
>>> Did you try pinning to different CPUs to observe numa effects?
>>> Please use perf or some other modern tool for this kind
>>> of benchmark. Thanks!
>>
>> The result is quite stable, and retest using perf stat：
> 
> How stable exactly?  Try with -r so we can find out.

Retest with "perf stat -r":

For unpatched one:
Performance counter stats for './ptr_ring_test -s 1000 -m 1 -N 100000000' (100 runs):

           6780.97 msec task-clock                #    2.000 CPUs utilized            ( +-  5.36% )
                73      context-switches          #    0.011 K/sec                    ( +-  5.07% )
                 0      cpu-migrations            #    0.000 K/sec                    ( +-100.00% )
                81      page-faults               #    0.012 K/sec                    ( +-  0.76% )
       17629544748      cycles                    #    2.600 GHz                      ( +-  5.36% )
       25496488950      instructions              #    1.45  insn per cycle           ( +-  0.26% )
   <not supported>      branches
          11489031      branch-misses                                                 ( +-  1.69% )

             3.391 +- 0.182 seconds time elapsed  ( +-  5.35% )

For patched one:
Performance counter stats for './ptr_ring_test_opt -s 1000 -m 1 -N 100000000' (100 runs):

           6567.83 msec task-clock                #    2.000 CPUs utilized            ( +-  5.53% )
                71      context-switches          #    0.011 K/sec                    ( +-  5.26% )
                 0      cpu-migrations            #    0.000 K/sec
                82      page-faults               #    0.012 K/sec                    ( +-  0.85% )
       17075489298      cycles                    #    2.600 GHz                      ( +-  5.53% )
       23861051578      instructions              #    1.40  insn per cycle           ( +-  0.07% )
   <not supported>      branches
          10473776      branch-misses                                                 ( +-  0.60% )

             3.284 +- 0.182 seconds time elapsed  ( +-  5.53% )


The result is more stable when using taskset to limit the running cpu, but I suppose
the above data is stable enough to justify the performance improvement?