linux-kernel - Re: [PATCH net-next v5 01/13] mm: page_frag: add a test module for page

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Date: Fri, 31 May 2024 16:50:35 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: <davem@...emloft.net>, <pabeni@...hat.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Alexander Duyck <alexander.duyck@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH net-next v5 01/13] mm: page_frag: add a test module for
 page_frag

On 2024/5/30 23:16, Jakub Kicinski wrote:
> On Thu, 30 May 2024 17:17:17 +0800 Yunsheng Lin wrote:
>>> Is this test actually meaningfully testing page_frag or rather
>>> the objpool construct and the scheduler? :S  
>>
>> For the objpool part, I guess it is ok to say that it is a
>> meaningfully testing for both page_frag and objpool if there is
>> changing to either of them.
> 
> Why guess when you can measure it. 
> Slow one down and see if it impacts the benchmark.

Before the slowing down on arm64 system:

 Performance counter stats for 'insmod ./page_frag_test.ko test_push_cpu=16 test_pop_cpu=17' (500 runs):

         19.420606      task-clock (msec)         #    0.001 CPUs utilized            ( +-  0.82% )
                 7      context-switches          #    0.377 K/sec                    ( +-  0.30% )
                 1      cpu-migrations            #    0.038 K/sec                    ( +-  2.82% )
                84      page-faults               #    0.004 M/sec                    ( +-  0.06% )
          50423999      cycles                    #    2.596 GHz                      ( +-  0.82% )
          35558295      instructions              #    0.71  insn per cycle           ( +-  0.09% )
           8340405      branches                  #  429.462 M/sec                    ( +-  0.08% )
             20669      branch-misses             #    0.25% of all branches          ( +-  0.10% )

      24.047641626 seconds time elapsed                                          ( +-  0.08% )


And there are 5120000 push and pop operations for each iteration,
so roughly each push and pop operation costs about 4687ns.

By adding 50ns delay in *__page_frag_alloc_va_align()
@@ -300,6 +297,8 @@ void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
 {
        unsigned int remaining = nc->remaining & align_mask;

+       ndelay(50);
+
        if (unlikely(fragsz > remaining)) {


We have:
 Performance counter stats for 'insmod ./page_frag_test.ko test_push_cpu=16 test_pop_cpu=17' (500 runs):

         18.012657      task-clock (msec)         #    0.001 CPUs utilized            ( +-  0.01% )
                 7      context-switches          #    0.395 K/sec                    ( +-  0.20% )
                 1      cpu-migrations            #    0.052 K/sec                    ( +-  1.35% )
                84      page-faults               #    0.005 M/sec                    ( +-  0.06% )
          46765406      cycles                    #    2.596 GHz                      ( +-  0.01% )
          35253336      instructions              #    0.75  insn per cycle           ( +-  0.00% )
           8277063      branches                  #  459.514 M/sec                    ( +-  0.00% )
             20558      branch-misses             #    0.25% of all branches          ( +-  0.07% )

      24.313647557 seconds time elapsed                                          ( +-  0.07% )


(24.313647557 - 24.047641626) * 1000000000 / 5120000 = 51ns, so the
testing seems correct.

> 
>> For the scheduler part, this test provides the below module param
>> to avoid the the noise from scheduler.
>>
>> +static int test_push_cpu;
>> +module_param(test_push_cpu, int, 0600);
>> +MODULE_PARM_DESC(test_push_cpu, "test cpu for pushing fragment");
>> +
>> +static int test_pop_cpu;
>> +module_param(test_pop_cpu, int, 0600);
>> +MODULE_PARM_DESC(test_pop_cpu, "test cpu for popping fragment");
>>
>> Or is there any better idea for testing page_frag?
> 
> .
>