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Subject: Re: [PATCH] sched/fair: provide u64 read for 32-bits arch helper
To:     vincent.donnefort@arm.com
Cc:     mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org,
        linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
        valentin.schneider@arm.com
References: <1595847564-239957-1-git-send-email-vincent.donnefort@arm.com>
From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <7131ec02-39ae-fe8a-e3d6-171a6e6c8103@arm.com>
Date:   Tue, 28 Jul 2020 10:09:29 +0100
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Hi Vincent,

On 7/27/20 11:59 AM, vincent.donnefort@arm.com wrote:
> From: Vincent Donnefort <vincent.donnefort@arm.com>
> 
> Introducing two macro helpers u64_32read() and u64_32read_set_copy() to
> factorize the u64 vminruntime and last_update_time read on a 32-bits
> architecture. Those new helpers encapsulate smp_rmb() and smp_wmb()
> synchronization and therefore, have a small penalty in set_task_rq_fair()
> and init_cfs_rq().
> 
> The choice of using a macro over an inline function is driven by the
> conditional u64 variable copy declarations.
> 
>    #ifndef CONFIG_64BIT
>       u64 [vminruntime|last_update_time]_copy;
>    #endif
> 
> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
> 

I've run it on 32-bit ARM big.LITTLE platform (odroid xu3) with
the CONFIG_FAIR_GROUP_SCHED and CONFIG_CGROUP_SCHED.

I haven't observed any issues while running hackbench or booting
the kernel, the performance of affected function
set_task_rq_fair() is the same.

Function profile results after hackbench test:
--------w/ Vincent's patch + performance cpufreq gov------------
root@odroid:/sys/kernel/debug/tracing# cat trace_stat/function?
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      4068    3753.185 us     0.922 
us        1.239 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      4492    4180.133 us     0.930 
us        2.318 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      4208    3991.386 us     0.948 
us        13.552 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      4753    4432.231 us     0.932 
us        5.875 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      7980    5037.096 us     0.631 
us        1.690 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      8143    5078.515 us     0.623 
us        2.930 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      9721    6477.904 us     0.666 
us        2.425 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      7743    4896.002 us     0.632 
us        1.188 us


-----------w/o Vincent patch, performance cpufreq gov------------
root@odroid:/sys/kernel/debug/tracing# cat trace_stat/function?
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      6525    5830.450 us     0.893 
us        3.213 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      6646    6069.444 us     0.913 
us        9.651 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      5988    5646.133 us     0.942 
us        7.685 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      5883    5390.103 us     0.916 
us        29.283 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      3844    2561.186 us     0.666 
us        0.933 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      5515    3491.011 us     0.633 
us        9.845 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      6947    4808.522 us     0.692 
us        5.822 us
   Function                               Hit    Time            Avg 
         s^2
   --------                               ---    ----            --- 
         ---
   set_task_rq_fair                      4530    2810.000 us     0.620 
us        0.554 us


Tested-by: Lukasz Luba <lukasz.luba@arm.com>

Regards,
Lukasz