linux-kernel - Re: [PATCH v5 03/11] cgroup, sched: let cpu serve the same files as cpuacct

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Date:	Tue, 15 Jan 2013 18:19:58 +0800
From:	Sha Zhengju <handai.szj@...il.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michal Hocko <mhocko@...e.cz>,
	Kay Sievers <kay.sievers@...y.org>,
	Lennart Poettering <mzxreary@...inter.de>,
	Dave Jones <davej@...hat.com>,
	Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH v5 03/11] cgroup, sched: let cpu serve the same files as cpuacct

On Mon, Jan 14, 2013 at 10:55 PM, Glauber Costa <glommer@...allels.com> wrote:
> On 01/14/2013 12:34 AM, Sha Zhengju wrote:
>>> +               struct kernel_cpustat *kcpustat = this_cpu_ptr(ca->cpustat);
>>> > +
>>> >                 kcpustat = this_cpu_ptr(ca->cpustat);
>> Is this reassignment unnecessary?
>>
>>
> No.
>

No?  No!

In task_group_account_field(), the following two hunks have the
similar behavior but different codes, there must be a trial in one of
them.

Hunk #1:
+#ifdef CONFIG_CGROUP_SCHED
+       rcu_read_lock();
+       tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
+                         struct task_group, css);

+       while (tg && (tg != &root_task_group)) {
+               struct kernel_cpustat *kcpustat =
this_cpu_ptr(tg->cpustat);   **HERE**
+
+               kcpustat->cpustat[index] += tmp;
+               tg = tg->parent;
+       }
+       rcu_read_unlock();
+#endif

Hunk #2:
#ifdef CONFIG_CGROUP_CPUACCT
        if (unlikely(!cpuacct_subsys.active))
                return;

        rcu_read_lock();
        ca = task_ca(p);
        while (ca && (ca != &root_cpuacct)) {
+               struct kernel_cpustat *kcpustat =
this_cpu_ptr(ca->cpustat); **HERE**
+
                kcpustat = this_cpu_ptr(ca->cpustat);   **HERE, which
is unnecessary**
                kcpustat->cpustat[index] += tmp;
                ca = parent_ca(ca);
        }
        rcu_read_unlock();
#endif


Also you can prove it by the following testcase.
#include <stdio.h>

int main(void)
{
        int i = 0;
        int array[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
        int *index = &array[0];

        while (i < 10) {
                int *ptr = index;

                printf("ptr=%d  %p, index = %d\n", *ptr, ptr, *index);
                index ++;
                i++;
                sleep(1);
        }

        return 0;
}

-- 
Thanks,
Sha
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