linux-kernel - Re: [PATCH] proc/schedstat: Expose /proc/<pid>/schedstat if delay accounting is enabled

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Date:	Fri, 29 May 2015 10:04:25 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:	Balbir Singh <bsingharora@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] proc/schedstat: Expose /proc/<pid>/schedstat if delay
 accounting is enabled


* Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com> wrote:

> /proc/<pid>/schedstat is currently only available if CONFIG_SCHEDSTATS is
> enabled. But, all the fields that this exposes are available and valid
> if CONFIG_TASK_DELAY_ACCT is enabled as well.
> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> ---
>  fs/proc/base.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 093ca14..3ece303 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -304,14 +304,17 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
>  }
>  #endif
>  
> +#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> +#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> +#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)

So such #ifdef parades are ugly and are usually a sign of some problem with the 
patch - as in this case.

But there's deeper problems as well:

/*
 * Provides /proc/PID/schedstat
 */
static int proc_pid_schedstat(struct seq_file *m, struct pid_namespace *ns,
                              struct pid *pid, struct task_struct *task)
{
        seq_printf(m, "%llu %llu %lu\n",
                   (unsigned long long)task->se.sum_exec_runtime,
                   (unsigned long long)task->sched_info.run_delay,
                   task->sched_info.pcount);

        return 0;
}

 - The sum_exec_runtime field is available unconditionally.

 - But the sched_info.run_delay field is only maintained if CONFIG_SCHEDSTATS is
   enabled.

 - Also, the sched_info.pcount field is again only maintained if CONFIG_SCHEDSTATS 
   is enabled.

So the claim in your changelog that these fields are maintained in the 
delayaccounting case is plain false: none of the fields are conditional on delay 
accounting.

So what's the purpose of your patch?

Thanks,

	Ingo
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