linux-kernel - Re: [PATCH] fix idle ticks in cpu summary line of /proc/stat

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Message-ID: <20120312153906.GE3994@tiehlicka.suse.cz>
Date:	Mon, 12 Mar 2012 16:39:06 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fix idle ticks in cpu summary line of /proc/stat

On Mon 12-03-12 20:18:33, Srivatsa S. Bhat wrote:
> On 03/12/2012 07:47 PM, Martin Schwidefsky wrote:
> 
> > On Mon, 12 Mar 2012 13:17:26 +0100
> > Michal Hocko <mhocko@...e.cz> wrote:
> > 
> >> Goot catch. But I think that the following fix should be better because
> >> it doesn't change the semantic of the function. What do you think?
> > ..
> >> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> >> index 7656642..dec767f 100644
> >> --- a/kernel/time/tick-sched.c
> >> +++ b/kernel/time/tick-sched.c
> >> @@ -221,7 +221,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
> >>  		update_ts_time_stats(cpu, ts, now, last_update_time);
> >>  		idle = ts->idle_sleeptime;
> >>  	} else {
> >> -		if (ts->idle_active && !nr_iowait_cpu(cpu)) {
> >> +		if (cpu_online(cpu) && ts->idle_active && !nr_iowait_cpu(cpu)) {
> >>  			ktime_t delta = ktime_sub(now, ts->idle_entrytime);
> >>
> >>  			idle = ktime_add(ts->idle_sleeptime, delta);
> >> @@ -262,7 +262,7 @@ u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
> >>  		update_ts_time_stats(cpu, ts, now, last_update_time);
> >>  		iowait = ts->iowait_sleeptime;
> >>  	} else {
> >> -		if (ts->idle_active && nr_iowait_cpu(cpu) > 0) {
> >> +		if (cpu_online(cpu) && ts->idle_active && nr_iowait_cpu(cpu) > 0) {
> >>  			ktime_t delta = ktime_sub(now, ts->idle_entrytime);
> >>
> >>  			iowait = ktime_add(ts->iowait_sleeptime, delta);
> > 
> > I would prefer an early exit from the functions. The target cpu is offline,
> > who guarantees that the "struct tick_sched" for the cpu contains anything
> > useful?

Hmm, the semantic is that the function either returns the sleeptime or
-1 if nohz is disabled. Bringing also online/offline into it seems
rather confusing. 
Maybe we shouldn't do the test layer up when we call the function
instead. This should be much cleaner IMO (it also reduced cpu_online
call from the governors call paths which might be a problem as well):

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 121f77c..d437258 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -24,7 +24,10 @@
 
 static u64 get_idle_time(int cpu)
 {
-	u64 idle, idle_time = get_cpu_idle_time_us(cpu, NULL);
+	u64 idle, idle_time = -1ULL;
+	
+	if (cpu_online(cpu))
+		idle_time = get_cpu_idle_time_us(cpu, NULL);
 
 	if (idle_time == -1ULL) {
 		/* !NO_HZ so we can rely on cpustat.idle */
@@ -38,7 +41,10 @@ static u64 get_idle_time(int cpu)
 
 static u64 get_iowait_time(int cpu)
 {
-	u64 iowait, iowait_time = get_cpu_iowait_time_us(cpu, NULL);
+	u64 iowait, iowait_time = -1ULL;
+	
+	if (cpu_online(cpu))
+		iowait_time = get_cpu_iowait_time_us(cpu, NULL);
 
 	if (iowait_time == -1ULL)
 		/* !NO_HZ so we can rely on cpustat.iowait */

> 
> Also, what about the case where last_update_time is non-NULL?
> With Martin's patch update_ts_time_stats() won't be called for offline cpus,

This is a separate issue AFAIU. If there is a possibility that somebody
is calling it like that then it is a bug. I am not familiar with cpu
governors (non-NULL update users) but we shouldn't paper over any bug with
this /proc/stat fix.

> whereas with Michal's patch it will be called and hence the counters will get
> updated.. We don't want to update counters for offline cpus right?
> 
> Regards,
> Srivatsa S. Bhat
> 
> --
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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