linux-kernel - Re: [REGRESSION] [Linux 3.2] top/htop and all other CPU usage

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Date:	Fri, 2 Dec 2011 11:39:21 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	"Artem S. Tashkinov" <t.artem@...os.com>, pomac@...or.com,
	linux-kernel@...r.kernel.org, tino.keitel@...ei.de,
	Len Brown <lenb@...nel.org>
Subject: Re: [REGRESSION] [Linux 3.2] top/htop and all other CPU usage

On Thu 01-12-11 15:07:49, Michal Hocko wrote:
[...]
> While implementation is not race free (we better not use locks in that
> path...) so we might race:
> 
> E.g.
> 
> CPU1			CPU2
> now = ktime_get
> 			tick_nohz_start_idle
> 				ts->idle_entrytime = now;
> if (ts->idle_active)
> 				ts->idle_active = 1
> [...]
> return idle_sleeptime
> 
> But this is OK because sleeptime will be more or less accurate. We just
> skip few ticks.
> 
> It would be worse if we had a race like:
> CPU1			CPU2
> now = ktime_get
> 			tick_nohz_start_idle
> 				now = ktime_get
> 				update_ts_time_stats()
> 				ts->idle_entrytime = now;
> 				ts->idle_active = 1
> if (ts->idle_active)
> 	delta = ktime_sub(now, idle_entrytime)
> 	ktime_add(idle_sleeptime, delta)
> 
> In this case we might get an overflow from ktime_sub but AFAIU the
> ktime_* magic the overflow should cause to get smaller idle_sleeptime
> in the end after ktime_add (we do not add a small number but rather
> subtract it), right?

Scratch that. Dunno why but I thought that ktime_t has unsigned values
but it is apparently not true (tv64 is s64). Anyway the above races should
be safe.

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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