Date:	Tue, 11 May 2010 21:46:20 -0500
From:	Xianghua Xiao <xiaoxianghua@...il.com>
To:	Suresh Rajashekara <suresh.raj+linuxomap@...il.com>
Cc:	linux-omap@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Issue with SCHED_FIFO app

On Sun, May 9, 2010 at 11:42 PM, Suresh Rajashekara
<suresh.raj+linuxomap@...il.com> wrote:
> Hi All,
>
> I had a couple of application (with real time priority SCHED_FIFO)
> which were working fine on 2.6.16. They have started behaving
> differently on 2.6.29.
>
> I will explain my problem briefly.
>
> Application A (my main application) is scheduled with SCHED_FIFO and priority 5.
> Application B (watchdog application) is also scheduled with SCHED_FIFO
> but with priority 54.
>
> A keeps putting the OMAP to sleep and wake up every 4 seconds and
> again puts it to sleep.
> B is supposed to be running every 1.25 seconds to kick watchdog, but
> since A keeps OMAP in sleep for 4 seconds, it should run as soon as
> OMAP wakes up.
>
> Since B is of a higher priority, its supposed to run whenever the OMAP
> wakes up and then A should again put it back to sleep. This happens
> perfectly on 2.6.16
>
> On 2.6.29, B fails to run when OMAP wakes up and before A puts it back
> to sleep. B only runs if there is atleast 1.5 seconds of delay between
> the awake-sleep cycle.
>
> On searching the internet, I figured out that CFS (completely fair
> scheduler) was introduced in 2.6.23, which makes some changes to the
> RT bandwidth (and many users started facing issues with they
> applications with SCHED_FIFO). Somewhere on the web I found that
> issuing
>
> echo -1 > /proc/sys/kernel/sched_rt_runtime_us
>
> should disable the changes which affects the RT bandwidth. It actually
> did help to an extent in solving some other problem (not described
> above. A's IOCTL call return was getting delayed), but this problem
> still persists.
>
> Any pointers to where I should look for the solution.
>
> Is there a way I can revert back to the scheduler behavior as it was on 2.6.16?
>
> I have disabled CONFIG_GROUP_SCHED and also CONFIG_CGROUPS. I am using
> 2.6.29 on an OMAP1 platform.
>
> Thanks in advance,
> Suresh
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>

I have seen similar things while upgrading a 2.6.18 RT kernel to
2.6.33 RT, actually exactly when CFS was introduced we found
performance issues, in that, our main application(a multi-thread
SCHED_FIFO / SCHED_RR mixed) runs with much higher overhead under CFS.
In 2.6.18RT, the cpu usage is close to 0% and on newer kernel with
CFS, the cpu usage is 12% when the application runs idle(i.e. sleeping
and waiting for input, WCHAN shows sched_timeout or futex_wait). When
the main application runs with real load, cpu usage gets much worse
with CFS.

I tried various methods, including the one you described above, and
made sure no sched_yield is used, etc, still the main application
spends 6% cpu in user space and 6% in kernel space while at idle. I
tried BFS schedule and it's actually better, about 8% in user space
and 0.6% in kernel space while the application runs idle. Again with
2.6.18 RT it's nearly 0% cpu usage.

Xianghua
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