Polling and kern.polling.idle_poll

[Luigi Rizzo]

On Thu, Jul 02, 2009 at 10:21:55AM -0400, Larry Baird wrote:
> Paul,
> 
> > Larry Baird wrote:
> > >The man page for polling states:
> > >    kern.polling.idle_poll
> > >	 Controls if polling is enabled in the idle loop.  There are no
> > >	 reasons (other than power saving or bugs in the scheduler's han-
> > >	 dling of idle priority kernel threads) to disable this.
> > >
> > >So why is it now disabled by default?  Looking back through cvs the
> > >change was made by Luigi way back in August of 2002.  
> > >
> > >  
> > Did you ever get a reply for this question on the -net list?  I may have 
> > missed it, but I don't think you did.
> > I'm very interested in why this was changed too.
> I got some off list mail from Luigi.  In 4.x, the idle_poll() was called
> from idle.  Now it is its own thread.  CPU statistics will now show high 100%
> utilization if your set kern.polling.idle_poll.  He also has some concerns
> about if locking was in place for SMP.  Looking at the code locking appears
> to be in place.
> 
> The performance improvment is too great for me to ignore, so I made a few
> mods to my kernels.  I modified kern_poll to set P_NOLOAD for the idle_poll
> thread. I then modified CPU statistics to view all processes with P_NOLOAD
> as the idle process.  So far working great for me.  Patches below.

just to understand -- the patch below is just cosmetics to avoid
seeing the polling thread appear as "SYS" time, right ?

cheers
luigi

> *** /sys/kern/kern_poll.c       2006-06-16 18:11:55.000000000 -0400
> --- kern_poll.c 2009-07-02 10:03:07.000000000 -0400
> ***************
> *** 577,583 ****
> --- 577,587 ----
>   {
>         struct thread *td = curthread;
>         struct rtprio rtp;
> +       struct proc *p = td->td_proc;
>   
> +       PROC_LOCK(p);
> +       p->p_flag |= P_NOLOAD;
> +       PROC_UNLOCK(p);
>         rtp.prio = RTP_PRIO_MAX;        /* lowest priority */
>         rtp.type = RTP_PRIO_IDLE;
>         mtx_lock_spin(&sched_lock);
> 
> *** /sys/kern/kern_clock.c      2008-06-04 06:31:41.000000000 -0400
> --- kern_clock.c        2009-07-02 10:05:01.000000000 -0400
> ***************
> *** 516,522 ****
>                                 thread_statclock(0);
>                         td->td_sticks++;
>                         p->p_rux.rux_sticks++;
> !                       if (td != PCPU_GET(idlethread)) {
>                                 cp_time[CP_SYS]++;
>                                 pcp_time[CP_SYS]++;
>                         } else {
> --- 516,522 ----
>                                 thread_statclock(0);
>                         td->td_sticks++;
>                         p->p_rux.rux_sticks++;
> !                       if ( ! (p->p_flag & P_NOLOAD) ) {
>                                 cp_time[CP_SYS]++;
>                                 pcp_time[CP_SYS]++;
>                         } else {
> 
> 
> 
> -- 
> ------------------------------------------------------------------------
> Larry Baird                        | http://www.gta.com
> Global Technology Associates, Inc. | Orlando, FL
> Email: lab at gta.com                 | TEL 407-380-0220, FAX 407-380-6080
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