Re: Does cli() need to be called before reading avenrun?

[Peter.Benie@mvhi.com (Peter Benie)]

Robert Love writes ("Re: Does cli() need to be called before reading avenrun?"):
> On Wed, 2003-01-01 at 19:39, Peter Benie wrote:
> > In kernel 2.4, in sys_sysinfo(), the code reads:
> > 
> >    cli();
> >    val.uptime = jiffies / HZ;
> > 
> >    val.loads[0] = avenrun[0] << (SI_LOAD_SHIFT - FSHIFT);
> >    val.loads[1] = avenrun[1] << (SI_LOAD_SHIFT - FSHIFT);
> >    val.loads[2] = avenrun[2] << (SI_LOAD_SHIFT - FSHIFT);
> > 
> >    val.procs = nr_threads-1;
> >    sti();
> > 
> > In loadavg_read_proc, the code is in essence the same, except that it
> > isn't wrapped in cli/sti.  
> > 
> > Is there a reason for the cli?
> 
> The reason we need some sort of protection is that there are the three
> array entries, so we need to make sure we read all three atomically (not
> that its a huge deal if we do not).

This is exactly what I have my doubts about. These are three load
averages with different time constants - it isn't meaningful to
compare them so it doesn't matter if they are not sampled at exactly
the same time.

What caused me to look at this code is was a complaint in the exim
source about how long sysinfo() took compared with reading
/proc/loadavg - the difference is a factor of 100 which is large
enough to seriously affect exim's performance. This looks like a
possible cause.

Peter