RE: [PATCH] Fix hotplug cpu on x86_64

["Protasevich, Natalie" <Natalie.Protasevich@UNISYS.com>]

> Brian Gerst wrote:
> > Brian Gerst wrote:
> >> I've been seeing bogus values from /proc/loadavg on an x86-64 SMP 
> >> kernel (but not UP).
> >>
> >> $ cat /proc/loadavg
> >> -1012098.26 922203.26 -982431.60 1/112 2688
> >>
> >> This is in the current git tree.  I'm also seeing strange values in
> >> /proc/stat:
> >>
> >> cpu  2489 40 920 60530 9398 171 288 1844674407350 cpu0 2509 60 940 
> >> 60550 9418 191 308 0
> >>
> >> The first line is the sum of all cpus (I only have one), so it's 
> >> picking up up bad data from the non-present cpus.  The last value, 
> >> stolen time, is completely bogus since that value is only 
> ever used 
> >> on s390.
> >>
> >> It looks to me like there is some problem with how the per-cpu 
> >> structures are being initialized, or are getting 
> corrupted.  I have 
> >> not been able to test i386 SMP yet to see if the problem is x86_64 
> >> specific.
> > 
> > I found the culprit: CPU hotplug.  The problem is that
> > prefill_possible_map() is called after setup_per_cpu_areas().  This 
> > leaves the per-cpu data sections for the future cpus uninitialized 
> > (still pointing to the original per-cpu data, which is initmem).  
> > Since the cpus exists in cpu_possible_map, for_each_cpu 
> will iterate 
> > over them even though the per-cpu data is invalid.
> 

I had to do the same in i386, but initially I was trying to avoid the
whole situation - allocating per_cpu data for all possible processors.
It seemed wasteful that on the system with NR_CPU=256 or 512 and brought
up as 4x everything per_cpu is (pre)allocated for all, although it's
sure convenient. I though at the time it would be great if
alloc_percpu() mechanism was able to dynamically re-create all the
per_cpu's for new processors, that way cpu_possible_map woun't probably
even be needed. Or is it too much trouble for too little gain...

Thanks,
--Natalie


> Initialize cpu_possible_map properly in the hotplug cpu case. 
>  Before this, the map was filled in after the per-cpu areas 
> were set up.  This left those cpus pointing to initmem and 
> causing invalid data in /proc/stat and elsewhere.
> 
> Signed-off-by: Brian Gerst <bgerst@didntduck.org>
> 
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