Re: [PATCH] Fix hotplug cpu on x86_64

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Protasevich, Natalie wrote:
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


It certainly is possible. In the hotplug cpu case, don't put the .data.percpu section in __initmem. It will then be preserved for any cpus that come online after boot.

--
				Brian Gerst
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux