Re: x86, perf, pmu: make reserve_ds_buffers() allocate memory dynamically

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-06-24 at 12:01 -0400, Don Zickus wrote:
> Hi guys,
> 
> Some kdump folks noticed that on large machines (say 80 cpu threads), the
> kernel reserved a good chunk of memory in the kdump kernel with the call
> to reserve_ds_buffers(). About 64k per cpu.
> 
> Normally the kdump kernel boots with maxcpus=1 to limit the amount of
> processing and memory usage it needs.  However, with reserve_ds_buffers()
> it seems to allocate a giant chunk of memory due to its use of
> for_each_possible_cpu().
> 
> I was wondering if it was possible to move some of that allocation to the
> cpu_prepare/cpu_starting routines and allocate during a cpu hotplug event.
> I am not sure if some of this allocation is on a per cpu basis (which
> includes multiple cores) or if there is some sort of other restriction
> that required the code to be statically allocated at boot time.

The only complication I can remember is that the BTS buffers are an
order 4 allocation, so the later you allocate them the less likely they
are to be available.

Other than that there isn't anything magical about any of it.