Re: [PATCH] perf_events: improve DS/BTS/PEBS buffer allocation

Stephane Eranian
2010-09-13T18:40:23Z

On Mon, Sep 13, 2010 at 7:35 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, 2010-09-13 at 17:55 +0200, Stephane Eranian wrote:
>>
>> Ok, so you're saying there is no allocator that will give non-contiguous
>> physical memory WITHOUT requiring a page fault to populate the pte.
>>
>> On the other hand, with vmalloc_node() the pte are populated when
>> you first touch the memory. That happens as part of memset() right after
>> the allocation and thus outside of NMI interrupt handler.
>>
>> Does this sound right?
>
> Nope, in particular read: http://lkml.org/lkml/2010/7/14/465
>
> The issue is that the vmalloc space can be mapped in different
> processes, and that memset() will only ensure its mapped in the current
> process, but the next one might need that fault to populate.
>
Ok, so can we play the same trick you're playing with the sampling
buffer, i.e., you use alloc_pages_node() for one page at a time, and
then you stitch them on demand via SW?