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

Stephane Eranian
2010-09-13T15:55:51Z

On Mon, Sep 13, 2010 at 5:41 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, 2010-09-13 at 17:31 +0200, Stephane Eranian wrote:
>> On Mon, Sep 13, 2010 at 5:24 PM, Peter Zijlstra <peterz@infradead.org> wrote:
>> > On Mon, 2010-09-13 at 17:20 +0200, Stephane Eranian wrote:
>> >
>> >> That is the case we the sizes you have chosen today. For DS, we
>> >> could round up to one page for now.
>> >
>> > Markus chose the BTS size, for PEBS a single page was plenty since we do
>> > single event things (although we could do multiple for attr.precise_ip <
>> > 2).
>> >
>> > For DS there's:
>> >  kmalloc_node(sizeof(struct ds), GFP_KERNEL | __GFP_ZERO, cpu_node(cpu));
>> >
>> Ok, let try again with alloc_pages_node() + kmalloc_node().
>> I think we can stick with kmalloc() for DS because we are far from
>> consuming a page.
>
> Thing is, if you're really seeing allocation failures,
> alloc_pages_node() isn't going to help. And the problem is, these
> allocations aren't movable, so memory compaction and all the other fancy
> stuff aren't really going to help much :/
>

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?

> There was some talk about a function that should sync all of vmalloc
> space, but iirc it was broken for some configs.
>