Re: [RFC PATCH v2 14/15] mm: Add alloc-free handshake to trigger memory region compaction

[Cody P Schafer <cody@linux.vnet.ibm.com>]

On 04/09/2013 02:48 PM, Srivatsa S. Bhat wrote:
> We need a way to decide when to trigger the worker threads to perform
> region evacuation/compaction. So the strategy used is as follows:
>
> Alloc path of page allocator:
> ----------------------------
>
> This accurately tracks the allocations and detects the first allocation
> in a new region and notes down that region number. Performing compaction
> rightaway is not going to be helpful because we need free pages in the
> lower regions to be able to do that. And the page allocator allocated in
> this region precisely because there was no memory available in lower regions.
> So the alloc path just notes down the freshly used region's id.
>
> Free path of page allocator:
> ---------------------------
>
> When we enter this path, we know that some memory is being freed. Here we
> check if the alloc path had noted down any region for compaction. If so,
> we trigger the worker function that tries to compact that memory.
>
> Also, we avoid any locking/synchronization overhead over this worker
> function in the alloc/free path, by attaching appropriate semantics to the
> available status flags etc, such that we won't need any special locking
> around them.
>

Can you explain why avoiding locking works in this case?

It appears the lack of locking is only on the worker side, and the 
mem_power_ctrl is implicitly protected by zone->lock on the alloc & free 
side.

In the previous patch I see smp_mb(), but no explanation is provided for 
why they are needed. Are they related to/necessary for this lack of locking?

What happens when a region is passed over for compaction because the 
worker is already compacting another region? Can this occur? Will the 
compaction re-trigger appropriately?

I recommend combining this patch and the previous patch to make the 
interface more clear, or make functions that explicitly handle the 
interface for accessing mem_power_ctrl.

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