Re: [RFC PATCH v2 2/3] xpfo: Only put previous userspace pages into the hot cache

On 09/02/2016 10:39 PM, Dave Hansen wrote:

On 09/02/2016 04:39 AM, Juerg Haefliger wrote:

quoted

Allocating a page to userspace that was previously allocated to the
kernel requires an expensive TLB shootdown. To minimize this, we only
put non-kernel pages into the hot cache to favor their allocation.

But kernel allocations do allocate from these pools, right?

Yes.

Does this
just mean that kernel allocations usually have to pay the penalty to
convert a page?

Only pages that are allocated for userspace (gfp & GFP_HIGHUSER == GFP_HIGHUSER) which were
previously allocated for the kernel (gfp & GFP_HIGHUSER != GFP_HIGHUSER) have to pay the penalty.

So, what's the logic here?  You're assuming that order-0 kernel
allocations are more rare than allocations for userspace?

The logic is to put reclaimed kernel pages into the cold cache to postpone their allocation as long
as possible to minimize (potential) TLB flushes.

...Juerg