Re: [rfc] lru_add_drain_all() vs isolation

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2009-09-08 at 10:03 -0400, Christoph Lameter wrote:
> On Tue, 8 Sep 2009, Peter Zijlstra wrote:
> 
> > This is about avoiding work when there is non, clearly when an
> > application does use the kernel it creates work.
> 
> Hmmm. The lru draining in page migration is to reduce the number of pages
> that are not on the lru to increase the chance of page migration to be
> successful. A page on a per cpu list cannot be drained.
> 
> Reducing the number of cpus where we perform the drain results in
> increased likelyhood that we cannot migrate a page because its on the per
> cpu lists of a cpu not covered.

Did you even read the patch?

There is _no_ functional difference between before and after, except
less wakeups on cpus that don't have any __lru_cache_add activity.

If there's pages on the per cpu lru_add_pvecs list it will be present in
the mask and will be send a drain request. If its not, then it won't be
send.