Re: [PATCH v3 RFC 14/14] mm: speedup page alloc for MPOL_PREFERRED_MANY by adding a NO_SLOWPATH gfp bit

[Ben Widawsky]

On 21-03-03 18:14:30, Michal Hocko wrote:
> On Wed 03-03-21 08:31:41, Ben Widawsky wrote:
> > On 21-03-03 14:59:35, Michal Hocko wrote:
> > > On Wed 03-03-21 21:46:44, Feng Tang wrote:
> > > > On Wed, Mar 03, 2021 at 09:18:32PM +0800, Tang, Feng wrote:
> > > > > On Wed, Mar 03, 2021 at 01:32:11PM +0100, Michal Hocko wrote:
> > > > > > On Wed 03-03-21 20:18:33, Feng Tang wrote:
> > > [...]
> > > > > > > One thing I tried which can fix the slowness is:
> > > > > > > 
> > > > > > > +	gfp_mask &= ~(__GFP_DIRECT_RECLAIM | __GFP_KSWAPD_RECLAIM);
> > > > > > > 
> > > > > > > which explicitly clears the 2 kinds of reclaim. And I thought it's too
> > > > > > > hacky and didn't mention it in the commit log.
> > > > > > 
> > > > > > Clearing __GFP_DIRECT_RECLAIM would be the right way to achieve
> > > > > > GFP_NOWAIT semantic. Why would you want to exclude kswapd as well? 
> > > > > 
> > > > > When I tried gfp_mask &= ~__GFP_DIRECT_RECLAIM, the slowness couldn't
> > > > > be fixed.
> > > > 
> > > > I just double checked by rerun the test, 'gfp_mask &= ~__GFP_DIRECT_RECLAIM'
> > > > can also accelerate the allocation much! though is still a little slower than
> > > > this patch. Seems I've messed some of the tries, and sorry for the confusion!
> > > > 
> > > > Could this be used as the solution? or the adding another fallback_nodemask way?
> > > > but the latter will change the current API quite a bit.
> > > 
> > > I haven't got to the whole series yet. The real question is whether the
> > > first attempt to enforce the preferred mask is a general win. I would
> > > argue that it resembles the existing single node preferred memory policy
> > > because that one doesn't push heavily on the preferred node either. So
> > > dropping just the direct reclaim mode makes some sense to me.
> > > 
> > > IIRC this is something I was recommending in an early proposal of the
> > > feature.
> > 
> > My assumption [FWIW] is that the usecases we've outlined for multi-preferred
> > would want more heavy pushing on the preference mask. However, maybe the uapi
> > could dictate how hard to try/not try.
> 
> What does that mean and what is the expectation from the kernel to be
> more or less cast in stone?
> 

(I'm not positive I've understood your question, so correct me if I
misunderstood)

I'm not sure there is a stone-cast way to define it nor should we. At the very
least though, something in uapi that has a general mapping to GFP flags
(specifically around reclaim) for the first round of allocation could make
sense.

In my head there are 3 levels of request possible for multiple nodes:
1. BIND: Those nodes or die.
2. Preferred hard: Those nodes and I'm willing to wait. Fallback if impossible.
3. Preferred soft: Those nodes but I don't want to wait.

Current UAPI in the series doesn't define a distinction between 2, and 3. As I
understand the change, Feng is defining the behavior to be #3, which makes #2
not an option. I sort of punted on defining it entirely, in the beginning.