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

[Michal Hocko]

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?

-- 
Michal Hocko
SUSE Labs