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

[Feng Tang]

On Wed, Mar 03, 2021 at 09:22:50AM -0800, Ben Widawsky wrote:
> 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.

As discussed earlier in the thread, one less hacky solution is to clear
__GFP_DIRECT_RECLAIM bit so that it won't go into direct reclaim, but still
wakeup the kswapd of target nodes and retry, which sits now between 'Preferred hard'
and 'Preferred soft' :)

For current MPOL_PREFERRED, its semantic is also 'Preferred hard', that it
will check free memory of other nodes before entering slowpath waiting.

Thanks,
Feng