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 08:07:17PM +0800, Tang, Feng wrote:
> Hi Michal,
> 
> On Wed, Mar 03, 2021 at 12:39:57PM +0100, Michal Hocko wrote:
> > On Wed 03-03-21 18:20:58, Feng Tang wrote:
> > > When doing broader test, we noticed allocation slowness in one test
> > > case that malloc memory with size which is slightly bigger than free
> > > memory of targeted nodes, but much less then the total free memory
> > > of system.
> > > 
> > > The reason is the code enters the slowpath of __alloc_pages_nodemask(),
> > > which takes quite some time. As alloc_pages_policy() will give it a 2nd
> > > try with NULL nodemask, so there is no need to enter the slowpath for
> > > the first try. Add a new gfp bit to skip the slowpath, so that user cases
> > > like this can leverage.
> > > 
> > > With it, the malloc in such case is much accelerated as it never enters
> > > the slowpath.
> > > 
> > > Adding a new gfp_mask bit is generally not liked, and another idea is to
> > > add another nodemask to struct 'alloc_context', so it has 2: 'preferred-nmask'
> > > and 'fallback-nmask', and they will be tried in turn if not NULL, with
> > > it we can call __alloc_pages_nodemask() only once.
> > 
> > Yes, it is very much disliked. Is there any reason why you cannot use
> > GFP_NOWAIT for that purpose?
> 
> I did try that at the first place, but it didn't obviously change the slowness.
> I assumed the direct claim was still involved as GFP_NOWAIT only impact kswapd
> reclaim.

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.

Thanks,
Feng