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

[Michal Hocko <mhocko@suse.com>]

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?
-- 
Michal Hocko
SUSE Labs