Re: [PATCH v2 2/4] mm: introduce cma_alloc_bulk API

[Minchan Kim <minchan@kernel.org>]

On Wed, Dec 02, 2020 at 09:22:36PM +0100, David Hildenbrand wrote:
> On 02.12.20 20:26, Minchan Kim wrote:
> > On Wed, Dec 02, 2020 at 07:51:07PM +0100, Michal Hocko wrote:
> >> On Wed 02-12-20 09:54:29, Minchan Kim wrote:
> >>> On Wed, Dec 02, 2020 at 05:48:34PM +0100, Michal Hocko wrote:
> >>>> On Wed 02-12-20 08:15:49, Minchan Kim wrote:
> >>>>> On Wed, Dec 02, 2020 at 04:49:15PM +0100, Michal Hocko wrote:
> >>>> [...]
> >>>>>> Well, what I can see is that this new interface is an antipatern to our
> >>>>>> allocation routines. We tend to control allocations by gfp mask yet you
> >>>>>> are introducing a bool parameter to make something faster... What that
> >>>>>> really means is rather arbitrary. Would it make more sense to teach
> >>>>>> cma_alloc resp. alloc_contig_range to recognize GFP_NOWAIT, GFP_NORETRY resp.
> >>>>>> GFP_RETRY_MAYFAIL instead?
> >>>>>
> >>>>> If we use cma_alloc, that interface requires "allocate one big memory
> >>>>> chunk". IOW, return value is just struct page and expected that the page
> >>>>> is a big contiguos memory. That means it couldn't have a hole in the
> >>>>> range.
> >>>>> However the idea here, what we asked is much smaller chunk rather
> >>>>> than a big contiguous memory so we could skip some of pages if they are
> >>>>> randomly pinned(long-term/short-term whatever) and search other pages
> >>>>> in the CMA area to avoid long stall. Thus, it couldn't work with exising
> >>>>> cma_alloc API with simple gfp_mak.
> >>>>
> >>>> I really do not see that as something really alient to the cma_alloc
> >>>> interface. All you should care about, really, is what size of the object
> >>>> you want and how hard the system should try. If you have a problem with
> >>>> an internal implementation of CMA and how it chooses a range and deal
> >>>> with pinned pages then it should be addressed inside the CMA allocator.
> >>>> I suspect that you are effectivelly trying to workaround those problems
> >>>> by a side implementation with a slightly different API. Or maybe I still
> >>>> do not follow the actual problem.
> >>>>  
> >>>>>> I am not deeply familiar with the cma allocator so sorry for a
> >>>>>> potentially stupid question. Why does a bulk interface performs better
> >>>>>> than repeated calls to cma_alloc? Is this because a failure would help
> >>>>>> to move on to the next pfn range while a repeated call would have to
> >>>>>> deal with the same range?
> >>>>>
> >>>>> Yub, true with other overheads(e.g., migration retrial, waiting writeback
> >>>>> PCP/LRU draining IPI)
> >>>>
> >>>> Why cannot this be implemented in the cma_alloc layer? I mean you can
> >>>> cache failed cases and optimize the proper pfn range search.
> >>>
> >>> So do you suggest this?
> >>>
> >>> enum cma_alloc_mode {
> >>> 	CMA_ALLOC_NORMAL,
> >>> 	CMA_ALLOC_FAIL_FAST,
> >>> };
> >>>
> >>> struct page *cma_alloc(struct cma *cma, size_t count, unsigned int
> >>> 	align, enum cma_alloc_mode mode);
> >>>
> >>> >From now on, cma_alloc will keep last failed pfn and then start to
> >>> search from the next pfn for both CMA_ALLOC_NORMAL and
> >>> CMA_ALLOC_FAIL_FAST if requested size from the cached pfn is okay
> >>> within CMA area and then wraparound it couldn't find right pages
> >>> from the cached pfn. Othewise, the cached pfn will reset to the zero
> >>> so that it starts the search from the 0. I like the idea since it's
> >>> general improvement, I think.
> >>
> >> Yes something like that. There are more options to be clever here - e.g.
> >> track ranges etc. but I am not sure this is worth the complexity.
> > 
> > Agree. Just last pfn caching would be good enough as simple start.
> > 
> >>
> >>> Furthemore, With CMA_ALLOC_FAIL_FAST, it could avoid several overheads
> >>> at the cost of sacrificing allocation success ratio like GFP_NORETRY.
> >>
> >> I am still not sure a specific flag is a good interface. Really can this
> >> be gfp_mask instead?
> > 
> > I am not strong(even, I did it with GFP_NORETRY) but David wanted to
> > have special mode and I agreed when he mentioned ALLOC_CONTIG_HARD as
> > one of options in future(it would be hard to indicate that mode with
> > gfp flags).
> 
> I can't tell regarding the CMA interface, but for the alloc_contig()
> interface I think modes make sense. Yes, it's different to other
> allocaters, but the contig range allocater is different already. E.g.,
> the CMA allocater mostly hides "which exact PFNs you try to allocate".
> 
> In the contig range allocater, gfp flags are currently used to express
> how to allocate pages used as migration targets. I don't think mangling
> in other gfp flags (or even overloading them) makes things a lot
> clearer. E.g., GFP_NORETRY: don't retry to allocate migration targets?
> don't retry to migrate pages? both?
> 
> As I said, other aspects might be harder to model (e.g., don't drain
> LRU) and hiding them behind generic gfp flags (e.g., GFP_NORETRY) feels
> wrong.

I also support a special flag/bool variable for cma_alloc rather than
relying on mixing original gfp_flags since it would be more clear
with preventing passing unhandled the other gfp_flags into cma_alloc.

> 
> With the mode, we're expressing details for the necessary page
> migration. Suggestions on how to model that are welcome.
> 
> -- 
> Thanks,
> 
> David / dhildenb
>