Re: [PATCH v2 2/2] mm: cma: try next MAX_ORDER_NR_PAGES during retry

[Dong Aisheng <dongas86@gmail.com>]

On Wed, Jan 26, 2022 at 12:33 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 12.01.22 14:15, Dong Aisheng wrote:
> > On an ARMv7 platform with 32M pageblock(MAX_ORDER 14), we observed a
>
> Did you actually intend to talk about pageblocks here (and below)?
>
> I assume you have to be clearer here that you talk about the maximum
> allocation granularity, which is usually bigger than actual pageblock size.
>

I'm talking about the ARM32 case where pageblock_order is equal to MAX_ORDER -1.
/* If huge pages are not used, group by MAX_ORDER_NR_PAGES */
#define pageblock_order         (MAX_ORDER-1)
In order to be clearer, maybe I can add this info into the commit message too.

> > huge number of repeat retries of CMA allocation (1k+) during booting
> > when allocating one page for each of 3 mmc instance probe.
> >
> > This is caused by CMA now supports cocurrent allocation since commit
> > a4efc174b382 ("mm/cma.c: remove redundant cma_mutex lock").
> > The pageblock or (MAX_ORDER -1) from which we are trying to allocate
> > memory may have already been acquired and isolated by others.
> > Current cma_alloc() will then retry the next area by the step of
> > bitmap_no + mask + 1 which are very likely within the same isolated range
> > and fail again. So when the pageblock or MAX_ORDER is big (e.g. 8192),
> > keep retrying in a small step become meaningless because it will be known
> > to fail at a huge number of times due to the pageblock has been isolated
> > by others, especially when allocating only one or two pages.
> >
> > Instread of looping in the same pageblock and wasting CPU mips a lot,
> > especially for big pageblock system (e.g. 16M or 32M),
> > we try the next MAX_ORDER_NR_PAGES directly.
> >
> > Doing this way can greatly mitigate the situtation.
> >
> > Below is the original error log during booting:
> > [    2.004804] cma: cma_alloc(cma (ptrval), count 1, align 0)
> > [    2.010318] cma: cma_alloc(cma (ptrval), count 1, align 0)
> > [    2.010776] cma: cma_alloc(): memory range at (ptrval) is busy, retrying
> > [    2.010785] cma: cma_alloc(): memory range at (ptrval) is busy, retrying
> > [    2.010793] cma: cma_alloc(): memory range at (ptrval) is busy, retrying
> > [    2.010800] cma: cma_alloc(): memory range at (ptrval) is busy, retrying
> > [    2.010807] cma: cma_alloc(): memory range at (ptrval) is busy, retrying
> > [    2.010814] cma: cma_alloc(): memory range at (ptrval) is busy, retrying
> > .... (+1K retries)
> >
> > After fix, the 1200+ reties can be reduced to 0.
> > Another test running 8 VPU decoder in parallel shows that 1500+ retries
> > dropped to ~145.
> >
> > IOW this patch can improve the CMA allocation speed a lot when there're
> > enough CMA memory by reducing retries significantly.
> >
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> > Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > CC: stable@vger.kernel.org # 5.11+
> > Fixes: a4efc174b382 ("mm/cma.c: remove redundant cma_mutex lock")
> > Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
> > ---
> > v1->v2:
> >  * change to align with MAX_ORDER_NR_PAGES instead of pageblock_nr_pages
> > ---
> >  mm/cma.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/cma.c b/mm/cma.c
> > index 1c13a729d274..1251f65e2364 100644
> > --- a/mm/cma.c
> > +++ b/mm/cma.c
> > @@ -500,7 +500,9 @@ struct page *cma_alloc(struct cma *cma, unsigned long count,
> >               trace_cma_alloc_busy_retry(cma->name, pfn, pfn_to_page(pfn),
> >                                          count, align);
> >               /* try again with a bit different memory target */
> > -             start = bitmap_no + mask + 1;
> > +             start = ALIGN(bitmap_no + mask + 1,
> > +                           MAX_ORDER_NR_PAGES >> cma->order_per_bit);
>
> Mind giving the reader a hint in the code why we went for
> MAX_ORDER_NR_PAGES?
>

Yes, good suggestion.
I could add one more line of code comments as follows:
"As alloc_contig_range() will isolate all pageblocks within the range
which are aligned
with max_t(MAX_ORDER_NR_PAGES, pageblock_nr_pages),
here we align with MAX_ORDER_NR_PAGES  which is usually bigger
than actual pageblock size"
Does this look ok to you?

> What would happen if the CMA granularity is bigger than
> MAX_ORDER_NR_PAGES? I'd assume no harm done, as we'd try aligning to 0.
>

I think yes.

Regards
Aisheng

> --
> Thanks,
>
> David / dhildenb
>