On 11/15/22 at 12:00pm, Dennis Zhou wrote:
> On Tue, Nov 15, 2022 at 05:08:52PM +0800, Baoquan He wrote:
> > Hi Dennis,
> >
> > On 11/14/22 at 08:13pm, Dennis Zhou wrote:
> > > Hi Vlastimil & Baoquan,
> > >
> > > On Mon, Nov 14, 2022 at 06:58:13PM +0100, Vlastimil Babka wrote:
> > > > On 11/14/22 08:44, Baoquan He wrote:
> > > > > Hi,
> > > > >
> > > > > I reproduced the build failure according to lkp report and made a patch
> > > > > as below to fix it.
> > > > >
> > > > > From dae7dd9705015ce36db757e88c78802584f949b1 Mon Sep 17 00:00:00 2001
> > > > > From: Baoquan He <bhe@xxxxxxxxxx>
> > > > > Date: Sun, 13 Nov 2022 18:08:27 +0800
> > > > > Subject: [PATCH] percpu: adjust the value of PERCPU_DYNAMIC_EARLY_SIZE
> > > > > Content-type: text/plain
> > > > >
> > > > > LKP reported a build failure as below on the patch "mm/slub, percpu:
> > > > > correct the calculation of early percpu allocation size"
> > > >
> > > > Since I have that patch in slab.git exposed to -next, should I take this fix
> > > > too, to make things simpler? Dennis?
> > > >
> > >
> > > I don't have any problems with you running a fix, but I'm not quite sure
> > > this is the right fix. Though this might cause a trivial merge conflict
> > > with: d667c94962c1 ("mm/percpu: remove unused PERCPU_DYNAMIC_EARLY_SLOTS")
> > > in my percpu#for-6.2 branch.
> > >
> > > If I'm understanding this correctly, slub requires additional percpu
> > > memory due to the use of 64k pages. By increasing
> > > PERCPU_DYNAMIC_EARLY_SIZE, we solve the problem for 64k page users, but
> > > require a few unnecessary pages that can bloat the size of subsequent
> > > percpu chunks. Though, I'm not sure if that's an issue today for
> > > embedded devices.
> >
> > Thanks for looking into this.
> >
> > I guess you are talking about PERCPU_DYNAMIC_EARLY_SIZE will impact the
> > first dynamic chunk size of page first chunk, because the embed first
> > chunk will take PERCPU_DYNAMIC_RESERVE. And the impact is done in below
> > max() invacation.
> >
> > static struct pcpu_alloc_info * __init __flatten pcpu_build_alloc_info(
> > size_t reserved_size, size_t dyn_size,
> > size_t atom_size,
> > pcpu_fc_cpu_distance_fn_t cpu_distance_fn)
> > {
> > ......
> > /* calculate size_sum and ensure dyn_size is enough for early alloc */
> > size_sum = PFN_ALIGN(static_size + reserved_size +
> > max_t(size_t, dyn_size, PERCPU_DYNAMIC_EARLY_SIZE));
> > ......
> > }
> >
> > >
> > > I think adding parity to PERCPU_DYNAMIC_EARLY_SIZE with
> > > PERCPU_DYNAMIC_RESERVE is defined by BITS_PER_LONG is a safer option
> > > here. A small TODO item would be to make PERCPU_DYNAMIC_RESERVE be a +
> > > value instead of a max() with PERCPU_DYNAMIC_EARLY_SIZE.
> >
> > Hmm, the below change may not take power arch into account. Please
> > check arch/powerpc/include/asm/page.h, seems the 32bit ppc could have
> > 256K pages too. Adding PERCPU_DYNAMIC_EARLY_SIZE to 20K may cost extra
> > memory during boot. But th left space of 1st dynamic chunk will join
> > the later percpu dynamic allocation, it's not wasted, right?
> >
> > Not sure if I got your point.
> >
> >
>
> Ah, I'm not familiar with all the PAGE_SIZE and word length
> combinations.
>
> The first chunk is smaller in the embedded case with the assumption that
> static percpu variables are highly accessed along with the limited
> initial allocations. While adding an additional 8KB is not the biggest
> deal to the first chunk, this can cause the unit_size for subsequent
> chunks to be larger. For example, x86 unit size jumps in powers of 2 due
> to alignment and packing against an allocation size of 2MB. So if we're
> at say 60KB for the first chunk, subsequent chunks could be 64KB. But
> adding 8KB, we'd go from 60KB -> 68KB and a chunk size of 64KB -> 128KB.
I could have misunderstanding about the first chunk usage and percpu
code. Below is my personal uderstanding about the 1st chunk size and
how PERCPU_DYNAMIC_EARLY_SIZE could impact it, please help point out
if I am wrong.
~~~
Abstract the definition of them here for reference.
/*
* Percpu allocator can serve percpu allocations before slab is
* initialized which allows slab to depend on the percpu allocator.
* The following parameter decide how much resource to preallocate
* for this. Keep PERCPU_DYNAMIC_RESERVE equal to or larger than
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* PERCPU_DYNAMIC_EARLY_SIZE.
~~~~~~~~~~~~~~~~~~~~~
*/
#define PERCPU_DYNAMIC_EARLY_SIZE (12 << 10)
......
#if BITS_PER_LONG > 32
#define PERCPU_DYNAMIC_RESERVE (28 << 10)
#else
#define PERCPU_DYNAMIC_RESERVE (20 << 10)
#endif
>From above definition, we can see that no matter how big
PERCPU_DYNAMIC_RESERVE is , it's >= PERCPU_DYNAMIC_EARLY_SIZE as the
code comment says. So the max() in pcpu_build_alloc_info() won't impact
the embeded 1st chunk at all.
So, PERCPU_DYNAMIC_EARLY_SIZE can only impact the page 1st chunk case,
namely when calling pcpu_page_first_chunk() to do that. In
pcpu_page_first_chunk(), we don't provide dyn_size, so with the help of
max(), it will get final dyn_size as PERCPU_DYNAMIC_EARLY_SIZE. This is
the only place where PERCPU_DYNAMIC_EARLY_SIZE takes effect on percpu.
However, the atom size of page 1st chunk is PAGE_SIZE, it doesn't have
the issue of possible bloating unit_size by the atom size, e.g 2M on
x86_64. Since pcpu_page_first_chunk() is the fallback of
pcpu_embed_first_chunk(), if we decide to provide PERCPU_DYNAMIC_RESERVE
as the current value, why we grudge setting it as the smaller value,
20K, whether it's 32bit or 64bit.
>
> If not `BITS_PER_LONG >32`, we could do `PAGE_SHIFT > 12`.
>
> Thanks,
> Dennis
>
> > >
> > > ---
> > > diff --git a/include/linux/percpu.h b/include/linux/percpu.h
> > > index f1ec5ad1351c..22ce3271eed2 100644
> > > --- a/include/linux/percpu.h
> > > +++ b/include/linux/percpu.h
> > > @@ -42,7 +42,11 @@
> > > * larger than PERCPU_DYNAMIC_EARLY_SIZE.
> > > */
> > > #define PERCPU_DYNAMIC_EARLY_SLOTS 128
> > > +#if BITS_PER_LONG > 32
> > > +#define PERCPU_DYNAMIC_EARLY_SIZE (20 << 10)
> > > +#else
> > > #define PERCPU_DYNAMIC_EARLY_SIZE (12 << 10)
> > > +#endif
> > >
> > > /*
> > > * PERCPU_DYNAMIC_RESERVE indicates the amount of free area to piggy
> > >
> >
> >
>
