On Wed, Nov 16, 2022 at 07:32:03PM +0800, Baoquan He wrote:
> 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.
>
I think I might be overindexing on the out of tree modifications here.
Currently, I think it's clear how modifying PERCPU_DYNAMIC_RESERVE
affects the system with the lower bound being dictated by
PERCPU_DYNAMIC_EARLY_SIZE. If we bump PERCPU_DYNAMIC_EARLY_SIZE, it's
not inherently obvious you can drop that value lower depending on your
system config.
Ultimately, it is only a few pages, so is saving it that big of a deal
today? Likely not, just a bit wasteful to potentially orphan a few extra
pages unnecessarily.
Let's just fix this now and I can massage this in the future if anything
comes up. I appreciate you taking the time to have this discussion with
me.
Vlastimil, can you please pick up this fix.
Acked-by: Dennis Zhou <dennis@xxxxxxxxxx>
Thanks,
Dennis
>
> >
> > 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
> > > >
> > >
> > >
> >
>
>
