Re: [PATCH RFC net-next v2 2/7] net: page_pool: place frag_* fields in one cacheline

[Ilias Apalodimas]

Apologies for the late reply, I was on vacation and start going
through my email piles...

On Tue, 18 Jul 2023 at 16:52, Alexander Lobakin
<aleksander.lobakin@xxxxxxxxx> wrote:
>
> From: Jesper Dangaard Brouer <jbrouer@xxxxxxxxxx>
> Date: Fri, 14 Jul 2023 20:37:39 +0200
>
> >
> >
> > On 14/07/2023 19.08, Alexander Lobakin wrote:
> >> On x86_64, frag_* fields of struct page_pool are scattered across two
> >> cachelines despite the summary size of 24 bytes. The last field,
> >> ::frag_users, is pushed out to the next one, sharing it with
> >> ::alloc_stats.
> >> All three fields are used in pretty much the same places. There are some
> >> holes and cold members to move around. Move frag_* one block up, placing
> >> them right after &page_pool_params perfectly at the beginning of CL2.
> >> This doesn't do any meaningful to the second block, as those are some
> >> destroy-path cold structures, and doesn't do anything to ::alloc_stats,
> >> which still starts at 200-byte offset, 8 bytes after CL3 (still fitting
> >> into 1 cacheline).
> >> On my setup, this yields 1-2% of Mpps when using PP frags actively.
> >> When it comes to 32-bit architectures with 32-byte CL: &page_pool_params
> >> plus ::pad is 44 bytes, the block taken care of is 16 bytes within one
> >> CL, so there should be at least no regressions from the actual change.
> >>
> >> Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> >> ---
> >>   include/net/page_pool.h | 10 +++++-----
> >>   1 file changed, 5 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> >> index 829dc1f8ba6b..212d72b5cfec 100644
> >> --- a/include/net/page_pool.h
> >> +++ b/include/net/page_pool.h
> >> @@ -130,16 +130,16 @@ static inline u64
> >> *page_pool_ethtool_stats_get(u64 *data, void *stats)
> >>   struct page_pool {
> >>       struct page_pool_params p;
> >>   +    long frag_users;
> >> +    struct page *frag_page;
> >> +    unsigned int frag_offset;
> >> +    u32 pages_state_hold_cnt;
> >
> > I think this is okay, but I want to highlight that:
> >  - pages_state_hold_cnt and pages_state_release_cnt
> > need to be kept on separate cache-lines.
>
> They're pretty far away from each other. I moved hold_cnt here as well
> to keep it stacked with frag_offset and avoid introducing 32-bit holes.

This is to prevent cache line bouncing and/or false sharing right?
The change seems fine to me as well but mind adding a comment about
this when you resend?

Thanks
/Ilias
>
> >
> >
> >> +
> >>       struct delayed_work release_dw;
> >>       void (*disconnect)(void *);
> >>       unsigned long defer_start;
> >>       unsigned long defer_warn;
> >>   -    u32 pages_state_hold_cnt;
> >> -    unsigned int frag_offset;
> >> -    struct page *frag_page;
> >> -    long frag_users;
> >> -
> >>   #ifdef CONFIG_PAGE_POOL_STATS
> >>       /* these stats are incremented while in softirq context */
> >>       struct page_pool_alloc_stats alloc_stats;
> >
>
> Thanks,
> Olek