Re: [PATCH] dma-debug: increase HASH_SIZE

[Geert Uytterhoeven]

On Wed, Oct 30, 2019 at 8:13 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> With modern NIC, it is not unusual having about ~256,000 active dma
> mappings. Hash size of 1024 buckets is too small.
>
> Forcing full cache line per bucket does not seem useful,
> especially now that we have a contention on free_entries_lock
> for allocations and freeing of entries. Better using space
> to fit more buckets.
>
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> ---
>  kernel/dma/debug.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 4ad74f5987ea9e95f9bb5e2d1592254e367d24fb..35e2a853bff9c482d789ab331d79aaee07753a97 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -27,7 +27,7 @@
>
>  #include <asm/sections.h>
>
> -#define HASH_SIZE       1024ULL
> +#define HASH_SIZE       16384ULL
>  #define HASH_FN_SHIFT   13
>  #define HASH_FN_MASK    (HASH_SIZE - 1)
>
> @@ -87,7 +87,7 @@ typedef bool (*match_fn)(struct dma_debug_entry *, struct dma_debug_entry *);
>  struct hash_bucket {
>         struct list_head list;
>         spinlock_t lock;
> -} ____cacheline_aligned_in_smp;
> +};
>
>  /* Hash list to save the allocated dma addresses */
>  static struct hash_bucket dma_entry_hash[HASH_SIZE];

JFTR, this increases dma_entry_hash size by 327680 bytes, and pushes
a few more boards beyond their bootloader-imposed kernel size limits.

Disabling CONFIG_DMA_API_DEBUG fixes that.
Of course the real fix is to fix the bootloaders...

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds