On Thu, 30 Sep 2010 19:28:05 +0200 Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> Le jeudi 30 septembre 2010 __ 09:45 -0700, Andrew Morton a __crit :
>
> > Could eliminate `p' I guess, but that would involve using
> > __get_cpu_var() as an lval, which looks vile and might generate worse
> > code.
> >
>
> Hmm, I see, please check this new patch, using the most modern stuff ;)
>
> > Readers of this code won't know why last_ino_get() was marked noinline.
> > It looks wrong, really.
>
> Oops sorry, this was a temporary hack of mine to ease disassembly
> analysis. Good catch !
>
> Here is the new generated code on i686 (with the noinline) :
> pretty good ;)
>
> c02e5930 <last_ino_get>:
> c02e5930: 55 push %ebp
> c02e5931: 89 e5 mov %esp,%ebp
> c02e5933: 64 a1 44 29 7d c0 mov %fs:0xc07d2944,%eax
> c02e5939: a9 ff 03 00 00 test $0x3ff,%eax
> c02e593e: 74 09 je c02e5949 <last_ino_get+0x19>
> c02e5940: 40 inc %eax
> c02e5941: 64 a3 44 29 7d c0 mov %eax,%fs:0xc07d2944
> c02e5947: c9 leave
> c02e5948: c3 ret
> c02e5949: b8 00 04 00 00 mov $0x400,%eax
> c02e594e: f0 0f c1 05 80 c8 92 c0 lock xadd %eax,0xc092c880
> c02e5956: eb e8 jmp c02e5940 <last_ino_get+0x10>
>
That uniprocessor, PREEMPT=n I guess.
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -624,6 +624,45 @@ void inode_add_to_lists(struct super_block *sb, struct inode *inode)
> }
> EXPORT_SYMBOL_GPL(inode_add_to_lists);
>
> +#define LAST_INO_BATCH 1024
> +
> +/*
> + * Each cpu owns a range of LAST_INO_BATCH numbers.
> + * 'shared_last_ino' is dirtied only once out of LAST_INO_BATCH allocations,
> + * to renew the exhausted range.
> + *
> + * This does not significantly increase overflow rate because every CPU can
> + * consume at most LAST_INO_BATCH-1 unused inode numbers. So there is
> + * NR_CPUS*(LAST_INO_BATCH-1) wastage. At 4096 and 1024, this is ~0.1% of the
> + * 2^32 range, and is a worst-case. Even a 50% wastage would only increase
> + * overflow rate by 2x, which does not seem too significant.
> + *
> + * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW
> + * error if st_ino won't fit in target struct field. Use 32bit counter
> + * here to attempt to avoid that.
> + */
> +static DEFINE_PER_CPU(unsigned int, last_ino);
> +
> +static unsigned int last_ino_get(void)
> +{
> + unsigned int res;
> +
> + get_cpu();
> + res = __this_cpu_read(last_ino);
> +#ifdef CONFIG_SMP
> + if (unlikely((res & (LAST_INO_BATCH - 1)) == 0)) {
> + static atomic_t shared_last_ino;
> + int next = atomic_add_return(LAST_INO_BATCH, &shared_last_ino);
> +
> + res = next - LAST_INO_BATCH;
> + }
> +#endif
> + res++;
> + __this_cpu_write(last_ino, res);
> + put_cpu();
> + return res;
> +}
Looks good ;)
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