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SubjectRe: [PATCH] kprobe: increase kprobe_hash_table size
On Fri, 07 Nov 2008 19:18:54 -0500 Masami Hiramatsu <mhiramat@redhat.com> wrote:

> Hi Andrew,
>
> Andrew Morton wrote:
> > On Fri, 07 Nov 2008 18:44:30 -0500 Masami Hiramatsu <mhiramat@redhat.com> wrote:
> >
> >> Increase the size of kprobe hash table to 512. It's useful when hundreds
> >> of kprobes were used in the kernel because current size is just 64.
> >>
> >
> > "useful" is a bit vague. How big is the problem which this solves, and
> > how well did it solve it?
>
> For example, when probing enters and exits of syscall-related functions,
> we need more than 500 probes. In that case, each hlist would have 8
> elements in average. With this patch, the hlist would have 1 element in
> average.
>
> I agree that there may be many opinions about what is the best suited size.
> Why I chose 512 was that I thought the table (byte) size was less than or
> equal 4096 even on 64-bit arch.

Well...

text data bss dec hex filename
7036 744 9380 17160 4308 kernel/kprobes.o
7048 744 73892 81684 13f14 kernel/kprobes.o

That's 64 kbytes more memory. It will be kretprobe_table_locks[] which
is hurting here, due to the ____cacheline_aligned.

I expected CONFIG_X86_VSMP=y to make this far worse, but fortunately
that only affects ____cacheline_internodealigned_in_smp.

btw, that array wastes a ton of memory on uniprocessor builds. Using
____cacheline_aligned_in_smp should fix that.

Please always check these thigns with /usr/bin/size.

btw2, could/should kprobe_table[] and kretprobe_inst_table[] be
aggregated into kretprobe_table_locks[]? That would save some memory
and might save some cache misses as well?


Anyway, enough pos-facto code review. Is this change which you're
proposing worth increasing kernel memory usage by 64k?


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