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SubjectRe: [PATCH v5 07/10] module: Optimize __module_address() using a latched RB-tree

* Peter Zijlstra <peterz@infradead.org> wrote:

> Currently __module_address() is using a linear search through all
> modules in order to find the module corresponding to the provided
> address. With a lot of modules this can take a lot of time.
>
> One of the users of this is kernel_text_address() which is employed
> in many stack unwinders; which in turn are used by perf-callchain
> and ftrace (possibly from NMI context).
>
> So by optimizing __module_address() we optimize many stack unwinders
> which are used by both perf and tracing in performance sensitive
> code.

So my (rather typical) workstation has 116 modules loaded currently -
but setups using in excess of 150 modules are not uncommon either.

A linear list walk of 100-150 entries for every single call chain
entry that hits some module, in 'perf record -g', can cause some
overhead!

> + /*
> + * If this is non-NULL, vfree after init() returns.

s/vfree/vfree()

> + /*
> + * We want mtn_core::{mod,node[0]} to be in the same cacheline as the
> + * above entries such that a regular lookup will only touch the one
> + * cacheline.

s/touch the one cacheline
/touch one cacheline

?

> +static __always_inline int
> +mod_tree_comp(void *key, struct latch_tree_node *n)
> +{
> + unsigned long val = (unsigned long)key;
> + unsigned long start, end;
> +
> + end = start = __mod_tree_val(n);
> + end += __mod_tree_size(n);
> +
> + if (val < start)
> + return -1;
> +
> + if (val >= end)
> + return 1;
> +
> + return 0;

So since we are counting nanoseconds, I suspect this could be written
more optimally as:

{
unsigned long val = (unsigned long)key;
unsigned long start, end;

start = __mod_tree_val(n);
if (val < start)
return -1;

end = start + __mod_tree_size(n);
if (val >= end)
return 1;

return 0;
}

right?

Thanks,

Ingo


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