On Tue, 10 Dec 2013 16:42:17 +0100
Petr Mladek <pmla...@suse.cz> wrote:
> probe_kernel_read is used when modifying function calls in
> ftrace_replace_code,
> see arch/x86/kernel/ftrace.c. On x86, the code is replaced using int3 guard.
> All functions are patched in parallel to reduce an expensive synchronization
> of all CPUs. The result is that all affected functions are called via
> ftrace_int3_handler during the process.
>
> ftrace_int3_handler is relatively slow because it has to check whether
> it is responsible for the handled IP. Therefore we should not modify
> functions that used during patching. It would slowdown the patching.
>
> I tried to switch between 7 tracers: blk, branch, function_graph, wakeup_rt,
> irqsoff, function, and nop. Every tracer has also been enabled and disabled.
> With 500 cycles, I got these times before this commit:
>
> real 18m2.477s 18m8.654s 18m9.196s
> user 0m0.008s 0m0.008s 0m0.012s
> sys 0m17.316s 0m17.104s 0m17.300s
>
> and after this commit:
>
> real 16m14.390s 16m15.200s 16m19.632s
> user 0m0.028s 0m0.024s 0m0.028s
> sys 0m23.788s 0m23.812s 0m23.804s
>
Actually no. The benefit of tracing a function outweighs the benefit
of speeding up the conversion. If you want a fast conversion, simply:
echo __probe_kernel_read > /sys/kernel/debug/tracing/set_ftrace_notrace
Try your numbers again with the above line.
-- Steve
> Signed-off-by: Petr Mladek <pmla...@suse.cz>
> ---
> mm/maccess.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/maccess.c b/mm/maccess.c
> index d53adf9ba84b..bed9ee854ea0 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -18,7 +18,7 @@
> long __weak probe_kernel_read(void *dst, const void *src, size_t size)
> __attribute__((alias("__probe_kernel_read")));
>
> -long __probe_kernel_read(void *dst, const void *src, size_t size)
> +long notrace __probe_kernel_read(void *dst, const void *src, size_t size)
> {
> long ret;
> mm_segment_t old_fs = get_fs();
--
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