Subject: Re: [PATCH net-next 1/8] perf: optimize perf_fetch_caller_regs

From: Alexei Starovoitov <[email-masked]>
Date: Tue, 5 Apr 2016 10:41:03 -0700

On 4/5/16 5:06 AM, Peter Zijlstra wrote:
> On Mon, Apr 04, 2016 at 09:52:47PM -0700, Alexei Starovoitov wrote:
>> avoid memset in perf_fetch_caller_regs, since it's the critical path of all tracepoints.
>> It's called from perf_sw_event_sched, perf_event_task_sched_in and all of perf_trace_##call
>> with this_cpu_ptr(&__perf_regs[..]) which are zero initialized by perpcu_alloc
>
> Its not actually allocated; but because its a static uninitialized
> variable we get .bss like behaviour and the initial value is copied to
> all CPUs when the per-cpu allocator thingy bootstraps SMP IIRC.

yes, it's .bss-like in a special section. I think static percpu still
goes through some fancy boot time init similar to dynamic.
What I tried to emphasize that either static or dynamic percpu areas
are guaranteed to be zero initialized.

>> and
>> subsequent call to perf_arch_fetch_caller_regs initializes the same fields on all archs,
>> so we can safely drop memset from all of the above cases and
>
> Indeed.
>
>> move it into
>> perf_ftrace_function_call that calls it with stack allocated pt_regs.
>
> Hmm, is there a reason that's still on-stack instead of using the
> per-cpu thing, Steve?
>
>> Signed-off-by: Alexei Starovoitov <[email-masked]>
>
> In any case,
>
> Acked-by: Peter Zijlstra (Intel) <[email-masked]>

Thanks for the quick review.