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

[Peter Zijlstra]

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.

> 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 <ast@xxxxxxxxxx>

In any case,

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>