linux-kernel - Re: [PATCH net-next 1/8] perf: optimize perf_fetch_caller

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Message-ID: <20160405120626.GM3448@twins.programming.kicks-ass.net>
Date:	Tue, 5 Apr 2016 14:06:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alexei Starovoitov <ast@...com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"David S . Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Wang Nan <wangnan0@...wei.com>, Josef Bacik <jbacik@...com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH net-next 1/8] perf: optimize perf_fetch_caller_regs

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@...nel.org>

In any case,

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>