linux-kernel - Re: [PATCH] trace: adjust code layout in get_recursion

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Message-ID: <20170822152025.wl664322yuu6esq4@hirez.programming.kicks-ass.net>
Date:   Tue, 22 Aug 2017 17:20:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] trace: adjust code layout in get_recursion_context

On Tue, Aug 22, 2017 at 05:14:10PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 22, 2017 at 04:40:24PM +0200, Jesper Dangaard Brouer wrote:
> > In an XDP redirect applications using tracepoint xdp:xdp_redirect to
> > diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
> > was consuming 2% CPU. This was reduced to 1.6% with this simple
> > change.
> 
> It is also incorrect. What do you suppose it now returns when the NMI
> hits a hard IRQ which hit during a Soft IRQ?

Does this help any? I can imagine the compiler could struggle to CSE
preempt_count() seeing how its an asm thing.

---
 kernel/events/internal.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 486fd78eb8d5..e0b5b8fa83a2 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -206,13 +206,14 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs);
 
 static inline int get_recursion_context(int *recursion)
 {
+	unsigned int pc = preempt_count();
 	int rctx;
 
-	if (in_nmi())
+	if (pc & NMI_MASK)
 		rctx = 3;
-	else if (in_irq())
+	else if (pc & HARDIRQ_MASK)
 		rctx = 2;
-	else if (in_softirq())
+	else if (pc & SOFTIRQ_OFFSET)
 		rctx = 1;
 	else
 		rctx = 0;