linux-kernel - [PATCH] tracing: Remove obsolete PREEMPTIRQ

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Message-ID: <20200409104034.GJ3818@techsingularity.net>
Date:   Thu, 9 Apr 2020 11:40:34 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH] tracing: Remove obsolete PREEMPTIRQ_EVENTS kconfig option

The PREEMPTIRQ_EVENTS option is unused after commit c3bc8fd637a9 ("tracing:
Centralize preemptirq tracepoints and unify their usage"). Remove it.

Note that this option is hazardous as it stands. It enables TRACE_IRQFLAGS
event on non-preempt configurations without the irqsoff tracer enabled.
TRACE_IRQFLAGS as it stands incurs significant overhead on each IRQ
entry/exit. This is because trace_hardirqs_[on|off] does all the per-cpu
manipulations and NMI checks even if tracing is completely disabled for
some insane reason.  For example, netperf running UDP_STREAM on localhost
incurs a 4-6% performance penalty without any tracing if IRQFLAGS is
set. It can be put behind a static brach but even the function entry/exit
costs a little bit.

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 kernel/trace/Kconfig | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 402eef84c859..568ad371e19c 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -258,15 +258,6 @@ config TRACE_PREEMPT_TOGGLE
 	  Enables hooks which will be called when preemption is first disabled,
 	  and last enabled.

-config PREEMPTIRQ_EVENTS
-	bool "Enable trace events for preempt and irq disable/enable"
-	select TRACE_IRQFLAGS
-	select TRACE_PREEMPT_TOGGLE if PREEMPTION
-	select GENERIC_TRACER
-	default n
-	help
-	  Enable tracing of disable and enable events for preemption and irqs.
-
 config IRQSOFF_TRACER
 	bool "Interrupts-off Latency Tracer"
 	default n