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Revision d0313e629f2690edfd269896b398048275227db0 authored by Ian Rogers on 12 August 2022, 23:09:49 UTC, committed by Arnaldo Carvalho de Melo on 13 August 2022, 18:03:09 UTC 
perf jevents: Fold strings optimization
 
If a shorter string ends a longer string then the shorter string may
reuse the longer string at an offset. For example, on x86 the event
arith.cycles_div_busy and cycles_div_busy can be folded, even though
they have difference names the strings are identical after 6
characters. cycles_div_busy can reuse the arith.cycles_div_busy string
at an offset of 6.

In pmu-events.c this looks like the following where the 'also:' lists
folded strings:

/* offset=177541 */ "arith.cycles_div_busy\000\000pipeline\000Cycles the divider is busy\000\000\000event=0x14,period=2000000,umask=0x1\000\000\000\000\000\000\000\000\000" /* also: cycles_div_busy\000\000pipeline\000Cycles the divider is busy\000\000\000event=0x14,period=2000000,umask=0x1\000\000\000\000\000\000\000\000\000 */

As jevents.py combines multiple strings for an event into a larger
string, the amount of folding is minimal as all parts of the event must
align. Other organizations can benefit more from folding, but lose space
by say recording more offsets.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.garry@huawei.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20220812230949.683239-15-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
1 parent 9118259
Raw File
backtracetest.c 
// SPDX-License-Identifier: GPL-2.0-only
/*
 * Simple stack backtrace regression test module
 *
 * (C) Copyright 2008 Intel Corporation
 * Author: Arjan van de Ven <arjan@linux.intel.com>
 */

#include <linux/completion.h>
#include <linux/delay.h>
#include <linux/interrupt.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/stacktrace.h>

static void backtrace_test_normal(void)
{
	pr_info("Testing a backtrace from process context.\n");
	pr_info("The following trace is a kernel self test and not a bug!\n");

	dump_stack();
}

static DECLARE_COMPLETION(backtrace_work);

static void backtrace_test_irq_callback(unsigned long data)
{
	dump_stack();
	complete(&backtrace_work);
}

static DECLARE_TASKLET_OLD(backtrace_tasklet, &backtrace_test_irq_callback);

static void backtrace_test_irq(void)
{
	pr_info("Testing a backtrace from irq context.\n");
	pr_info("The following trace is a kernel self test and not a bug!\n");

	init_completion(&backtrace_work);
	tasklet_schedule(&backtrace_tasklet);
	wait_for_completion(&backtrace_work);
}

#ifdef CONFIG_STACKTRACE
static void backtrace_test_saved(void)
{
	unsigned long entries[8];
	unsigned int nr_entries;

	pr_info("Testing a saved backtrace.\n");
	pr_info("The following trace is a kernel self test and not a bug!\n");

	nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
	stack_trace_print(entries, nr_entries, 0);
}
#else
static void backtrace_test_saved(void)
{
	pr_info("Saved backtrace test skipped.\n");
}
#endif

static int backtrace_regression_test(void)
{
	pr_info("====[ backtrace testing ]===========\n");

	backtrace_test_normal();
	backtrace_test_irq();
	backtrace_test_saved();

	pr_info("====[ end of backtrace testing ]====\n");
	return 0;
}

static void exitf(void)
{
}

module_init(backtrace_regression_test);
module_exit(exitf);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Arjan van de Ven <arjan@linux.intel.com>");
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