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cee4393989
Commit e31d28b6ab
("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs(). However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled. This situation results
in hangs when running the trace benchmarks.
A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function. All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.
This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
230 lines
5.2 KiB
C
230 lines
5.2 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/delay.h>
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#include <linux/module.h>
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#include <linux/kthread.h>
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#include <linux/trace_clock.h>
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#define CREATE_TRACE_POINTS
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#include "trace_benchmark.h"
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static struct task_struct *bm_event_thread;
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static char bm_str[BENCHMARK_EVENT_STRLEN] = "START";
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static u64 bm_total;
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static u64 bm_totalsq;
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static u64 bm_last;
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static u64 bm_max;
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static u64 bm_min;
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static u64 bm_first;
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static u64 bm_cnt;
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static u64 bm_stddev;
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static unsigned int bm_avg;
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static unsigned int bm_std;
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static bool ok_to_run;
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/*
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* This gets called in a loop recording the time it took to write
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* the tracepoint. What it writes is the time statistics of the last
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* tracepoint write. As there is nothing to write the first time
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* it simply writes "START". As the first write is cold cache and
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* the rest is hot, we save off that time in bm_first and it is
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* reported as "first", which is shown in the second write to the
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* tracepoint. The "first" field is writen within the statics from
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* then on but never changes.
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*/
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static void trace_do_benchmark(void)
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{
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u64 start;
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u64 stop;
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u64 delta;
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u64 stddev;
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u64 seed;
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u64 last_seed;
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unsigned int avg;
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unsigned int std = 0;
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/* Only run if the tracepoint is actually active */
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if (!trace_benchmark_event_enabled() || !tracing_is_on())
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return;
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local_irq_disable();
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start = trace_clock_local();
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trace_benchmark_event(bm_str);
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stop = trace_clock_local();
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local_irq_enable();
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bm_cnt++;
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delta = stop - start;
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/*
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* The first read is cold cached, keep it separate from the
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* other calculations.
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*/
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if (bm_cnt == 1) {
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bm_first = delta;
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scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
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"first=%llu [COLD CACHED]", bm_first);
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return;
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}
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bm_last = delta;
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if (delta > bm_max)
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bm_max = delta;
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if (!bm_min || delta < bm_min)
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bm_min = delta;
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/*
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* When bm_cnt is greater than UINT_MAX, it breaks the statistics
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* accounting. Freeze the statistics when that happens.
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* We should have enough data for the avg and stddev anyway.
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*/
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if (bm_cnt > UINT_MAX) {
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scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
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"last=%llu first=%llu max=%llu min=%llu ** avg=%u std=%d std^2=%lld",
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bm_last, bm_first, bm_max, bm_min, bm_avg, bm_std, bm_stddev);
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return;
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}
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bm_total += delta;
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bm_totalsq += delta * delta;
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if (bm_cnt > 1) {
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/*
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* Apply Welford's method to calculate standard deviation:
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* s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
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*/
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stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total;
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do_div(stddev, (u32)bm_cnt);
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do_div(stddev, (u32)bm_cnt - 1);
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} else
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stddev = 0;
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delta = bm_total;
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do_div(delta, bm_cnt);
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avg = delta;
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if (stddev > 0) {
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int i = 0;
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/*
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* stddev is the square of standard deviation but
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* we want the actualy number. Use the average
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* as our seed to find the std.
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*
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* The next try is:
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* x = (x + N/x) / 2
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*
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* Where N is the squared number to find the square
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* root of.
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*/
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seed = avg;
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do {
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last_seed = seed;
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seed = stddev;
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if (!last_seed)
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break;
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do_div(seed, last_seed);
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seed += last_seed;
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do_div(seed, 2);
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} while (i++ < 10 && last_seed != seed);
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std = seed;
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}
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scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
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"last=%llu first=%llu max=%llu min=%llu avg=%u std=%d std^2=%lld",
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bm_last, bm_first, bm_max, bm_min, avg, std, stddev);
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bm_std = std;
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bm_avg = avg;
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bm_stddev = stddev;
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}
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static int benchmark_event_kthread(void *arg)
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{
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/* sleep a bit to make sure the tracepoint gets activated */
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msleep(100);
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while (!kthread_should_stop()) {
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trace_do_benchmark();
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/*
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* We don't go to sleep, but let others run as well.
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* This is bascially a "yield()" to let any task that
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* wants to run, schedule in, but if the CPU is idle,
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* we'll keep burning cycles.
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*
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* Note the tasks_rcu_qs() version of cond_resched() will
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* notify synchronize_rcu_tasks() that this thread has
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* passed a quiescent state for rcu_tasks. Otherwise
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* this thread will never voluntarily schedule which would
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* block synchronize_rcu_tasks() indefinitely.
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*/
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cond_resched_tasks_rcu_qs();
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}
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return 0;
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}
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/*
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* When the benchmark tracepoint is enabled, it calls this
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* function and the thread that calls the tracepoint is created.
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*/
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int trace_benchmark_reg(void)
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{
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if (!ok_to_run) {
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pr_warning("trace benchmark cannot be started via kernel command line\n");
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return -EBUSY;
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}
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bm_event_thread = kthread_run(benchmark_event_kthread,
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NULL, "event_benchmark");
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if (IS_ERR(bm_event_thread)) {
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pr_warning("trace benchmark failed to create kernel thread\n");
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return PTR_ERR(bm_event_thread);
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}
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return 0;
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}
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/*
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* When the benchmark tracepoint is disabled, it calls this
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* function and the thread that calls the tracepoint is deleted
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* and all the numbers are reset.
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*/
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void trace_benchmark_unreg(void)
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{
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if (!bm_event_thread)
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return;
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kthread_stop(bm_event_thread);
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bm_event_thread = NULL;
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strcpy(bm_str, "START");
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bm_total = 0;
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bm_totalsq = 0;
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bm_last = 0;
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bm_max = 0;
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bm_min = 0;
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bm_cnt = 0;
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/* These don't need to be reset but reset them anyway */
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bm_first = 0;
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bm_std = 0;
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bm_avg = 0;
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bm_stddev = 0;
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}
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static __init int ok_to_run_trace_benchmark(void)
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{
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ok_to_run = true;
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return 0;
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}
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early_initcall(ok_to_run_trace_benchmark);
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