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51816e9e11
The kernel test robot has reported that the use of __this_cpu_add()
causes bug messages like:
BUG: using __this_cpu_add() in preemptible [00000000] code: ...
Given the imprecise nature of the count and the possibility of resetting
the count and doing the measurement again, this is not really a big
problem to use the unprotected __this_cpu_*() functions.
To make the preemption checking code happy, the this_cpu_*() functions
will be used if CONFIG_DEBUG_PREEMPT is defined.
The imprecise nature of the locking counts are also documented with
the suggestion that we should run the measurement a few times with the
counts reset in between to get a better picture of what is going on
under the hood.
Fixes: a8654596f0 ("locking/rwsem: Enable lock event counting")
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
98 lines
3.4 KiB
C
98 lines
3.4 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* Authors: Waiman Long <longman@redhat.com>
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*/
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#ifndef __LOCKING_LOCK_EVENTS_H
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#define __LOCKING_LOCK_EVENTS_H
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enum lock_events {
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#include "lock_events_list.h"
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lockevent_num, /* Total number of lock event counts */
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LOCKEVENT_reset_cnts = lockevent_num,
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};
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#ifdef CONFIG_LOCK_EVENT_COUNTS
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/*
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* Per-cpu counters
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*/
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DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]);
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/*
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* The purpose of the lock event counting subsystem is to provide a low
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* overhead way to record the number of specific locking events by using
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* percpu counters. It is the percpu sum that matters, not specifically
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* how many of them happens in each cpu.
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*
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* It is possible that the same percpu counter may be modified in both
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* the process and interrupt contexts. For architectures that perform
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* percpu operation with multiple instructions, it is possible to lose
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* count if a process context percpu update is interrupted in the middle
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* and the same counter is updated in the interrupt context. Therefore,
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* the generated percpu sum may not be precise. The error, if any, should
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* be small and insignificant.
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*
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* For those architectures that do multi-instruction percpu operation,
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* preemption in the middle and moving the task to another cpu may cause
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* a larger error in the count. Again, this will be few and far between.
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* Given the imprecise nature of the count and the possibility of resetting
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* the count and doing the measurement again, this is not really a big
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* problem.
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*
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* To get a better picture of what is happening under the hood, it is
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* suggested that a few measurements should be taken with the counts
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* reset in between to stamp out outliner because of these possible
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* error conditions.
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*
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* To minimize overhead, we use __this_cpu_*() in all cases except when
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* CONFIG_DEBUG_PREEMPT is defined. In this particular case, this_cpu_*()
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* will be used to avoid the appearance of unwanted BUG messages.
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*/
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#ifdef CONFIG_DEBUG_PREEMPT
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#define lockevent_percpu_inc(x) this_cpu_inc(x)
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#define lockevent_percpu_add(x, v) this_cpu_add(x, v)
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#else
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#define lockevent_percpu_inc(x) __this_cpu_inc(x)
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#define lockevent_percpu_add(x, v) __this_cpu_add(x, v)
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#endif
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/*
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* Increment the PV qspinlock statistical counters
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*/
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static inline void __lockevent_inc(enum lock_events event, bool cond)
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{
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if (cond)
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lockevent_percpu_inc(lockevents[event]);
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}
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#define lockevent_inc(ev) __lockevent_inc(LOCKEVENT_ ##ev, true)
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#define lockevent_cond_inc(ev, c) __lockevent_inc(LOCKEVENT_ ##ev, c)
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static inline void __lockevent_add(enum lock_events event, int inc)
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{
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lockevent_percpu_add(lockevents[event], inc);
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}
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#define lockevent_add(ev, c) __lockevent_add(LOCKEVENT_ ##ev, c)
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#else /* CONFIG_LOCK_EVENT_COUNTS */
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#define lockevent_inc(ev)
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#define lockevent_add(ev, c)
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#define lockevent_cond_inc(ev, c)
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#endif /* CONFIG_LOCK_EVENT_COUNTS */
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#endif /* __LOCKING_LOCK_EVENTS_H */
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