[PATCH] locking/lock_events: Use this_cpu_add() when necessary

[PATCH] locking/lock_events: Use this_cpu_add() when necessary

[Waiman Long]

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: ...

This is only an issue on preempt kernel where preemption can happen
in the middle of the multi-instruction percpu operation. It is not an
issue on x86 as the percpu operation is a single instruction.  The lock
events code is updated to use the slower this_cpu_add() for non-x86
preempt kernel or when CONFIG_DEBUG_PREEMPT is defined.

Fixes: a8654596f0371 ("locking/rwsem: Enable lock event counting")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/lock_events.h | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/lock_events.h b/kernel/locking/lock_events.h
index feb1acc54611..2b6c8b7588dc 100644
--- a/kernel/locking/lock_events.h
+++ b/kernel/locking/lock_events.h
@@ -30,13 +30,36 @@ enum lock_events {
  */
 DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]);
 
+/*
+ * The purpose of the lock event counting subsystem is to provide a low
+ * overhead way to record the number of specific locking events by using
+ * percpu counters. It is the percpu sum that matters, not specifically
+ * how many of them happens in each cpu.
+ *
+ * In !preempt kernel, we can just use __this_cpu_{inc|add}() as preemption
+ * won't happen in the middle of the percpu operation. In preempt kernel,
+ * it depends on whether the percpu operation is atomic (1 instruction)
+ * or not. We know x86 generates a single instruction to do percpu op, but
+ * we can't guarantee that for other architectures. We also need to use
+ * the slower this_cpu_{inc|add}() when CONFIG_DEBUG_PREEMPT is defined
+ * to make the checking code happy.
+ */
+#if defined(CONFIG_PREEMPT) && \
+   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
+#define lockevent_percpu_inc(x)		this_cpu_inc(x)
+#define lockevent_percpu_add(x, v)	this_cpu_add(x, v)
+#else
+#define lockevent_percpu_inc(x)		__this_cpu_inc(x)
+#define lockevent_percpu_add(x, v)	__this_cpu_add(x, v)
+#endif
+
 /*
  * Increment the PV qspinlock statistical counters
  */
 static inline void __lockevent_inc(enum lock_events event, bool cond)
 {
 	if (cond)
-		__this_cpu_inc(lockevents[event]);
+		lockevent_percpu_inc(lockevents[event]);
 }
 
 #define lockevent_inc(ev)	  __lockevent_inc(LOCKEVENT_ ##ev, true)
@@ -44,7 +67,7 @@ static inline void __lockevent_inc(enum lock_events event, bool cond)
 
 static inline void __lockevent_add(enum lock_events event, int inc)
 {
-	__this_cpu_add(lockevents[event], inc);
+	lockevent_percpu_add(lockevents[event], inc);
 }
 
 #define lockevent_add(ev, c)	__lockevent_add(LOCKEVENT_ ##ev, c)
-- 
2.18.1

Re: [PATCH] locking/lock_events: Use this_cpu_add() when necessary

[Linus Torvalds]

On Wed, May 22, 2019 at 8:40 AM Waiman Long <longman@redhat.com> wrote:
>
> +#if defined(CONFIG_PREEMPT) && \
> +   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
> +#define lockevent_percpu_inc(x)                this_cpu_inc(x)
> +#define lockevent_percpu_add(x, v)     this_cpu_add(x, v)

Why that CONFIG_X86 special case?

On x86, the regular non-underscore versionm is perfectly fine, and the
underscore is no faster or simpler.

So just make it be

   #if defined(CONFIG_PREEMPT)
     .. non-underscore versions..
   #else
     .. underscore versions ..
   #endif

and realize that x86 simply doesn't _care_. On x86, it will be one
single instruction regardless.

Non-x86 may prefer the underscore versions for the non-preempt case.

             Linus

Re: [PATCH] locking/lock_events: Use this_cpu_add() when necessary

[Waiman Long]

On 5/22/19 3:54 PM, Linus Torvalds wrote:
> On Wed, May 22, 2019 at 8:40 AM Waiman Long <longman@redhat.com> wrote:
>> +#if defined(CONFIG_PREEMPT) && \
>> +   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
>> +#define lockevent_percpu_inc(x)                this_cpu_inc(x)
>> +#define lockevent_percpu_add(x, v)     this_cpu_add(x, v)
> Why that CONFIG_X86 special case?
>
> On x86, the regular non-underscore versionm is perfectly fine, and the
> underscore is no faster or simpler.

The condition is to use non-underscore version only when

1) It is a preempt kernel; AND
2) It either have CONFIG_DEBUG_PREEMPT on, OR it is a non-x86 system.


> So just make it be
>
>    #if defined(CONFIG_PREEMPT)
>      .. non-underscore versions..
>    #else
>      .. underscore versions ..
>    #endif
>
> and realize that x86 simply doesn't _care_. On x86, it will be one
> single instruction regardless.
>
> Non-x86 may prefer the underscore versions for the non-preempt case.

I was thinking of doing that originally, but then change it so x86
preempt kernel will also use the underscore version as long as
CONFIG_DEBUG_PREEMPT is not set.

I can change it back if that makes it less confusing.

Cheers,
Longman

Re: [PATCH] locking/lock_events: Use this_cpu_add() when necessary

[Will Deacon]

On Wed, May 22, 2019 at 12:54:13PM -0700, Linus Torvalds wrote:
> On Wed, May 22, 2019 at 8:40 AM Waiman Long <longman@redhat.com> wrote:
> >
> > +#if defined(CONFIG_PREEMPT) && \
> > +   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
> > +#define lockevent_percpu_inc(x)                this_cpu_inc(x)
> > +#define lockevent_percpu_add(x, v)     this_cpu_add(x, v)
> 
> Why that CONFIG_X86 special case?
> 
> On x86, the regular non-underscore versionm is perfectly fine, and the
> underscore is no faster or simpler.
> 
> So just make it be
> 
>    #if defined(CONFIG_PREEMPT)
>      .. non-underscore versions..
>    #else
>      .. underscore versions ..
>    #endif
> 
> and realize that x86 simply doesn't _care_. On x86, it will be one
> single instruction regardless.
> 
> Non-x86 may prefer the underscore versions for the non-preempt case.

To be honest, given this depends on LOCK_EVENT_COUNTS, I'd be inclined to
keep things simple and drop the underscore versions entirely. Saves having
to worry about things like "could I take an interrupt during the add?".

Will

Re: [PATCH] locking/lock_events: Use this_cpu_add() when necessary

[Waiman Long]

On 5/23/19 10:58 AM, Will Deacon wrote:
> On Wed, May 22, 2019 at 12:54:13PM -0700, Linus Torvalds wrote:
>> On Wed, May 22, 2019 at 8:40 AM Waiman Long <longman@redhat.com> wrote:
>>> +#if defined(CONFIG_PREEMPT) && \
>>> +   (defined(CONFIG_DEBUG_PREEMPT) || !defined(CONFIG_X86))
>>> +#define lockevent_percpu_inc(x)                this_cpu_inc(x)
>>> +#define lockevent_percpu_add(x, v)     this_cpu_add(x, v)
>> Why that CONFIG_X86 special case?
>>
>> On x86, the regular non-underscore versionm is perfectly fine, and the
>> underscore is no faster or simpler.
>>
>> So just make it be
>>
>>    #if defined(CONFIG_PREEMPT)
>>      .. non-underscore versions..
>>    #else
>>      .. underscore versions ..
>>    #endif
>>
>> and realize that x86 simply doesn't _care_. On x86, it will be one
>> single instruction regardless.
>>
>> Non-x86 may prefer the underscore versions for the non-preempt case.
> To be honest, given this depends on LOCK_EVENT_COUNTS, I'd be inclined to
> keep things simple and drop the underscore versions entirely. Saves having
> to worry about things like "could I take an interrupt during the add?".
>
I have sent out the v2 patch that simplifies the condition. Now the
underscore versions will be used for !preempt kernel and non-underscore
version used in preempt kernel. The non-underscore versions may generate
a lot more unnecessary code when CONFIG_PREEMPT_COUNT is defined.

Cheers,
Longman