linux-kernel - Re: [PATCH 05/13] locking/qspinlock: be less clever with the preprocessor

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Date:   Tue, 12 Jul 2022 10:33:57 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 05/13] locking/qspinlock: be less clever with the
 preprocessor

Excerpts from Waiman Long's message of July 6, 2022 6:02 am:
> On 7/4/22 10:38, Nicholas Piggin wrote:
>> Stop qspinlock.c including itself and avoid most of the function
>> renaming with the preprocessor.
>>
>> This is mostly done by having the common slowpath code take a 'bool
>> paravirt' argument and adjusting code based on that.
>>
>> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
>> ---
>>   kernel/locking/qspinlock.c          | 116 ++++++++++++----------------
>>   kernel/locking/qspinlock_paravirt.h |  10 +--
>>   2 files changed, 52 insertions(+), 74 deletions(-)
>>
>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index 8f2173e22479..b96c58ca51de 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -11,8 +11,6 @@
>>    *          Peter Zijlstra <peterz@...radead.org>
>>    */
>>   
>> -#ifndef _GEN_PV_LOCK_SLOWPATH
>> -
>>   #include <linux/smp.h>
>>   #include <linux/bug.h>
>>   #include <linux/cpumask.h>
>> @@ -285,35 +283,21 @@ static __always_inline void set_locked(struct qspinlock *lock)
>>   	WRITE_ONCE(lock->locked, _Q_LOCKED_VAL);
>>   }
>>   
>> -
>> -/*
>> - * Generate the native code for queued_spin_unlock_slowpath(); provide NOPs for
>> - * all the PV callbacks.
>> - */
>> -
>> -static __always_inline void __pv_init_node(struct qnode *node) { }
>> -static __always_inline void __pv_wait_node(struct qnode *node,
>> -					   struct qnode *prev) { }
>> -static __always_inline void __pv_kick_node(struct qspinlock *lock,
>> -					   struct qnode *node) { }
>> -static __always_inline u32  __pv_wait_head_or_lock(struct qspinlock *lock,
>> -						   struct qnode *node)
>> -						   { return 0; }
>> -
>> -#define pv_enabled()		false
>> -
>> -#define pv_init_node		__pv_init_node
>> -#define pv_wait_node		__pv_wait_node
>> -#define pv_kick_node		__pv_kick_node
>> -#define pv_wait_head_or_lock	__pv_wait_head_or_lock
>> -
>>   #ifdef CONFIG_PARAVIRT_SPINLOCKS
>> -#define queued_spin_lock_slowpath	native_queued_spin_lock_slowpath
>> -#endif
>> -
>> -#endif /* _GEN_PV_LOCK_SLOWPATH */
>> +#include "qspinlock_paravirt.h"
>> +#else /* CONFIG_PARAVIRT_SPINLOCKS */
>> +static __always_inline void pv_init_node(struct qnode *node) { }
>> +static __always_inline void pv_wait_node(struct qnode *node,
>> +					 struct qnode *prev) { }
>> +static __always_inline void pv_kick_node(struct qspinlock *lock,
>> +					 struct qnode *node) { }
>> +static __always_inline u32  pv_wait_head_or_lock(struct qspinlock *lock,
>> +						 struct qnode *node)
>> +						   { return 0; }
>> +static __always_inline bool pv_hybrid_queued_unfair_trylock(struct qspinlock *lock) { BUILD_BUG(); }
>> +#endif /* CONFIG_PARAVIRT_SPINLOCKS */
>>   
>> -static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
>> +static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, bool paravirt)
> 
> Using "const bool paravirt" may help the compiler generating better code 
> by eliminating dead one, if it is not doing that already.

I think adding always_inline per Peter's suggestion should be enough.
The compiler should be able to see it's constant. Actually I'm surprised
attribute pure is helpful within a compilation unit, I would think the
compiler should be able to deduce that too.

That said, no problem with massaging things to help the compiler DTRT.

> 
>>   {
>>   	struct qnode *prev, *next, *node;
>>   	u32 val, old, tail;
>> @@ -338,8 +322,13 @@ static inline void queued_spin_lock_mcs_queue(struct qspinlock *lock)
>>   	 */
>>   	if (unlikely(idx >= MAX_NODES)) {
>>   		lockevent_inc(lock_no_node);
>> -		while (!queued_spin_trylock(lock))
>> -			cpu_relax();
>> +		if (paravirt) {
>> +			while (!pv_hybrid_queued_unfair_trylock(lock))
>> +				cpu_relax();
>> +		} else {
>> +			while (!queued_spin_trylock(lock))
>> +				cpu_relax();
>> +		}
> 
> The code will look a bit better if you add the following helper function 
> and use it instead.
> 
> static inline bool queued_spin_trylock_common(struct qspinlock *lock, 
> const bool paravirt)
> {
>          if (paravirt)
>                  return pv_hybrid_queued_unfair_trylock(lock);
>          else
>                  return queued_spin_trylock(lock);
> }

I did change that later on actually to always just use the basic 
trylock. This was trying to be a more mechanical conversion that didn't 
change too much.

Thanks,
Nick