Date:   Tue, 14 Jun 2022 22:28:31 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Vincent Donnefort <vdonnefort@...gle.com>
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        qperret@...gle.com, kernel-team@...roid.com,
        Tao Zhou <tao.zhou@...ux.dev>
Subject: Re: [PATCH v10 1/7] sched/fair: Provide u64 read for 32-bits arch
 helper

On Tue, Jun 14, 2022 at 12:45:50PM +0100, Vincent Donnefort wrote:

> On Thu, Jun 09, 2022 at 11:23:07PM +0800, Tao Zhou wrote:
> > On Tue, Jun 07, 2022 at 01:32:48PM +0100, Vincent Donnefort wrote:
> > 
> > > From: Vincent Donnefort <vincent.donnefort@....com>
> > > 
> > > Introducing macro helpers u64_u32_{store,load}() to factorize lockless
> > > accesses to u64 variables for 32-bits architectures.
> > > 
> > > Users are for now cfs_rq.min_vruntime and sched_avg.last_update_time. To
> > > accommodate the later where the copy lies outside of the structure
> > > (cfs_rq.last_udpate_time_copy instead of sched_avg.last_update_time_copy),
> > > use the _copy() version of those helpers.
> > > 
> > > Those new helpers encapsulate smp_rmb() and smp_wmb() synchronization and
> > > therefore, have a small penalty for 32-bits machines in set_task_rq_fair()
> > > and init_cfs_rq().
> > > 
> > > Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
> > > Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> > > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
> > > 
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 77b2048a9326..05614d9b919c 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -612,11 +612,8 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq)
> > >  	}
> > >  
> > >  	/* ensure we never gain time by being placed backwards. */
> > > -	cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
> > > -#ifndef CONFIG_64BIT
> > > -	smp_wmb();
> > > -	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> > > -#endif
> > > +	u64_u32_store(cfs_rq->min_vruntime,
> > > +		      max_vruntime(cfs_rq->min_vruntime, vruntime));
> > >  }
> > >  
> > >  static inline bool __entity_less(struct rb_node *a, const struct rb_node *b)
> > > @@ -3313,6 +3310,11 @@ static inline void cfs_rq_util_change(struct cfs_rq *cfs_rq, int flags)
> > >  }
> > >  
> > >  #ifdef CONFIG_SMP
> > > +static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> > > +{
> > > +	return u64_u32_load_copy(cfs_rq->avg.last_update_time,
> > > +				 cfs_rq->last_update_time_copy);
> > > +}
> > >  #ifdef CONFIG_FAIR_GROUP_SCHED
> > >  /*
> > >   * Because list_add_leaf_cfs_rq always places a child cfs_rq on the list
> > > @@ -3423,27 +3425,9 @@ void set_task_rq_fair(struct sched_entity *se,
> > >  	if (!(se->avg.last_update_time && prev))
> > >  		return;
> > >  
> > > -#ifndef CONFIG_64BIT
> > > -	{
> > > -		u64 p_last_update_time_copy;
> > > -		u64 n_last_update_time_copy;
> > > -
> > > -		do {
> > > -			p_last_update_time_copy = prev->load_last_update_time_copy;
> > > -			n_last_update_time_copy = next->load_last_update_time_copy;
> > > -
> > > -			smp_rmb();
> > > -
> > > -			p_last_update_time = prev->avg.last_update_time;
> > > -			n_last_update_time = next->avg.last_update_time;
> > > +	p_last_update_time = cfs_rq_last_update_time(prev);
> > > +	n_last_update_time = cfs_rq_last_update_time(next);
> > >  
> > > -		} while (p_last_update_time != p_last_update_time_copy ||
> > > -			 n_last_update_time != n_last_update_time_copy);
> > > -	}
> > > -#else
> > > -	p_last_update_time = prev->avg.last_update_time;
> > > -	n_last_update_time = next->avg.last_update_time;
> > > -#endif
> > >  	__update_load_avg_blocked_se(p_last_update_time, se);
> > >  	se->avg.last_update_time = n_last_update_time;
> > >  }
> > > @@ -3796,12 +3780,9 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
> > >  	}
> > >  
> > >  	decayed |= __update_load_avg_cfs_rq(now, cfs_rq);
> > > -
> > > -#ifndef CONFIG_64BIT
> > > -	smp_wmb();
> > > -	cfs_rq->load_last_update_time_copy = sa->last_update_time;
> > > -#endif
> > > -
> > > +	u64_u32_store_copy(sa->last_update_time,
> > > +			   cfs_rq->last_update_time_copy,
> > > +			   sa->last_update_time);
> > >  	return decayed;
> > >  }
> > >  
> > > @@ -3933,27 +3914,6 @@ static inline void update_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s
> > >  	}
> > >  }
> > >  
> > > -#ifndef CONFIG_64BIT
> > > -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> > > -{
> > > -	u64 last_update_time_copy;
> > > -	u64 last_update_time;
> > > -
> > > -	do {
> > > -		last_update_time_copy = cfs_rq->load_last_update_time_copy;
> > > -		smp_rmb();
> > > -		last_update_time = cfs_rq->avg.last_update_time;
> > > -	} while (last_update_time != last_update_time_copy);
> > > -
> > > -	return last_update_time;
> > > -}
> > > -#else
> > > -static inline u64 cfs_rq_last_update_time(struct cfs_rq *cfs_rq)
> > > -{
> > > -	return cfs_rq->avg.last_update_time;
> > > -}
> > > -#endif
> > > -
> > >  /*
> > >   * Synchronize entity load avg of dequeued entity without locking
> > >   * the previous rq.
> > > @@ -6960,21 +6920,8 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > >  	if (READ_ONCE(p->__state) == TASK_WAKING) {
> > >  		struct sched_entity *se = &p->se;
> > >  		struct cfs_rq *cfs_rq = cfs_rq_of(se);
> > > -		u64 min_vruntime;
> > > -
> > > -#ifndef CONFIG_64BIT
> > > -		u64 min_vruntime_copy;
> > > -
> > > -		do {
> > > -			min_vruntime_copy = cfs_rq->min_vruntime_copy;
> > > -			smp_rmb();
> > > -			min_vruntime = cfs_rq->min_vruntime;
> > > -		} while (min_vruntime != min_vruntime_copy);
> > > -#else
> > > -		min_vruntime = cfs_rq->min_vruntime;
> > > -#endif
> > >  
> > > -		se->vruntime -= min_vruntime;
> > > +		se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> > >  	}
> > >  
> > >  	if (p->on_rq == TASK_ON_RQ_MIGRATING) {
> > > @@ -11422,10 +11369,7 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
> > >  void init_cfs_rq(struct cfs_rq *cfs_rq)
> > >  {
> > >  	cfs_rq->tasks_timeline = RB_ROOT_CACHED;
> > > -	cfs_rq->min_vruntime = (u64)(-(1LL << 20));
> > > -#ifndef CONFIG_64BIT
> > > -	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> > > -#endif
> > > +	u64_u32_store(cfs_rq->min_vruntime, (u64)(-(1LL << 20)));
> > >  #ifdef CONFIG_SMP
> > >  	raw_spin_lock_init(&cfs_rq->removed.lock);
> > >  #endif
> > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > > index 1f97f357aacd..bf4a0ec98678 100644
> > > --- a/kernel/sched/sched.h
> > > +++ b/kernel/sched/sched.h
> > > @@ -520,6 +520,45 @@ struct cfs_bandwidth { };
> > >  
> > >  #endif	/* CONFIG_CGROUP_SCHED */
> > >  
> > > +/*
> > > + * u64_u32_load/u64_u32_store
> > > + *
> > > + * Use a copy of a u64 value to protect against data race. This is only
> > > + * applicable for 32-bits architectures.
> > > + */
> > > +#ifdef CONFIG_64BIT
> > > +# define u64_u32_load_copy(var, copy)       var
> > > +# define u64_u32_store_copy(var, copy, val) (var = val)
> > > +#else
> > > +# define u64_u32_load_copy(var, copy)					\
> > > +({									\
> > > +	u64 __val, __val_copy;						\
> > > +	do {								\
> > > +		__val_copy = copy;					\
> > > +		/*							\
> > > +		 * paired with u64_u32_store, ordering access		\
> > 
> > s/u64_u32_store/u64_u32_store_copy()/
> 
> Ack.
> 
> > 
> > > +		 * to var and copy.					\
> > > +		 */							\
> > > +		smp_rmb();						\
> > > +		__val = var;						\
> > > +	} while (__val != __val_copy);					\
> > > +	__val;								\
> > > +})
> > > +# define u64_u32_store_copy(var, copy, val)				\
> > > +do {									\
> > > +	typeof(val) __val = (val);					\
        ^^^^^^^^^^^^^^^^^^^^^^^^^^
        Instructs to store a local __val..(I missed this sorry)
        The time for these cycles is possible that eg. cfs_rq->min_vruntime
        has been updated in this period. But here locally store the
        old value.

> > > +	var = __val;							\
        ^^^^^^^^^^^^
        Original code here will load the even updated(more recently)
        eg. cfs_rq->min_vruntime from other cpus update(the load here
        is also need a read pair barrier, so I reference that use the
        smp_mb, but I do not know).

        I mean the __val here is more possible to be stale. That the
        ->min_vruntime or ->lut is not latest.

Your code simplify something but not lead to be more accurate compared to
original code. Is it possible to delete that local store and restore
the Data Dependancies that the original code do if your code is not DD.

All the above may be wrong. Thanks for your time.

> > > +	/*								\
> > > +	 * paired with u64_u32_load, ordering access to var and		\
> > 
> > s/u64_u32_load/u64_u32_load_copy()/
> 
> Ack.
> 
> > 
> > > +	 * copy.							\
> > > +	 */								\
> > > +	smp_wmb();							\
> > > +	copy = __val;							\
> > > +} while (0)
> > 
> > Try again here from me.
> > The semantics of this macro is different from the original code.
> > To be consistent with the original code, the 'copy = __val;' should
> > be changed to 'copy = var'. Why not original code here.
> > 
> > They are different in that the var can be changed currently on another
> > CPU and copy can be different from __val.
> 
> I'm not sure I understand your point here. We are using a temporary variable to
> set both var and copy. They'll end-up with the same value which is what we want.
> If what you worries is a concurrent write, you need a lock to modify the
> last_update_time or the min_vruntime (and the other values introduced
> by this patchset).
> 
> This is not about reading an up to date value, it is about reading a sane one.
> 
> > 
> > Documentation/memory-barriers.txt(line 1351, 5.19.0-rc1-rt1):
> > In section:
> > (*) What are memory barriers?
> >      - Multicopy atomicity.
> > The example there can be referenced here I think.
> > 
> > And I feel that here the smp_wmb() should be changed to smp_mb(). One user
> > of this macro have the load/write  write. Another user have write  write
> > if I am not wrong.
> 
> I do not get this part. the _store_copy() only store var and copy. Hence the
> smp_wmb. While the _load_copy() only load var and and copy, hence the smp_rmb.
> 
> This is actually described in the section "SMP BARRIER PAIRING".
> 
> > 
> > Also,
> > 
> > 'Memory Barriers: a Hardware View for Software Hackers' (July 23, 2010)
> > Section: 6.3 Example 2, I just scrolled to the code in table 3 and check
> > that the semantics there is almost the same with the semantics here.
> > 
> > I do not have confidence to convince anything just after some days when I
> > read it.
> > 
> > According to those two examples, if the small penalty is small, this
> > u64_u32_store_copy() macro should be changed like this:
> > 
> > # define u64_u32_store_copy(var, copy, val)				\
> > do {									\
> > 	typeof(val) __val = (val);					\
> > 	var = __val;							\
> > 	/*								\
> > 	 * paired with u64_u32_load_copy(), ordering access to var and	\
> > 	 * copy.							\
> > 	 */								\
> > 	smp_mb();							\
> > 	copy = var;							\
> > } while (0)
> > 
> > Not sure and this is the continuation to my reply to this patch in v4.
> > 
> > Thanks,
> > Tao
> > > +#endif
> > > +# define u64_u32_load(var)      u64_u32_load_copy(var, var##_copy)
> > > +# define u64_u32_store(var, val) u64_u32_store_copy(var, var##_copy, val)
> > > +
> > >  /* CFS-related fields in a runqueue */
> > >  struct cfs_rq {	
> > >  	struct load_weight	load;
> > > @@ -560,7 +599,7 @@ struct cfs_rq {
> > >  	 */
> > >  	struct sched_avg	avg;
> > >  #ifndef CONFIG_64BIT
> > > -	u64			load_last_update_time_copy;
> > > +	u64			last_update_time_copy;
> > >  #endif
> > >  	struct {
> > >  		raw_spinlock_t	lock ____cacheline_aligned;
> > > -- 
> > > 2.36.1.255.ge46751e96f-goog
> > 
> > -- 
> > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@...roid.com.
> > 

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