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Date: Tue, 14 Jun 2022 12:45:50 +0100
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Tao Zhou <tao.zhou@...ux.dev>
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
Subject: Re: [PATCH v10 1/7] sched/fair: Provide u64 read for 32-bits arch
helper
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); \
> > + var = __val; \
> > + /* \
> > + * 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
>
> --
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