lkml.org 
[lkml]   [2020]   [Jun]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v5 2/2] sched/uclamp: Protect uclamp fast path code with static key
Hi Patrick

On 06/30/20 10:11, Patrick Bellasi wrote:
>
> Hi Qais,
> here are some more 2c from me...
>
> On Mon, Jun 29, 2020 at 18:26:33 +0200, Qais Yousef <qais.yousef@arm.com> wrote...
>
> [...]
>
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 235b2cae00a0..8d80d6091d86 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -794,6 +794,26 @@ unsigned int sysctl_sched_uclamp_util_max = SCHED_CAPACITY_SCALE;
> > /* All clamps are required to be less or equal than these values */
> > static struct uclamp_se uclamp_default[UCLAMP_CNT];
> >
> > +/*
> > + * This static key is used to reduce the uclamp overhead in the fast path. It
> > + * primarily disables the call to uclamp_rq_{inc, dec}() in
> > + * enqueue/dequeue_task().
> > + *
> > + * This allows users to continue to enable uclamp in their kernel config with
> > + * minimum uclamp overhead in the fast path.
> > + *
> > + * As soon as userspace modifies any of the uclamp knobs, the static key is
> > + * enabled, since we have an actual users that make use of uclamp
> > + * functionality.
> > + *
> > + * The knobs that would enable this static key are:
> > + *
> > + * * A task modifying its uclamp value with sched_setattr().
> > + * * An admin modifying the sysctl_sched_uclamp_{min, max} via procfs.
> > + * * An admin modifying the cgroup cpu.uclamp.{min, max}
>
> I guess this list can be obtained with a grep or git changelog, moreover
> this text will require maintenance.
>
> What about replacing this full comment with something shorted like:
>
> ---8<---
> Static key to reduce uclamp overhead in the fast path by disabling
> calls to uclamp_rq_{inc, dec}().
> ---8<---

If you don't mind, I rather more verbose info. As a relatively new comer, lack
of comments about expectation of some functions is still a challenge.

>
> > + */
> > +DEFINE_STATIC_KEY_FALSE(sched_uclamp_used);
> > +
> > /* Integer rounded range for each bucket */
> > #define UCLAMP_BUCKET_DELTA DIV_ROUND_CLOSEST(SCHED_CAPACITY_SCALE, UCLAMP_BUCKETS)
> >
> > @@ -994,9 +1014,30 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
> > lockdep_assert_held(&rq->lock);
> >
> > bucket = &uc_rq->bucket[uc_se->bucket_id];
> > - SCHED_WARN_ON(!bucket->tasks);
> > +
> > + /*
> > + * bucket->tasks could be zero if sched_uclamp_used was enabled while
> > + * the current task was running, hence we could end up with unbalanced
> > + * call to uclamp_rq_dec_id().
> > + *
> > + * Need to be careful of the following enqeueue/dequeue order
> > + * problem too
> > + *
> > + * enqueue(taskA)
> > + * // sched_uclamp_used gets enabled
> > + * enqueue(taskB)
> > + * dequeue(taskA)
> > + * // bucket->tasks is now 0
> > + * dequeue(taskB)
> > + *
> > + * where we could end up with uc_se->active of the task set to true and
> > + * the wrong bucket[uc_se->bucket_id].value.
> > + *
> > + * Hence always make sure we reset things properly.
> > + */
> > if (likely(bucket->tasks))
> > bucket->tasks--;
> > +
> > uc_se->active = false;
>
> Better than v4, what about just using this active flag?
>
> ---8<---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8f360326861e..465a7645713b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -990,6 +990,13 @@ static inline void uclamp_rq_dec_id(struct rq *rq, struct task_struct *p,
>
> lockdep_assert_held(&rq->lock);
>
> + /*
> + * If a task was already enqueue at uclamp enable time
> + * nothing has been accounted for it.
> + */
> + if (unlikely(!uc_se->active))
> + return;
> +
> bucket = &uc_rq->bucket[uc_se->bucket_id];
> SCHED_WARN_ON(!bucket->tasks);
> if (likely(bucket->tasks))
> ---8<---
>
> This will allow also to keep in all the ref count checks we have,
> e.g. the SChed_WARN_ON().

Works for me. Though I'd like to expand on the comment more just because there
were few things that were caught out and worth documenting IMO.

>
>
> > /*
> > @@ -1032,6 +1073,13 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
> > {
> > enum uclamp_id clamp_id;
> >
> > + /*
> > + * Avoid any overhead until uclamp is actually used by the userspace.
> > + * Including the branch if we use static_branch_likely()
>
> I still find this last sentence hard to parse, but perhaps it's just me
> still missing a breakfast :)

It used to be

* Including the JMP if we use static_branch_likely()

Note s/branch/JMP/

Effectively the condition is written such that we produce a NOP when uclamp is
not used. I'll rephrase.

>
> > + */
> > + if (!static_branch_unlikely(&sched_uclamp_used))
> > + return;
>
> I'm also still wondering if the optimization is still working when we
> have that ! in front.

It does. I looked at the generated code before posting.

>
> Had a check at:
>
> https://elixir.bootlin.com/linux/latest/source/include/linux/jump_label.h#L399
>
> and AFAIU, it all boils down to cook a __branch_check()'s compiler hint,
> and ISTR that those are "anti-patterns"?
>
> That said we do have some usages for this pattern too:
>
> $ git grep '!static_branch_unlikely' | wc -l 36
> $ git grep 'static_branch_unlikely' | wc -l 220
>
> ?
>
> > +
> > if (unlikely(!p->sched_class->uclamp_enabled))
> > return;
> >
>
> [...]
>
> > +/**
> > + * uclamp_rq_util_with - clamp @util with @rq and @p effective uclamp values.
> > + * @rq: The rq to clamp against. Must not be NULL.
> > + * @util: The util value to clamp.
> > + * @p: The task to clamp against. Can be NULL if you want to clamp
> > + * against @rq only.
> > + *
> > + * Clamps the passed @util to the max(@rq, @p) effective uclamp values.
> > + *
> > + * If sched_uclamp_used static key is disabled, then just return the util
> > + * without any clamping since uclamp aggregation at the rq level in the fast
> > + * path is disabled, rendering this operation a NOP.
> > + *
> > + * Use uclamp_eff_value() if you don't care about uclamp values at rq level. It
> > + * will return the correct effective uclamp value of the task even if the
> > + * static key is disabled.
>
> Well, if you don't care about rq, you don't call a uclamp_rq_* method.
>
> I would say that the above paragraph is redundant, moreover it adds some
> cross-reference to a different method (name) which required maintenance.
>
> What about removing it?

I'd rather keep this one too. It helps explaining what the expected way to use
this code. I don't think the maintenance is a big issue? We have to maintain
the code anyway?

>
> > + */
> > static __always_inline
> > unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> > struct task_struct *p)
> > {
> > - unsigned long min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> > - unsigned long max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
> > + unsigned long min_util;
> > + unsigned long max_util;
> > +
> > + if (!static_branch_likely(&sched_uclamp_used))
> > + return util;
> > +
> > + min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
> > + max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
>
> I think moving the initialization is not required, the compiler should
> be smart enough to place theme where's better.

I did look at the generated code before posting. The compiler doesn't optimize
the reads. Likely because of the READ_ONCE() which implies volatile access.

>
> > if (p) {
> > min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
> > @@ -2371,6 +2396,11 @@ unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> >
> > return clamp(util, min_util, max_util);
> > }
> > +
> > +static inline bool uclamp_is_enabled(void)
> > +{
> > + return static_branch_likely(&sched_uclamp_used);
> > +}
>
> Looks like here we mix up terms, which can be confusing.
> AFAIKS, we use:
> - *_enabled for the sched class flags (compile time)
> - *_used for the user-space opting in (run time)

I wanted to add a comment here.

I can rename it to uclamp_is_used() if you want.

Thanks

--
Qais Yousef

>
> Thus, perhaps we can just use the same pattern used by the
> sched_numa_balancing static key:
>
> $ git grep sched_numa_balancing
> kernel/sched/core.c:DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
> kernel/sched/core.c: static_branch_enable(&sched_numa_balancing);
> kernel/sched/core.c: static_branch_disable(&sched_numa_balancing);
> kernel/sched/core.c: int state = static_branch_likely(&sched_numa_balancing);
> kernel/sched/fair.c: if (!static_branch_likely(&sched_numa_balancing))
> kernel/sched/fair.c: if (!static_branch_likely(&sched_numa_balancing))
> kernel/sched/fair.c: if (!static_branch_likely(&sched_numa_balancing))
> kernel/sched/fair.c: if (static_branch_unlikely(&sched_numa_balancing))
> kernel/sched/sched.h:extern struct static_key_false sched_numa_balancing;
>
> IOW: unconditionally define sched_uclamp_used as non static in core.c,
> and use it directly on schedutil too.
>
> > #else /* CONFIG_UCLAMP_TASK */
> > static inline
> > unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> > @@ -2378,6 +2408,11 @@ unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
> > {
> > return util;
> > }
> > +
> > +static inline bool uclamp_is_enabled(void)
> > +{
> > + return false;
> > +}
> > #endif /* CONFIG_UCLAMP_TASK */
> >
> > #ifdef arch_scale_freq_capacity
>
> Best,
> Patrick

\
 
 \ /
  Last update: 2020-06-30 11:47    [W:0.095 / U:27.556 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site