Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v5 2/2] sched/uclamp: Protect uclamp fast path code with static key | Date | Tue, 30 Jun 2020 10:44:15 +0100 |
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On 30/06/20 09:11, 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<--- >
Having some sense of when that key gets flipped is worthwhile IMO; though it may not have to be exhaustive list.
>> + */ >> +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) >> [...] >> @@ -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 :) >
AIUI this tries to explain why we use 'unlikely' rather than 'likely' - it's to prevent !uclamp users from having the branch overhead (see include/linux/jump_label.h, there's a nice fat comment just above static_branch_likely()).
IMO that point is already covered by the first sentence, as I blabbered on some previous version.
>> + */ >> + 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. > > 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 > > ? >
We use it for e.g. the sched_asym_cpucapacity key, and that works (and I've been down into the asm a few times).
>> + >> 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? > >> + */ >> 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. >
Not so sure with the READ_ONCE() & the volatile underneath; a quick compiler test with a volatile read before a branch tells me we still do the read before the branch, even if the value is only used after the branch.
>> 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) > > 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
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