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sched/fair: Fix low cpu usage with high throttling by removing expira…
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…tion of cpu-local slices

It has been observed, that highly-threaded, non-cpu-bound applications
running under cpu.cfs_quota_us constraints can hit a high percentage of
periods throttled while simultaneously not consuming the allocated
amount of quota. This use case is typical of user-interactive non-cpu
bound applications, such as those running in kubernetes or mesos when
run on multiple cpu cores.

This has been root caused to cpu-local run queue being allocated per cpu
bandwidth slices, and then not fully using that slice within the period.
At which point the slice and quota expires. This expiration of unused
slice results in applications not being able to utilize the quota for
which they are allocated.

The non-expiration of per-cpu slices was recently fixed by
'commit 512ac99 ("sched/fair: Fix bandwidth timer clock drift
condition")'. Prior to that it appears that this had been broken since
at least 'commit 51f2176 ("sched/fair: Fix unlocked reads of some
cfs_b->quota/period")' which was introduced in v3.16-rc1 in 2014. That
added the following conditional which resulted in slices never being
expired.

if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
	/* extend local deadline, drift is bounded above by 2 ticks */
	cfs_rq->runtime_expires += TICK_NSEC;

Because this was broken for nearly 5 years, and has recently been fixed
and is now being noticed by many users running kubernetes
(kubernetes/kubernetes#67577) it is my opinion
that the mechanisms around expiring runtime should be removed
altogether.

This allows quota already allocated to per-cpu run-queues to live longer
than the period boundary. This allows threads on runqueues that do not
use much CPU to continue to use their remaining slice over a longer
period of time than cpu.cfs_period_us. However, this helps prevent the
above condition of hitting throttling while also not fully utilizing
your cpu quota.

This theoretically allows a machine to use slightly more than its
allotted quota in some periods. This overflow would be bounded by the
remaining quota left on each per-cpu runqueueu. This is typically no
more than min_cfs_rq_runtime=1ms per cpu. For CPU bound tasks this will
change nothing, as they should theoretically fully utilize all of their
quota in each period. For user-interactive tasks as described above this
provides a much better user/application experience as their cpu
utilization will more closely match the amount they requested when they
hit throttling. This means that cpu limits no longer strictly apply per
period for non-cpu bound applications, but that they are still accurate
over longer timeframes.

This greatly improves performance of high-thread-count, non-cpu bound
applications with low cfs_quota_us allocation on high-core-count
machines. In the case of an artificial testcase (10ms/100ms of quota on
80 CPU machine), this commit resulted in almost 30x performance
improvement, while still maintaining correct cpu quota restrictions.
That testcase is available at https://github.com/indeedeng/fibtest.

Fixes: 512ac99 ("sched/fair: Fix bandwidth timer clock drift condition")
Signed-off-by: Dave Chiluk <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Phil Auld <[email protected]>
Reviewed-by: Ben Segall <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: John Hammond <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Kyle Anderson <[email protected]>
Cc: Gabriel Munos <[email protected]>
Cc: Peter Oskolkov <[email protected]>
Cc: Cong Wang <[email protected]>
Cc: Brendan Gregg <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
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Dave Chiluk authored and Peter Zijlstra committed Aug 8, 2019
1 parent 139d025 commit de53fd7
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74 changes: 60 additions & 14 deletions Documentation/scheduler/sched-bwc.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -9,15 +9,16 @@ CFS bandwidth control is a CONFIG_FAIR_GROUP_SCHED extension which allows the
specification of the maximum CPU bandwidth available to a group or hierarchy.

The bandwidth allowed for a group is specified using a quota and period. Within
each given "period" (microseconds), a group is allowed to consume only up to
"quota" microseconds of CPU time. When the CPU bandwidth consumption of a
group exceeds this limit (for that period), the tasks belonging to its
hierarchy will be throttled and are not allowed to run again until the next
period.

A group's unused runtime is globally tracked, being refreshed with quota units
above at each period boundary. As threads consume this bandwidth it is
transferred to cpu-local "silos" on a demand basis. The amount transferred
each given "period" (microseconds), a task group is allocated up to "quota"
microseconds of CPU time. That quota is assigned to per-cpu run queues in
slices as threads in the cgroup become runnable. Once all quota has been
assigned any additional requests for quota will result in those threads being
throttled. Throttled threads will not be able to run again until the next
period when the quota is replenished.

A group's unassigned quota is globally tracked, being refreshed back to
cfs_quota units at each period boundary. As threads consume this bandwidth it
is transferred to cpu-local "silos" on a demand basis. The amount transferred
within each of these updates is tunable and described as the "slice".

Management
Expand All @@ -35,12 +36,12 @@ The default values are::

A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
bandwidth restriction in place, such a group is described as an unconstrained
bandwidth group. This represents the traditional work-conserving behavior for
bandwidth group. This represents the traditional work-conserving behavior for
CFS.

Writing any (valid) positive value(s) will enact the specified bandwidth limit.
The minimum quota allowed for the quota or period is 1ms. There is also an
upper bound on the period length of 1s. Additional restrictions exist when
The minimum quota allowed for the quota or period is 1ms. There is also an
upper bound on the period length of 1s. Additional restrictions exist when
bandwidth limits are used in a hierarchical fashion, these are explained in
more detail below.

Expand All @@ -53,8 +54,8 @@ unthrottled if it is in a constrained state.
System wide settings
--------------------
For efficiency run-time is transferred between the global pool and CPU local
"silos" in a batch fashion. This greatly reduces global accounting pressure
on large systems. The amount transferred each time such an update is required
"silos" in a batch fashion. This greatly reduces global accounting pressure
on large systems. The amount transferred each time such an update is required
is described as the "slice".

This is tunable via procfs::
Expand Down Expand Up @@ -97,6 +98,51 @@ There are two ways in which a group may become throttled:
In case b) above, even though the child may have runtime remaining it will not
be allowed to until the parent's runtime is refreshed.

CFS Bandwidth Quota Caveats
---------------------------
Once a slice is assigned to a cpu it does not expire. However all but 1ms of
the slice may be returned to the global pool if all threads on that cpu become
unrunnable. This is configured at compile time by the min_cfs_rq_runtime
variable. This is a performance tweak that helps prevent added contention on
the global lock.

The fact that cpu-local slices do not expire results in some interesting corner
cases that should be understood.

For cgroup cpu constrained applications that are cpu limited this is a
relatively moot point because they will naturally consume the entirety of their
quota as well as the entirety of each cpu-local slice in each period. As a
result it is expected that nr_periods roughly equal nr_throttled, and that
cpuacct.usage will increase roughly equal to cfs_quota_us in each period.

For highly-threaded, non-cpu bound applications this non-expiration nuance
allows applications to briefly burst past their quota limits by the amount of
unused slice on each cpu that the task group is running on (typically at most
1ms per cpu or as defined by min_cfs_rq_runtime). This slight burst only
applies if quota had been assigned to a cpu and then not fully used or returned
in previous periods. This burst amount will not be transferred between cores.
As a result, this mechanism still strictly limits the task group to quota
average usage, albeit over a longer time window than a single period. This
also limits the burst ability to no more than 1ms per cpu. This provides
better more predictable user experience for highly threaded applications with
small quota limits on high core count machines. It also eliminates the
propensity to throttle these applications while simultanously using less than
quota amounts of cpu. Another way to say this, is that by allowing the unused
portion of a slice to remain valid across periods we have decreased the
possibility of wastefully expiring quota on cpu-local silos that don't need a
full slice's amount of cpu time.

The interaction between cpu-bound and non-cpu-bound-interactive applications
should also be considered, especially when single core usage hits 100%. If you
gave each of these applications half of a cpu-core and they both got scheduled
on the same CPU it is theoretically possible that the non-cpu bound application
will use up to 1ms additional quota in some periods, thereby preventing the
cpu-bound application from fully using its quota by that same amount. In these
instances it will be up to the CFS algorithm (see sched-design-CFS.rst) to
decide which application is chosen to run, as they will both be runnable and
have remaining quota. This runtime discrepancy will be made up in the following
periods when the interactive application idles.

Examples
--------
1. Limit a group to 1 CPU worth of runtime::
Expand Down
72 changes: 7 additions & 65 deletions kernel/sched/fair.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4371,8 +4371,6 @@ void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b)

now = sched_clock_cpu(smp_processor_id());
cfs_b->runtime = cfs_b->quota;
cfs_b->runtime_expires = now + ktime_to_ns(cfs_b->period);
cfs_b->expires_seq++;
}

static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
Expand All @@ -4394,8 +4392,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
{
struct task_group *tg = cfs_rq->tg;
struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(tg);
u64 amount = 0, min_amount, expires;
int expires_seq;
u64 amount = 0, min_amount;

/* note: this is a positive sum as runtime_remaining <= 0 */
min_amount = sched_cfs_bandwidth_slice() - cfs_rq->runtime_remaining;
Expand All @@ -4412,61 +4409,17 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
cfs_b->idle = 0;
}
}
expires_seq = cfs_b->expires_seq;
expires = cfs_b->runtime_expires;
raw_spin_unlock(&cfs_b->lock);

cfs_rq->runtime_remaining += amount;
/*
* we may have advanced our local expiration to account for allowed
* spread between our sched_clock and the one on which runtime was
* issued.
*/
if (cfs_rq->expires_seq != expires_seq) {
cfs_rq->expires_seq = expires_seq;
cfs_rq->runtime_expires = expires;
}

return cfs_rq->runtime_remaining > 0;
}

/*
* Note: This depends on the synchronization provided by sched_clock and the
* fact that rq->clock snapshots this value.
*/
static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
{
struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);

/* if the deadline is ahead of our clock, nothing to do */
if (likely((s64)(rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires) < 0))
return;

if (cfs_rq->runtime_remaining < 0)
return;

/*
* If the local deadline has passed we have to consider the
* possibility that our sched_clock is 'fast' and the global deadline
* has not truly expired.
*
* Fortunately we can check determine whether this the case by checking
* whether the global deadline(cfs_b->expires_seq) has advanced.
*/
if (cfs_rq->expires_seq == cfs_b->expires_seq) {
/* extend local deadline, drift is bounded above by 2 ticks */
cfs_rq->runtime_expires += TICK_NSEC;
} else {
/* global deadline is ahead, expiration has passed */
cfs_rq->runtime_remaining = 0;
}
}

static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec)
{
/* dock delta_exec before expiring quota (as it could span periods) */
cfs_rq->runtime_remaining -= delta_exec;
expire_cfs_rq_runtime(cfs_rq);

if (likely(cfs_rq->runtime_remaining > 0))
return;
Expand Down Expand Up @@ -4661,8 +4614,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
resched_curr(rq);
}

static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
u64 remaining, u64 expires)
static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
{
struct cfs_rq *cfs_rq;
u64 runtime;
Expand All @@ -4684,7 +4636,6 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
remaining -= runtime;

cfs_rq->runtime_remaining += runtime;
cfs_rq->runtime_expires = expires;

/* we check whether we're throttled above */
if (cfs_rq->runtime_remaining > 0)
Expand All @@ -4709,7 +4660,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
*/
static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags)
{
u64 runtime, runtime_expires;
u64 runtime;
int throttled;

/* no need to continue the timer with no bandwidth constraint */
Expand Down Expand Up @@ -4737,8 +4688,6 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u
/* account preceding periods in which throttling occurred */
cfs_b->nr_throttled += overrun;

runtime_expires = cfs_b->runtime_expires;

/*
* This check is repeated as we are holding onto the new bandwidth while
* we unthrottle. This can potentially race with an unthrottled group
Expand All @@ -4751,8 +4700,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u
cfs_b->distribute_running = 1;
raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
/* we can't nest cfs_b->lock while distributing bandwidth */
runtime = distribute_cfs_runtime(cfs_b, runtime,
runtime_expires);
runtime = distribute_cfs_runtime(cfs_b, runtime);
raw_spin_lock_irqsave(&cfs_b->lock, flags);

cfs_b->distribute_running = 0;
Expand Down Expand Up @@ -4834,8 +4782,7 @@ static void __return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
return;

raw_spin_lock(&cfs_b->lock);
if (cfs_b->quota != RUNTIME_INF &&
cfs_rq->runtime_expires == cfs_b->runtime_expires) {
if (cfs_b->quota != RUNTIME_INF) {
cfs_b->runtime += slack_runtime;

/* we are under rq->lock, defer unthrottling using a timer */
Expand Down Expand Up @@ -4868,7 +4815,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
{
u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
unsigned long flags;
u64 expires;

/* confirm we're still not at a refresh boundary */
raw_spin_lock_irqsave(&cfs_b->lock, flags);
Expand All @@ -4886,7 +4832,6 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
if (cfs_b->quota != RUNTIME_INF && cfs_b->runtime > slice)
runtime = cfs_b->runtime;

expires = cfs_b->runtime_expires;
if (runtime)
cfs_b->distribute_running = 1;

Expand All @@ -4895,11 +4840,10 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
if (!runtime)
return;

runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
runtime = distribute_cfs_runtime(cfs_b, runtime);

raw_spin_lock_irqsave(&cfs_b->lock, flags);
if (expires == cfs_b->runtime_expires)
lsub_positive(&cfs_b->runtime, runtime);
lsub_positive(&cfs_b->runtime, runtime);
cfs_b->distribute_running = 0;
raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
}
Expand Down Expand Up @@ -5056,8 +5000,6 @@ void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)

cfs_b->period_active = 1;
overrun = hrtimer_forward_now(&cfs_b->period_timer, cfs_b->period);
cfs_b->runtime_expires += (overrun + 1) * ktime_to_ns(cfs_b->period);
cfs_b->expires_seq++;
hrtimer_start_expires(&cfs_b->period_timer, HRTIMER_MODE_ABS_PINNED);
}

Expand Down
4 changes: 0 additions & 4 deletions kernel/sched/sched.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -335,8 +335,6 @@ struct cfs_bandwidth {
u64 quota;
u64 runtime;
s64 hierarchical_quota;
u64 runtime_expires;
int expires_seq;

u8 idle;
u8 period_active;
Expand Down Expand Up @@ -557,8 +555,6 @@ struct cfs_rq {

#ifdef CONFIG_CFS_BANDWIDTH
int runtime_enabled;
int expires_seq;
u64 runtime_expires;
s64 runtime_remaining;

u64 throttled_clock;
Expand Down

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