[PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Cong Wang]

Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
we should sync its ->expires_seq too. However it is missing
for distribute_cfs_runtime(), especially the slack timer call path.

Fixes: 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition")
Cc: Xunlei Pang <xlpang@linux.alibaba.com>
Cc: Ben Segall <bsegall@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
---
 kernel/sched/fair.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2f0a0be4d344..910c50db3d74 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4857,7 +4857,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 }
 
 static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
-		u64 remaining, u64 expires)
+		u64 remaining, u64 expires, int expires_seq)
 {
 	struct cfs_rq *cfs_rq;
 	u64 runtime;
@@ -4880,6 +4880,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
 
 		cfs_rq->runtime_remaining += runtime;
 		cfs_rq->runtime_expires = expires;
+		cfs_rq->expires_seq = expires_seq;
 
 		/* we check whether we're throttled above */
 		if (cfs_rq->runtime_remaining > 0)
@@ -4905,7 +4906,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
 static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 {
 	u64 runtime, runtime_expires;
-	int throttled;
+	int throttled, expires_seq;
 
 	/* no need to continue the timer with no bandwidth constraint */
 	if (cfs_b->quota == RUNTIME_INF)
@@ -4933,6 +4934,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 	cfs_b->nr_throttled += overrun;
 
 	runtime_expires = cfs_b->runtime_expires;
+	expires_seq = cfs_b->expires_seq;
 
 	/*
 	 * This check is repeated as we are holding onto the new bandwidth while
@@ -4946,7 +4948,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 		raw_spin_unlock(&cfs_b->lock);
 		/* we can't nest cfs_b->lock while distributing bandwidth */
 		runtime = distribute_cfs_runtime(cfs_b, runtime,
-						 runtime_expires);
+						 runtime_expires, expires_seq);
 		raw_spin_lock(&cfs_b->lock);
 
 		throttled = !list_empty(&cfs_b->throttled_cfs_rq);
@@ -5055,6 +5057,7 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
 {
 	u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
+	int expires_seq;
 	u64 expires;
 
 	/* confirm we're still not at a refresh boundary */
@@ -5068,12 +5071,13 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
 		runtime = cfs_b->runtime;
 
 	expires = cfs_b->runtime_expires;
+	expires_seq = cfs_b->expires_seq;
 	raw_spin_unlock(&cfs_b->lock);
 
 	if (!runtime)
 		return;
 
-	runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
+	runtime = distribute_cfs_runtime(cfs_b, runtime, expires, expires_seq);
 
 	raw_spin_lock(&cfs_b->lock);
 	if (expires == cfs_b->runtime_expires)
-- 
2.14.4

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Xunlei Pang]

Hi Cong,

On 7/28/18 8:24 AM, Cong Wang wrote:
> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
> we should sync its ->expires_seq too. However it is missing
> for distribute_cfs_runtime(), especially the slack timer call path.

I don't think it's a problem, as expires_seq will get synced in
assign_cfs_rq_runtime().

Thanks,
Xunlei

> 
> Fixes: 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition")
> Cc: Xunlei Pang <xlpang@linux.alibaba.com>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> ---
>  kernel/sched/fair.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2f0a0be4d344..910c50db3d74 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4857,7 +4857,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>  }
>  
>  static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
> -		u64 remaining, u64 expires)
> +		u64 remaining, u64 expires, int expires_seq)
>  {
>  	struct cfs_rq *cfs_rq;
>  	u64 runtime;
> @@ -4880,6 +4880,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>  
>  		cfs_rq->runtime_remaining += runtime;
>  		cfs_rq->runtime_expires = expires;
> +		cfs_rq->expires_seq = expires_seq;
>  
>  		/* we check whether we're throttled above */
>  		if (cfs_rq->runtime_remaining > 0)
> @@ -4905,7 +4906,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>  static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  {
>  	u64 runtime, runtime_expires;
> -	int throttled;
> +	int throttled, expires_seq;
>  
>  	/* no need to continue the timer with no bandwidth constraint */
>  	if (cfs_b->quota == RUNTIME_INF)
> @@ -4933,6 +4934,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  	cfs_b->nr_throttled += overrun;
>  
>  	runtime_expires = cfs_b->runtime_expires;
> +	expires_seq = cfs_b->expires_seq;
>  
>  	/*
>  	 * This check is repeated as we are holding onto the new bandwidth while
> @@ -4946,7 +4948,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  		raw_spin_unlock(&cfs_b->lock);
>  		/* we can't nest cfs_b->lock while distributing bandwidth */
>  		runtime = distribute_cfs_runtime(cfs_b, runtime,
> -						 runtime_expires);
> +						 runtime_expires, expires_seq);
>  		raw_spin_lock(&cfs_b->lock);
>  
>  		throttled = !list_empty(&cfs_b->throttled_cfs_rq);
> @@ -5055,6 +5057,7 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>  static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  {
>  	u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
> +	int expires_seq;
>  	u64 expires;
>  
>  	/* confirm we're still not at a refresh boundary */
> @@ -5068,12 +5071,13 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  		runtime = cfs_b->runtime;
>  
>  	expires = cfs_b->runtime_expires;
> +	expires_seq = cfs_b->expires_seq;
>  	raw_spin_unlock(&cfs_b->lock);
>  
>  	if (!runtime)
>  		return;
>  
> -	runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
> +	runtime = distribute_cfs_runtime(cfs_b, runtime, expires, expires_seq);
>  
>  	raw_spin_lock(&cfs_b->lock);
>  	if (expires == cfs_b->runtime_expires)
> 

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[bsegall]

Xunlei Pang <xlpang@linux.alibaba.com> writes:

> Hi Cong,
>
> On 7/28/18 8:24 AM, Cong Wang wrote:
>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
>> we should sync its ->expires_seq too. However it is missing
>> for distribute_cfs_runtime(), especially the slack timer call path.
>
> I don't think it's a problem, as expires_seq will get synced in
> assign_cfs_rq_runtime().
>
> Thanks,
> Xunlei

It does seem unlikely to actually come up since the cfs_rq would have to
not run until the period was expired-locally-but-not-globally, but
there's no reason to not fix it.


>
>> 
>> Fixes: 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition")
>> Cc: Xunlei Pang <xlpang@linux.alibaba.com>
>> Cc: Ben Segall <bsegall@google.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
>> ---
>>  kernel/sched/fair.c | 12 ++++++++----
>>  1 file changed, 8 insertions(+), 4 deletions(-)
>> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 2f0a0be4d344..910c50db3d74 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4857,7 +4857,7 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>>  }
>>  
>>  static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>> -		u64 remaining, u64 expires)
>> +		u64 remaining, u64 expires, int expires_seq)
>>  {
>>  	struct cfs_rq *cfs_rq;
>>  	u64 runtime;
>> @@ -4880,6 +4880,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>>  
>>  		cfs_rq->runtime_remaining += runtime;
>>  		cfs_rq->runtime_expires = expires;
>> +		cfs_rq->expires_seq = expires_seq;
>>  
>>  		/* we check whether we're throttled above */
>>  		if (cfs_rq->runtime_remaining > 0)
>> @@ -4905,7 +4906,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>>  static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>>  {
>>  	u64 runtime, runtime_expires;
>> -	int throttled;
>> +	int throttled, expires_seq;
>>  
>>  	/* no need to continue the timer with no bandwidth constraint */
>>  	if (cfs_b->quota == RUNTIME_INF)
>> @@ -4933,6 +4934,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>>  	cfs_b->nr_throttled += overrun;
>>  
>>  	runtime_expires = cfs_b->runtime_expires;
>> +	expires_seq = cfs_b->expires_seq;
>>  
>>  	/*
>>  	 * This check is repeated as we are holding onto the new bandwidth while
>> @@ -4946,7 +4948,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>>  		raw_spin_unlock(&cfs_b->lock);
>>  		/* we can't nest cfs_b->lock while distributing bandwidth */
>>  		runtime = distribute_cfs_runtime(cfs_b, runtime,
>> -						 runtime_expires);
>> +						 runtime_expires, expires_seq);
>>  		raw_spin_lock(&cfs_b->lock);
>>  
>>  		throttled = !list_empty(&cfs_b->throttled_cfs_rq);
>> @@ -5055,6 +5057,7 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>>  static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>>  {
>>  	u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
>> +	int expires_seq;
>>  	u64 expires;
>>  
>>  	/* confirm we're still not at a refresh boundary */
>> @@ -5068,12 +5071,13 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>>  		runtime = cfs_b->runtime;
>>  
>>  	expires = cfs_b->runtime_expires;
>> +	expires_seq = cfs_b->expires_seq;
>>  	raw_spin_unlock(&cfs_b->lock);
>>  
>>  	if (!runtime)
>>  		return;
>>  
>> -	runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
>> +	runtime = distribute_cfs_runtime(cfs_b, runtime, expires, expires_seq);
>>  
>>  	raw_spin_lock(&cfs_b->lock);
>>  	if (expires == cfs_b->runtime_expires)
>> 

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Cong Wang]

On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>
> Hi Cong,
>
> On 7/28/18 8:24 AM, Cong Wang wrote:
> > Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
> > we should sync its ->expires_seq too. However it is missing
> > for distribute_cfs_runtime(), especially the slack timer call path.
>
> I don't think it's a problem, as expires_seq will get synced in
> assign_cfs_rq_runtime().

Sure, but there is a small window during which they are not synced.
Why do you want to wait until the next assign_cfs_rq_runtime() when
you already know runtime_expires is synced?

Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime()
inside __account_cfs_rq_runtime(), which means the check of
cfs_rq->expires_seq is not accurate for unthrottling case if the clock
drift happens soon enough?

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Xunlei Pang]

On 7/31/18 1:55 AM, Cong Wang wrote:
> On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>>
>> Hi Cong,
>>
>> On 7/28/18 8:24 AM, Cong Wang wrote:
>>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
>>> we should sync its ->expires_seq too. However it is missing
>>> for distribute_cfs_runtime(), especially the slack timer call path.
>>
>> I don't think it's a problem, as expires_seq will get synced in
>> assign_cfs_rq_runtime().
> 
> Sure, but there is a small window during which they are not synced.
> Why do you want to wait until the next assign_cfs_rq_runtime() when
> you already know runtime_expires is synced?
> 
> Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime()
> inside __account_cfs_rq_runtime(), which means the check of
> cfs_rq->expires_seq is not accurate for unthrottling case if the clock
> drift happens soon enough?
> 

expire_cfs_rq_runtime():
    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;
    }

So if clock drift happens soon, then expires_seq decides the correct
thing we should do: if cfs_b->expires_seq advanced, then clear the stale
cfs_rq->runtime_remaining from the slack timer of the past period, then
assign_cfs_rq_runtime() will refresh them afterwards, otherwise it is a
real clock drift. I am still not getting where the race is?

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[bsegall]

Xunlei Pang <xlpang@linux.alibaba.com> writes:

> On 7/31/18 1:55 AM, Cong Wang wrote:
>> On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>>>
>>> Hi Cong,
>>>
>>> On 7/28/18 8:24 AM, Cong Wang wrote:
>>>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
>>>> we should sync its ->expires_seq too. However it is missing
>>>> for distribute_cfs_runtime(), especially the slack timer call path.
>>>
>>> I don't think it's a problem, as expires_seq will get synced in
>>> assign_cfs_rq_runtime().
>> 
>> Sure, but there is a small window during which they are not synced.
>> Why do you want to wait until the next assign_cfs_rq_runtime() when
>> you already know runtime_expires is synced?
>> 
>> Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime()
>> inside __account_cfs_rq_runtime(), which means the check of
>> cfs_rq->expires_seq is not accurate for unthrottling case if the clock
>> drift happens soon enough?
>> 
>
> expire_cfs_rq_runtime():
>     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;
>     }
>
> So if clock drift happens soon, then expires_seq decides the correct
> thing we should do: if cfs_b->expires_seq advanced, then clear the stale
> cfs_rq->runtime_remaining from the slack timer of the past period, then
> assign_cfs_rq_runtime() will refresh them afterwards, otherwise it is a
> real clock drift. I am still not getting where the race is?

Nothing /important/ goes wrong because distribute_cfs_runtime only fills
runtime_remaining up to 1, not a real amount.

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Cong Wang]

On Tue, Jul 31, 2018 at 10:13 AM <bsegall@google.com> wrote:
>
> Xunlei Pang <xlpang@linux.alibaba.com> writes:
>
> > On 7/31/18 1:55 AM, Cong Wang wrote:
> >> On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
> >>>
> >>> Hi Cong,
> >>>
> >>> On 7/28/18 8:24 AM, Cong Wang wrote:
> >>>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
> >>>> we should sync its ->expires_seq too. However it is missing
> >>>> for distribute_cfs_runtime(), especially the slack timer call path.
> >>>
> >>> I don't think it's a problem, as expires_seq will get synced in
> >>> assign_cfs_rq_runtime().
> >>
> >> Sure, but there is a small window during which they are not synced.
> >> Why do you want to wait until the next assign_cfs_rq_runtime() when
> >> you already know runtime_expires is synced?
> >>
> >> Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime()
> >> inside __account_cfs_rq_runtime(), which means the check of
> >> cfs_rq->expires_seq is not accurate for unthrottling case if the clock
> >> drift happens soon enough?
> >>
> >
> > expire_cfs_rq_runtime():
> >     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;
> >     }
> >
> > So if clock drift happens soon, then expires_seq decides the correct
> > thing we should do: if cfs_b->expires_seq advanced, then clear the stale
> > cfs_rq->runtime_remaining from the slack timer of the past period, then
> > assign_cfs_rq_runtime() will refresh them afterwards, otherwise it is a
> > real clock drift. I am still not getting where the race is?

But expires_seq is supposed to be the same here, after
distribute_cfs_runtime(), therefore runtime_remaining is not supposed
to be cleared.

Which part do I misunderstand? expires_seq should not be same here?
Or you are saying a wrongly clear of runtime_remaning is fine?


>
> Nothing /important/ goes wrong because distribute_cfs_runtime only fills
> runtime_remaining up to 1, not a real amount.

No, runtime_remaining is updated right before expire_cfs_rq_runtime():

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);

so almost certainly it can't be 1.

Which means the following check could be passed:

 4655         if (cfs_rq->runtime_remaining < 0)
 4656                 return;

therefore we are reaching the clock drift logic code inside
expire_cfs_rq_runtime()
where expires_seq is supposed to be same as they should be sync'ed.
Therefore without patch, we wrongly clear the runtime_remainng?

Thanks.

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Xunlei Pang]

On 8/1/18 4:55 AM, Cong Wang wrote:
> On Tue, Jul 31, 2018 at 10:13 AM <bsegall@google.com> wrote:
>>
>> Xunlei Pang <xlpang@linux.alibaba.com> writes:
>>
>>> On 7/31/18 1:55 AM, Cong Wang wrote:
>>>> On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>>>>>
>>>>> Hi Cong,
>>>>>
>>>>> On 7/28/18 8:24 AM, Cong Wang wrote:
>>>>>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
>>>>>> we should sync its ->expires_seq too. However it is missing
>>>>>> for distribute_cfs_runtime(), especially the slack timer call path.
>>>>>
>>>>> I don't think it's a problem, as expires_seq will get synced in
>>>>> assign_cfs_rq_runtime().
>>>>
>>>> Sure, but there is a small window during which they are not synced.
>>>> Why do you want to wait until the next assign_cfs_rq_runtime() when
>>>> you already know runtime_expires is synced?
>>>>
>>>> Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime()
>>>> inside __account_cfs_rq_runtime(), which means the check of
>>>> cfs_rq->expires_seq is not accurate for unthrottling case if the clock
>>>> drift happens soon enough?
>>>>
>>>
>>> expire_cfs_rq_runtime():
>>>     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;
>>>     }
>>>
>>> So if clock drift happens soon, then expires_seq decides the correct
>>> thing we should do: if cfs_b->expires_seq advanced, then clear the stale
>>> cfs_rq->runtime_remaining from the slack timer of the past period, then
>>> assign_cfs_rq_runtime() will refresh them afterwards, otherwise it is a
>>> real clock drift. I am still not getting where the race is?
> 
> But expires_seq is supposed to be the same here, after
> distribute_cfs_runtime(), therefore runtime_remaining is not supposed
> to be cleared.
> 
> Which part do I misunderstand? expires_seq should not be same here?
> Or you are saying a wrongly clear of runtime_remaning is fine?
> 

Let's see the unthrottle cases.
1. for the periodic timer
distribute_cfs_runtime updates the throttled cfs_rq->runtime_expires to
be a new value, so expire_cfs_rq_runtime does nothing because of:
  rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires < 0

Afterwards assign_cfs_rq_runtime() will sync its expires_seq.

2. for the slack timer
the two expires_seq should be the same, so if clock drift happens soon,
expire_cfs_rq_runtime regards it as true clock drift:
  cfs_rq->runtime_expires += TICK_NSEC
If it happens that global expires_seq advances, it also doesn't matter,
expire_cfs_rq_runtime will clear the stale expire_cfs_rq_runtime as
expected.

> 
>>
>> Nothing /important/ goes wrong because distribute_cfs_runtime only fills
>> runtime_remaining up to 1, not a real amount.
> 
> No, runtime_remaining is updated right before expire_cfs_rq_runtime():
> 
> 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);
> 
> so almost certainly it can't be 1.

I think Ben means it firstly gets a distributtion of 1 to run after
unthrottling, soon it will have a negative runtime_remaining, and go
to assign_cfs_rq_runtime().

Thanks,
Xunlei

> 
> Which means the following check could be passed:
> 
>  4655         if (cfs_rq->runtime_remaining < 0)
>  4656                 return;
> 
> therefore we are reaching the clock drift logic code inside
> expire_cfs_rq_runtime()
> where expires_seq is supposed to be same as they should be sync'ed.
> Therefore without patch, we wrongly clear the runtime_remainng?
> 
> Thanks.
> 

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[bsegall]

Cong Wang <xiyou.wangcong@gmail.com> writes:

> On Tue, Jul 31, 2018 at 10:13 AM <bsegall@google.com> wrote:
>>
>> Xunlei Pang <xlpang@linux.alibaba.com> writes:
>>
>> > On 7/31/18 1:55 AM, Cong Wang wrote:
>> >> On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>> >>>
>> >>> Hi Cong,
>> >>>
>> >>> On 7/28/18 8:24 AM, Cong Wang wrote:
>> >>>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires,
>> >>>> we should sync its ->expires_seq too. However it is missing
>> >>>> for distribute_cfs_runtime(), especially the slack timer call path.
>> >>>
>> >>> I don't think it's a problem, as expires_seq will get synced in
>> >>> assign_cfs_rq_runtime().
>> >>
>> >> Sure, but there is a small window during which they are not synced.
>> >> Why do you want to wait until the next assign_cfs_rq_runtime() when
>> >> you already know runtime_expires is synced?
>> >>
>> >> Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime()
>> >> inside __account_cfs_rq_runtime(), which means the check of
>> >> cfs_rq->expires_seq is not accurate for unthrottling case if the clock
>> >> drift happens soon enough?
>> >>
>> >
>> > expire_cfs_rq_runtime():
>> >     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;
>> >     }
>> >
>> > So if clock drift happens soon, then expires_seq decides the correct
>> > thing we should do: if cfs_b->expires_seq advanced, then clear the stale
>> > cfs_rq->runtime_remaining from the slack timer of the past period, then
>> > assign_cfs_rq_runtime() will refresh them afterwards, otherwise it is a
>> > real clock drift. I am still not getting where the race is?
>
> But expires_seq is supposed to be the same here, after
> distribute_cfs_runtime(), therefore runtime_remaining is not supposed
> to be cleared.
>
> Which part do I misunderstand? expires_seq should not be same here?
> Or you are saying a wrongly clear of runtime_remaning is fine?
>
>
>>
>> Nothing /important/ goes wrong because distribute_cfs_runtime only fills
>> runtime_remaining up to 1, not a real amount.
>
> No, runtime_remaining is updated right before expire_cfs_rq_runtime():
>
> 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);
>
> so almost certainly it can't be 1.

Yes, in practice what's actually going to happen is that the
runtime_remaining will be put to 1 by distribute, the cfs_rq will be
unthrottled, and then when it runs it will go negative immediately and
hit the negative check in expires, so expires_seq being wrong will not
actually matter. In addition, the worst thing that will happen if one of
the account_cfs_rq_runtime(cfs_rq, 0) paths is hit first is that it will
lose 1ns of quota, which also doesn't really matter.

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Cong Wang]

On Tue, Jul 31, 2018 at 8:24 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote:
>
> Let's see the unthrottle cases.
> 1. for the periodic timer
> distribute_cfs_runtime updates the throttled cfs_rq->runtime_expires to
> be a new value, so expire_cfs_rq_runtime does nothing because of:
>   rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires < 0
>
> Afterwards assign_cfs_rq_runtime() will sync its expires_seq.

Is there any guarantee rq_clock(cfs_rq) is always ahead of
cfs_rq->runtime_expires in this case?

I doubt, because cfs_rq->runtime_expires could be assigned
by a sched_clock() on a different CPU running the periodic timer.

Also, rq_clock() is behind sched_clock() on the same CPU too,
sometimes it is merely hundreds of nanoseconds, sometimes it is
tens of thousands nanoseconds in my environment. (I have a
different patch to address this, but still not sure if it is correct.)


>
> 2. for the slack timer
> the two expires_seq should be the same, so if clock drift happens soon,
> expire_cfs_rq_runtime regards it as true clock drift:
>   cfs_rq->runtime_expires += TICK_NSEC
> If it happens that global expires_seq advances, it also doesn't matter,
> expire_cfs_rq_runtime will clear the stale expire_cfs_rq_runtime as
> expected.

Hmm, looks like due to the runtime_refresh_within() check in
slack timer.



>
> >
> >>
> >> Nothing /important/ goes wrong because distribute_cfs_runtime only fills
> >> runtime_remaining up to 1, not a real amount.
> >
> > No, runtime_remaining is updated right before expire_cfs_rq_runtime():
> >
> > 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);
> >
> > so almost certainly it can't be 1.
>
> I think Ben means it firstly gets a distributtion of 1 to run after
> unthrottling, soon it will have a negative runtime_remaining, and go
> to assign_cfs_rq_runtime().

That is obvious, being 1 in distribute_cfs_runtime is not relevant to the
discussion here.

Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Cong Wang]

On Wed, Aug 1, 2018 at 10:17 AM <bsegall@google.com> wrote:
> Yes, in practice what's actually going to happen is that the
> runtime_remaining will be put to 1 by distribute, the cfs_rq will be
> unthrottled, and then when it runs it will go negative immediately and
> hit the negative check in expires, so expires_seq being wrong will not
> actually matter. In addition, the worst thing that will happen if one of
> the account_cfs_rq_runtime(cfs_rq, 0) paths is hit first is that it will
> lose 1ns of quota, which also doesn't really matter.

Ah, I see.

Thanks!