From: Tim Chen <[email protected]>
In traces on newidle_balance(), this_rq->next_balance
time goes backward and earlier than current time jiffies, e.g.
11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
13.856 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
13.910 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
14.637 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73c
14.666 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
It doesn't make sense to have a next_balance in the past.
Fix newidle_balance() and update_next_balance() so the next
balance time is at least jiffies+1.
Signed-off-by: Tim Chen <[email protected]>
[Rebase]
Signed-off-by: Vincent Guittot <[email protected]>
---
kernel/sched/fair.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a162b0ec8963..1050037578a9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10138,7 +10138,10 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
/* used by idle balance, so cpu_busy = 0 */
interval = get_sd_balance_interval(sd, 0);
- next = sd->last_balance + interval;
+ if (time_after(jiffies+1, sd->last_balance + interval))
+ next = jiffies+1;
+ else
+ next = sd->last_balance + interval;
if (time_after(*next_balance, next))
*next_balance = next;
@@ -10974,6 +10977,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
out:
/* Move the next balance forward */
+ if (time_after(jiffies+1, this_rq->next_balance))
+ this_rq->next_balance = jiffies+1;
if (time_after(this_rq->next_balance, next_balance))
this_rq->next_balance = next_balance;
--
2.17.1
On Fri, Nov 12, 2021 at 11:04:58AM +0100, Vincent Guittot wrote:
> From: Tim Chen <[email protected]>
>
> In traces on newidle_balance(), this_rq->next_balance
> time goes backward and earlier than current time jiffies, e.g.
>
> 11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
> 11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
> 13.856 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
> 13.910 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
> 14.637 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73c
> 14.666 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
No explanation of what these numbers mean, or where they're taken from.
> It doesn't make sense to have a next_balance in the past.
> Fix newidle_balance() and update_next_balance() so the next
> balance time is at least jiffies+1.
The changelog is deficient in that it doesn't explain how the times end
up in the past, therefore we cannot evaluate if the provided solution is
sufficient etc..
> Signed-off-by: Tim Chen <[email protected]>
> Signed-off-by: Vincent Guittot <[email protected]>
> ---
> kernel/sched/fair.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a162b0ec8963..1050037578a9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10138,7 +10138,10 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>
> /* used by idle balance, so cpu_busy = 0 */
> interval = get_sd_balance_interval(sd, 0);
> - next = sd->last_balance + interval;
> + if (time_after(jiffies+1, sd->last_balance + interval))
> + next = jiffies+1;
> + else
> + next = sd->last_balance + interval;
>
> if (time_after(*next_balance, next))
> *next_balance = next;
> @@ -10974,6 +10977,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>
> out:
> /* Move the next balance forward */
> + if (time_after(jiffies+1, this_rq->next_balance))
> + this_rq->next_balance = jiffies+1;
jiffies roll over here..
Also, what's the point of the update_next_balance() addition in the face
of this one? AFAICT this hunk completely renders the other hunk useless.
> if (time_after(this_rq->next_balance, next_balance))
> this_rq->next_balance = next_balance;
and you've violated your own premise :-)
Now, this pattern is repeated throughout, if it's a problem here, why
isn't it a problem in say rebalance_domains() ?
Can we please unify the code across sites instead of growing different
hacks in different places?
On 11/12/21 7:21 AM, Peter Zijlstra wrote:
> On Fri, Nov 12, 2021 at 11:04:58AM +0100, Vincent Guittot wrote:
>> From: Tim Chen <[email protected]>
>>
>> In traces on newidle_balance(), this_rq->next_balance
>> time goes backward and earlier than current time jiffies, e.g.
>>
>> 11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
>> 11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>> 13.856 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73b
>> 13.910 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73b
>> 14.637 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb73c
>> 14.666 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb73c
>
> No explanation of what these numbers mean, or where they're taken from.
Sorry I should have added more explanation. I put a probe on newidle_balance and dump
out the values of this_rq pointer, this_rq->next_balance and jiffies entering
newidle_balance using the following commands:
perf probe 'newidle_balance this_rq this_rq->next_balance jiffies'
perf trace -e probe:newidle_balance
In the first line of the trace, next_balance start off at 0x1004fb76c:
11.602 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb76c jiffies=0x1004fb739
and in the second line, next_balance actually goes backward to 0x1004fb731, and becomes less than the jiffies value 0x1004fb739.
11.624 ( ): probe:newidle_balance:(ffffffff810d2470) this_rq=0xffff88fe7f8aae00 next_balance=0x1004fb731 jiffies=0x1004fb739
>
>> It doesn't make sense to have a next_balance in the past.
>> Fix newidle_balance() and update_next_balance() so the next
>> balance time is at least jiffies+1.
>
> The changelog is deficient in that it doesn't explain how the times end
> up in the past, therefore we cannot evaluate if the provided solution is
> sufficient etc..
>
>> Signed-off-by: Tim Chen <[email protected]>
>> Signed-off-by: Vincent Guittot <[email protected]>
>> ---
>> kernel/sched/fair.c | 7 ++++++-
>> 1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index a162b0ec8963..1050037578a9 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10138,7 +10138,10 @@ update_next_balance(struct sched_domain *sd, unsigned long *next_balance)
>>
>> /* used by idle balance, so cpu_busy = 0 */
>> interval = get_sd_balance_interval(sd, 0);
>> - next = sd->last_balance + interval;
>> + if (time_after(jiffies+1, sd->last_balance + interval))
>> + next = jiffies+1;
>> + else
>> + next = sd->last_balance + interval;
>>
>> if (time_after(*next_balance, next))
>> *next_balance = next;
>> @@ -10974,6 +10977,8 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
>>
>> out:
>> /* Move the next balance forward */
>> + if (time_after(jiffies+1, this_rq->next_balance))
>> + this_rq->next_balance = jiffies+1;
>
> jiffies roll over here..
>
> Also, what's the point of the update_next_balance() addition in the face
> of this one? AFAICT this hunk completely renders the other hunk useless.
>
>> if (time_after(this_rq->next_balance, next_balance))
>> this_rq->next_balance = next_balance;
>
> and you've violated your own premise :-)
Agree that this hunk is redundant. Should only keep the update_next_balance() hunk.
>
> Now, this pattern is repeated throughout, if it's a problem here, why
> isn't it a problem in say rebalance_domains() ?
In rebalance_domains, next_balance is assigned an initial value of jiffies+60*HZ
and could only increase.
So when we update this_rq-next_balance with next_balance computed,
it should always be more than current jiffies.
>
> Can we please unify the code across sites instead of growing different
> hacks in different places?
>
I'll take a closer look at the next_balance computation in other places.
Tim