Messages in this thread Patch in this message | | | Subject | Re: [PATCH 2/2] sched: sched: Fix rq->next_balance time updated to earlier than current time | From | Tim Chen <> | Date | Mon, 15 Nov 2021 11:40:03 -0800 |
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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 <tim.c.chen@linux.intel.com> >> >> 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 <tim.c.chen@linux.intel.com> >> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >> --- >> 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
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