Messages in this thread Patch in this message | | | Date | Thu, 23 Jan 2014 15:39:36 +0100 | From | Daniel Lezcano <> | Subject | Re: [PATCH 3/9] sched: Move idle_stamp up to the core |
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On 01/23/2014 01:58 PM, Peter Zijlstra wrote: > On Tue, Jan 21, 2014 at 12:17:57PM +0100, Peter Zijlstra wrote: >> From: Daniel Lezcano <daniel.lezcano@linaro.org> >> >> The idle_balance modifies the idle_stamp field of the rq, making this >> information to be shared across core.c and fair.c. As we can know if the >> cpu is going to idle or not with the previous patch, let's encapsulate the >> idle_stamp information in core.c by moving it up to the caller. The >> idle_balance function returns true in case a balancing occured and the cpu >> won't be idle, false if no balance happened and the cpu is going idle. >> >> Cc: alex.shi@linaro.org >> Cc: peterz@infradead.org >> Cc: mingo@kernel.org >> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org> >> Signed-off-by: Peter Zijlstra <peterz@infradead.org> >> Link: http://lkml.kernel.org/r/1389949444-14821-3-git-send-email-daniel.lezcano@linaro.org >> --- >> kernel/sched/core.c | 2 +- >> kernel/sched/fair.c | 14 ++++++-------- >> kernel/sched/sched.h | 2 +- >> 3 files changed, 8 insertions(+), 10 deletions(-) >> >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -2680,7 +2680,7 @@ static void __sched __schedule(void) >> pre_schedule(rq, prev); >> >> if (unlikely(!rq->nr_running)) >> - idle_balance(rq); >> + rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq); > > OK, spotted a problem here.. > > So previously idle_stamp was set _before_ actually doing idle_balance(), > and that was RIGHT, because that way we include the cost of actually > doing idle_balance() into the idle time. > > By not including the cost of idle_balance() you underestimate the 'idle' > time in that if idle_balance() filled the entire idle time we account 0 > idle, even though we had 'plenty' of time to run the entire thing. > > This leads to not running idle_balance() even though we have the time > for it.
Good catch. How did you notice that ?
> So we very much want something like: > > > if (!rq->nr_running) > rq->idle_stamp = rq_clock(rq); > > p = pick_next_task(rq, prev); > > if (!is_idle_task(p)) > rq->idle_stamp = 0;
Is this code assuming idle_balance() is in pick_next_task ?
For this specific patch 3/9, will be ok the following ?
+ if (unlikely(!rq->nr_running)) { + rq->idle_stamp = rq_clock(rq); + if (idle_balance(rq)) + rq->idle_stamp = 0; + }
So the patch 9/9 is wrong also because it does not fill idle_stamp before idle_balance, the fix would be.
kernel/sched/core.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
Index: cpuidle-next/kernel/sched/core.c =================================================================== --- cpuidle-next.orig/kernel/sched/core.c +++ cpuidle-next/kernel/sched/core.c @@ -2579,15 +2579,17 @@ again: } }
+ rq->idle_stamp = rq_clock(rq); + /* * If there is a task balanced on this cpu, pick the next task, * otherwise fall in the optimization by picking the idle task * directly. */ - if (idle_balance(rq)) + if (idle_balance(rq)) { + rq->idle_stamp = 0; goto again; - - rq->idle_stamp = rq_clock(rq); + }
return idle_sched_class.pick_next_task(rq, prev); }
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