On Mon, Jun 16, 2014 at 11:16:52AM +0200, Mike Galbraith wrote:
> (disregard patch of same name from that enterprise weenie;)
>
> If a task has been dequeued, it has been accounted. Do not project
> cycles that may or may not ever be accounted to a dequeued task, as
> that may make clock_gettime() both inaccurate and non-monotonic.
>
> Protect update_rq_clock() from slight TSC skew while at it.
>
> Signed-off-by: Mike Galbraith <[email protected]>
> ---
> kernel/sched/core.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -144,6 +144,8 @@ void update_rq_clock(struct rq *rq)
> return;
>
> delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
> + if (delta < 0)
> + return;
> rq->clock += delta;
> update_rq_clock_task(rq, delta);
> }
Have you actually observed this? If TSC is stable this should not
happen, if TSC is not stable we should be using kernel/sched/clock.c
which should also avoid this, because while sched_clock_cpu(x) -
sched_clock_cpu(y) < 0 is possible, sched_clock_cpu(x) -
sched_clock_cpu(x) should always be >= 0.
I suppose it can happen when the TSC gets screwed and we haven't
switched to the slow path yet.
> @@ -2533,7 +2535,12 @@ static u64 do_task_delta_exec(struct tas
> {
> u64 ns = 0;
>
> - if (task_current(rq, p)) {
> + /*
> + * Must be ->curr, ->on_cpu _and_ ->on_rq. If dequeued, we
> + * would project cycles that may never be accounted to this
> + * thread, breaking clock_gettime().
> + */
> + if (task_current(rq, p) && p->on_cpu && p->on_rq) {
do we still need ->on_cpu in this case?
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