Messages in this thread Patch in this message | | | Subject | [patch] sched: Fix clock_gettime(CLOCK_[PROCESS/THREAD]_CPUTIME_ID) monotonicity | From | Mike Galbraith <> | Date | Mon, 16 Jun 2014 11:16:52 +0200 |
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(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 <umgwanakikbuti@gmail.com> --- 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); } @@ -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) { update_rq_clock(rq); ns = rq_clock_task(rq) - p->se.exec_start; if ((s64)ns < 0) @@ -2576,8 +2583,10 @@ unsigned long long task_sched_runtime(st * If we race with it leaving cpu, we'll take a lock. So we're correct. * If we race with it entering cpu, unaccounted time is 0. This is * indistinguishable from the read occurring a few cycles earlier. + * If we see ->on_cpu without ->on_rq, the task is leaving, and has + * been accounted, so we're correct here as well. */ - if (!p->on_cpu) + if (!p->on_cpu || !p->on_rq) return p->se.sum_exec_runtime; #endif
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