> From: linux-rdma-owner@xxxxxxxxxxxxxxx [mailto:linux-rdma- > owner@xxxxxxxxxxxxxxx] On Behalf Of Thomas Gleixner > Sent: Tuesday, April 24, 2018 3:22 PM > To: Frederic Weisbecker <frederic@xxxxxxxxxx> > Cc: Wan, Kaike <kaike.wan@xxxxxxxxx>; Marciniszyn, Mike > <mike.marciniszyn@xxxxxxxxx>; Dalessandro, Dennis > <dennis.dalessandro@xxxxxxxxx>; Weiny, Ira <ira.weiny@xxxxxxxxx>; Fleck, > John <john.fleck@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux- > rdma@xxxxxxxxxxxxxxx; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Anna-Maria > Gleixner <anna-maria@xxxxxxxxxxxxx>; Frederic Weisbecker > <fweisbec@xxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx> > Subject: [PATCH] tick/sched: Do not mess with an enqueued hrtimer > > Kaike reported that in tests rdma hrtimers occasionaly stopped working. He > did great debugging, which provided enough context to decode the problem. > > CPU 3 CPU 2 > > idle > start sched_timer expires = 712171000000 queue->next = sched_timer > start rdmavt timer. expires = > 712172915662 > lock(baseof(CPU3)) > tick_nohz_stop_tick() > tick = 716767000000 timerqueue_add(tmr) > > hrtimer_set_expires(sched_timer, tick); > sched_timer->expires = 716767000000 <---- FAIL > if (tmr->expires < queue->next- > >expires) > hrtimer_start(sched_timer) queue->next = tmr; > lock(baseof(CPU3)) > unlock(baseof(CPU3)) > timerqueue_remove() > timerqueue_add() > > ts->sched_timer is queued and queue->next is pointing to it, but then > ts->sched_timer.expires is modified. > > This not only corrupts the ordering of the timerqueue RB tree, it also makes > CPU2 see the new expiry time of timerqueue->next->expires when checking > whether timerqueue->next needs to be updated. So CPU2 sees that the > rdma timer is earlier than timerqueue->next and sets the rdma timer as new > next. > > Depending on whether it had also seen the new time at RB tree enqueue, it > might have queued the rdma timer at the wrong place and then after > removing the sched_timer the RB tree is completely hosed. > > The problem was introduced with a commit which tried to solve > inconsistency between the hrtimer in the tick_sched data and the underlying > hardware clockevent. It split out hrtimer_set_expires() to store the new tick > time in both the NOHZ and the NOHZ + HIGHRES case, but missed the fact > that in the NOHZ + HIGHRES case the hrtimer might still be queued. > > Use hrtimer_start(timer, tick...) for the NOHZ + HIGHRES case which sets > timer->expires after canceling the timer and move the > timer->hrtimer_set_expires() > invocation into the NOHZ only code path which is not affected as it merily > uses the hrtimer as next event storage so code pathes can be shared with > the NOHZ + HIGHRES case. > > Fixes: d4af6d933ccf ("nohz: Fix spurious warning when hrtimer and > clockevent get out of sync") > Reported-by: "Wan Kaike" <kaike.wan@xxxxxxxxx> > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: "Marciniszyn Mike" <mike.marciniszyn@xxxxxxxxx> > Cc: Anna-Maria Gleixner <anna-maria@xxxxxxxxxxxxx> > Cc: linux-rdma@xxxxxxxxxxxxxxx > Cc: "Dalessandro Dennis" <dennis.dalessandro@xxxxxxxxx> > Cc: "Fleck John" <john.fleck@xxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> > Cc: "Weiny Ira" <ira.weiny@xxxxxxxxx> > Cc: "linux-rdma@xxxxxxxxxxxxxxx" > Cc: stable@xxxxxxxxxxxxxxx > Link: > https://lkml.kernel.org/r/alpine.DEB.2.21.1804241637390.1679@xxxxxxxxxxxxx > utronix.de > > --- > kernel/time/tick-sched.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -804,12 +804,12 @@ static void tick_nohz_stop_tick(struct t > return; > } > > - hrtimer_set_expires(&ts->sched_timer, tick); > - > - if (ts->nohz_mode == NOHZ_MODE_HIGHRES) > - hrtimer_start_expires(&ts->sched_timer, > HRTIMER_MODE_ABS_PINNED); > - else > + if (ts->nohz_mode == NOHZ_MODE_HIGHRES) { > + hrtimer_start(&ts->sched_timer, tick, > HRTIMER_MODE_ABS_PINNED); > + } else { > + hrtimer_set_expires(&ts->sched_timer, tick); > tick_program_event(tick, 1); > + } > } > > static void tick_nohz_retain_tick(struct tick_sched *ts) > -- > To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the > body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at > http://vger.kernel.org/majordomo-info.html Tested-by: Kaike Wan <Kaike.wan@xxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
