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Date: Thu, 26 Mar 2020 10:27:59 -0700 From: bsegall@...gle.com To: Huaixin Chang <changhuaixin@...ux.alibaba.com> Cc: linux-kernel@...r.kernel.org, shanpeic@...ux.alibaba.com, yun.wang@...ux.alibaba.com, xlpang@...ux.alibaba.com, peterz@...radead.org, mingo@...hat.com, bsegall@...gle.com, chiluk+linux@...eed.com, vincent.guittot@...aro.org Subject: Re: [PATCH v2] sched: Fix race between runtime distribution and assignment Huaixin Chang <changhuaixin@...ux.alibaba.com> writes: > Currently, there is a potential race between distribute_cfs_runtime() > and assign_cfs_rq_runtime(). Race happens when cfs_b->runtime is read, > distributes without holding lock and finds out there is not enough > runtime to charge against after distribution. Because > assign_cfs_rq_runtime() might be called during distribution, and use > cfs_b->runtime at the same time. > > Fibtest is the tool to test this race. Assume all gcfs_rq is throttled > and cfs period timer runs, slow threads might run and sleep, returning > unused cfs_rq runtime and keeping min_cfs_rq_runtime in their local > pool. If all this happens sufficiently quickly, cfs_b->runtime will drop > a lot. If runtime distributed is large too, over-use of runtime happens. > > A runtime over-using by about 70 percent of quota is seen when we > test fibtest on a 96-core machine. We run fibtest with 1 fast thread and > 95 slow threads in test group, configure 10ms quota for this group and > see the CPU usage of fibtest is 17.0%, which is far more than the > expected 10%. > > On a smaller machine with 32 cores, we also run fibtest with 96 > threads. CPU usage is more than 12%, which is also more than expected > 10%. This shows that on similar workloads, this race do affect CPU > bandwidth control. > > Solve this by holding lock inside distribute_cfs_runtime(). Some minor requests below, other than that Reviewed-by: Ben Segall <bsegall@...gle.com> > > Fixes: c06f04c70489 ("sched: Fix potential near-infinite distribute_cfs_runtime() loop") > Signed-off-by: Huaixin Chang <changhuaixin@...ux.alibaba.com> > --- > v2: fix spelling, initialize variable rumaining in distribute_cfs_runtime() > --- > kernel/sched/fair.c | 31 ++++++++++++------------------- > 1 file changed, 12 insertions(+), 19 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index c1217bfe5e81..fd30e06a7ffc 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -4629,11 +4629,11 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq) > resched_curr(rq); > } > > -static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining) > +static void distribute_cfs_runtime(struct cfs_bandwidth *cfs_b) > { > struct cfs_rq *cfs_rq; > - u64 runtime; > - u64 starting_runtime = remaining; > + u64 runtime, remaining = 1; > + unsigned long flags; > > rcu_read_lock(); > list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq, > @@ -4648,10 +4648,13 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining) > /* By the above check, this should never be true */ > SCHED_WARN_ON(cfs_rq->runtime_remaining > 0); > > + raw_spin_lock_irqsave(&cfs_b->lock, flags); No need for _irqsave/_irqrestore, the rqlock already did. > runtime = -cfs_rq->runtime_remaining + 1; > - if (runtime > remaining) > - runtime = remaining; > - remaining -= runtime; > + if (runtime > cfs_b->runtime) > + runtime = cfs_b->runtime; > + cfs_b->runtime -= runtime; > + remaining = cfs_b->runtime; > + raw_spin_unlock_irqrestore(&cfs_b->lock, flags); > > cfs_rq->runtime_remaining += runtime; > > @@ -4666,8 +4669,6 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining) > break; > } > rcu_read_unlock(); > - > - return starting_runtime - remaining; > } > > /* > @@ -4678,7 +4679,6 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining) > */ > static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags) > { > - u64 runtime; > int throttled; > > /* no need to continue the timer with no bandwidth constraint */ > @@ -4708,23 +4708,17 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u > > /* > * This check is repeated as we are holding onto the new bandwidth while > - * we unthrottle. This can potentially race with an unthrottled group > - * trying to acquire new bandwidth from the global pool. This can result > - * in us over-using our runtime if it is all used during this loop, but > - * only by limited amounts in that extreme case. > + * we unthrottle. "This check is repeated as we release cfs_b->lock while we unthrottle." or something like that. This code is no longer even holding onto the new bandwidth on its own. > */ > while (throttled && cfs_b->runtime > 0 && !cfs_b->distribute_running) { > - runtime = cfs_b->runtime; > cfs_b->distribute_running = 1; > raw_spin_unlock_irqrestore(&cfs_b->lock, flags); > /* we can't nest cfs_b->lock while distributing bandwidth */ > - runtime = distribute_cfs_runtime(cfs_b, runtime); > + distribute_cfs_runtime(cfs_b); > raw_spin_lock_irqsave(&cfs_b->lock, flags); > > cfs_b->distribute_running = 0; > throttled = !list_empty(&cfs_b->throttled_cfs_rq); > - > - lsub_positive(&cfs_b->runtime, runtime); > } > > /* > @@ -4858,10 +4852,9 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b) > if (!runtime) > return; > > - runtime = distribute_cfs_runtime(cfs_b, runtime); > + distribute_cfs_runtime(cfs_b); > > raw_spin_lock_irqsave(&cfs_b->lock, flags); > - lsub_positive(&cfs_b->runtime, runtime); > cfs_b->distribute_running = 0; > raw_spin_unlock_irqrestore(&cfs_b->lock, flags); > }
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