From baa9be4ffb55876923dc9716abc0a448e510ba30 Mon Sep 17 00:00:00 2001 From: Phil Auld Date: Mon, 8 Oct 2018 10:36:40 -0400 Subject: sched/fair: Fix throttle_list starvation with low CFS quota With a very low cpu.cfs_quota_us setting, such as the minimum of 1000, distribute_cfs_runtime may not empty the throttled_list before it runs out of runtime to distribute. In that case, due to the change from c06f04c7048 to put throttled entries at the head of the list, later entries on the list will starve. Essentially, the same X processes will get pulled off the list, given CPU time and then, when expired, get put back on the head of the list where distribute_cfs_runtime will give runtime to the same set of processes leaving the rest. Fix the issue by setting a bit in struct cfs_bandwidth when distribute_cfs_runtime is running, so that the code in throttle_cfs_rq can decide to put the throttled entry on the tail or the head of the list. The bit is set/cleared by the callers of distribute_cfs_runtime while they hold cfs_bandwidth->lock. This is easy to reproduce with a handful of CPU consumers. I use 'crash' on the live system. In some cases you can simply look at the throttled list and see the later entries are not changing: crash> list cfs_rq.throttled_list -H 0xffff90b54f6ade40 -s cfs_rq.runtime_remaining | paste - - | awk '{print $1" "$4}' | pr -t -n3 1 ffff90b56cb2d200 -976050 2 ffff90b56cb2cc00 -484925 3 ffff90b56cb2bc00 -658814 4 ffff90b56cb2ba00 -275365 5 ffff90b166a45600 -135138 6 ffff90b56cb2da00 -282505 7 ffff90b56cb2e000 -148065 8 ffff90b56cb2fa00 -872591 9 ffff90b56cb2c000 -84687 10 ffff90b56cb2f000 -87237 11 ffff90b166a40a00 -164582 crash> list cfs_rq.throttled_list -H 0xffff90b54f6ade40 -s cfs_rq.runtime_remaining | paste - - | awk '{print $1" "$4}' | pr -t -n3 1 ffff90b56cb2d200 -994147 2 ffff90b56cb2cc00 -306051 3 ffff90b56cb2bc00 -961321 4 ffff90b56cb2ba00 -24490 5 ffff90b166a45600 -135138 6 ffff90b56cb2da00 -282505 7 ffff90b56cb2e000 -148065 8 ffff90b56cb2fa00 -872591 9 ffff90b56cb2c000 -84687 10 ffff90b56cb2f000 -87237 11 ffff90b166a40a00 -164582 Sometimes it is easier to see by finding a process getting starved and looking at the sched_info: crash> task ffff8eb765994500 sched_info PID: 7800 TASK: ffff8eb765994500 CPU: 16 COMMAND: "cputest" sched_info = { pcount = 8, run_delay = 697094208, last_arrival = 240260125039, last_queued = 240260327513 }, crash> task ffff8eb765994500 sched_info PID: 7800 TASK: ffff8eb765994500 CPU: 16 COMMAND: "cputest" sched_info = { pcount = 8, run_delay = 697094208, last_arrival = 240260125039, last_queued = 240260327513 }, Signed-off-by: Phil Auld Reviewed-by: Ben Segall Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: stable@vger.kernel.org Fixes: c06f04c70489 ("sched: Fix potential near-infinite distribute_cfs_runtime() loop") Link: http://lkml.kernel.org/r/20181008143639.GA4019@pauld.bos.csb Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 22 +++++++++++++++++++--- kernel/sched/sched.h | 2 ++ 2 files changed, 21 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7fc4a371bdd2..f88e00705b55 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4476,9 +4476,13 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq) /* * Add to the _head_ of the list, so that an already-started - * distribute_cfs_runtime will not see us + * distribute_cfs_runtime will not see us. If disribute_cfs_runtime is + * not running add to the tail so that later runqueues don't get starved. */ - list_add_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq); + if (cfs_b->distribute_running) + list_add_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq); + else + list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq); /* * If we're the first throttled task, make sure the bandwidth @@ -4622,14 +4626,16 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun) * in us over-using our runtime if it is all used during this loop, but * only by limited amounts in that extreme case. */ - while (throttled && cfs_b->runtime > 0) { + while (throttled && cfs_b->runtime > 0 && !cfs_b->distribute_running) { runtime = cfs_b->runtime; + cfs_b->distribute_running = 1; raw_spin_unlock(&cfs_b->lock); /* we can't nest cfs_b->lock while distributing bandwidth */ runtime = distribute_cfs_runtime(cfs_b, runtime, runtime_expires); raw_spin_lock(&cfs_b->lock); + cfs_b->distribute_running = 0; throttled = !list_empty(&cfs_b->throttled_cfs_rq); cfs_b->runtime -= min(runtime, cfs_b->runtime); @@ -4740,6 +4746,11 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b) /* confirm we're still not at a refresh boundary */ raw_spin_lock(&cfs_b->lock); + if (cfs_b->distribute_running) { + raw_spin_unlock(&cfs_b->lock); + return; + } + if (runtime_refresh_within(cfs_b, min_bandwidth_expiration)) { raw_spin_unlock(&cfs_b->lock); return; @@ -4749,6 +4760,9 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b) runtime = cfs_b->runtime; expires = cfs_b->runtime_expires; + if (runtime) + cfs_b->distribute_running = 1; + raw_spin_unlock(&cfs_b->lock); if (!runtime) @@ -4759,6 +4773,7 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b) raw_spin_lock(&cfs_b->lock); if (expires == cfs_b->runtime_expires) cfs_b->runtime -= min(runtime, cfs_b->runtime); + cfs_b->distribute_running = 0; raw_spin_unlock(&cfs_b->lock); } @@ -4867,6 +4882,7 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b) cfs_b->period_timer.function = sched_cfs_period_timer; hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL); cfs_b->slack_timer.function = sched_cfs_slack_timer; + cfs_b->distribute_running = 0; } static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 455fa330de04..9683f458aec7 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -346,6 +346,8 @@ struct cfs_bandwidth { int nr_periods; int nr_throttled; u64 throttled_time; + + bool distribute_running; #endif }; -- cgit v1.2.3 From 9845c49cc9bbb317a0bc9e9cf78d8e09d54c9af0 Mon Sep 17 00:00:00 2001 From: Song Muchun Date: Sun, 14 Oct 2018 19:26:12 +0800 Subject: sched/fair: Fix the min_vruntime update logic in dequeue_entity() The comment and the code around the update_min_vruntime() call in dequeue_entity() are not in agreement. From commit: b60205c7c558 ("sched/fair: Fix min_vruntime tracking") I think that we want to update min_vruntime when a task is sleeping/migrating. So, the check is inverted there - fix it. Signed-off-by: Song Muchun Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Fixes: b60205c7c558 ("sched/fair: Fix min_vruntime tracking") Link: http://lkml.kernel.org/r/20181014112612.2614-1-smuchun@gmail.com Signed-off-by: Ingo Molnar --- kernel/sched/fair.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index f88e00705b55..908c9cdae2f0 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4001,7 +4001,7 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags) * put back on, and if we advance min_vruntime, we'll be placed back * further than we started -- ie. we'll be penalized. */ - if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) == DEQUEUE_SAVE) + if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE) update_min_vruntime(cfs_rq); } -- cgit v1.2.3