Ingo Molnar <mi...@kernel.org> writes: > I've Cc:-ed a handful of gents who worked on CFS bandwidth details to widen > the discussion. > Patch quoted below. > > Looks like a real bug that needs to be fixed - and at first sight the quota > of 1000 looks very > low - could we improve the arithmetics perhaps? > > A low quota of 1000 is used because there's many VMs or containers > provisioned on the system > that is triggering the bug, right? > > Thanks, > > Ingo > > * Phil Auld <pa...@redhat.com> wrote: > >> From: "Phil Auld" <pa...@redhat.com> >> >> sched/fair: Avoid 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. >> >> Signed-off-by: Phil Auld <pa...@redhat.com> >> Fixes: c06f04c70489 ("sched: Fix potential near-infinite >> distribute_cfs_runtime() loop") >> Cc: Peter Zijlstra <pet...@infradead.org> >> Cc: Ingo Molnar <mi...@kernel.org> >> Cc: sta...@vger.kernel.org
Reviewed-by: Ben Segall <bseg...@google.com> In theory this does mean the unfairness could still happen if distribute is still running, but while a tiny quota makes it more likely, the fact that we're not getting through much of the list makes it not really a worry. If you wanted to be even more careful there could be some generation counter or something, but it doesn't seem necessary. >> --- >> >> 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 >> }, >> >> >> fair.c | 22 +++++++++++++++++++--- >> 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 >> }; >> >> >> >> --