kernel.git/kernel/sched/fair.c, branch linux-5.16.y

sched/fair: Improve consistency of allowed NUMA balance calculations

2022-04-08T12:05:43Z

[ Upstream commit 2cfb7a1b031b0e816af7a6ee0c6ab83b0acdf05a ] There are inconsistencies when determining if a NUMA imbalance is allowed that should be corrected. o allow_numa_imbalance changes types and is not always examining the destination group so both the type should be corrected as well as the naming. o find_idlest_group uses the sched_domain's weight instead of the group weight which is different to find_busiest_group o find_busiest_group uses the source group instead of the destination which is different to task_numa_find_cpu o Both find_idlest_group and find_busiest_group should account for the number of running tasks if a move was allowed to be consistent with task_numa_find_cpu Fixes: 7d2b5dd0bcc4 ("sched/numa: Allow a floating imbalance between NUMA nodes") Signed-off-by: Mel Gorman Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Gautham R. Shenoy Link: https://lore.kernel.org/r/20220208094334.16379-2-mgorman@techsingularity.net Signed-off-by: Sasha Levin

sched/pelt: Relax the sync of util_sum with util_avg

2022-02-01T16:29:13Z

[ Upstream commit 98b0d890220d45418cfbc5157b3382e6da5a12ab ] Rick reported performance regressions in bugzilla because of cpu frequency being lower than before: https://bugzilla.kernel.org/show_bug.cgi?id=215045 He bisected the problem to: commit 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent") This commit forces util_sum to be synced with the new util_avg after removing the contribution of a task and before the next periodic sync. By doing so util_sum is rounded to its lower bound and might lost up to LOAD_AVG_MAX-1 of accumulated contribution which has not yet been reflected in util_avg. Instead of always setting util_sum to the low bound of util_avg, which can significantly lower the utilization of root cfs_rq after propagating the change down into the hierarchy, we revert the change of util_sum and propagate the difference. In addition, we also check that cfs's util_sum always stays above the lower bound for a given util_avg as it has been observed that sched_entity's util_sum is sometimes above cfs one. Fixes: 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent") Reported-by: Rick Yiu Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Tested-by: Sachin Sant Link: https://lkml.kernel.org/r/20220111134659.24961-2-vincent.guittot@linaro.org Signed-off-by: Sasha Levin

sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity

2022-01-27T11:01:40Z

[ Upstream commit 014ba44e8184e1acf93e0cbb7089ee847802f8f0 ] select_idle_sibling() has a special case for tasks woken up by a per-CPU kthread where the selected CPU is the previous one. For asymmetric CPU capacity systems, the assumption was that the wakee couldn't have a bigger utilization during task placement than it used to have during the last activation. That was not considering uclamp.min which can completely change between two task activations and as a consequence mandates the fitness criterion asym_fits_capacity(), even for the exit path described above. Fixes: b4c9c9f15649 ("sched/fair: Prefer prev cpu in asymmetric wakeup path") Signed-off-by: Vincent Donnefort Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Valentin Schneider Reviewed-by: Dietmar Eggemann Link: https://lkml.kernel.org/r/20211129173115.4006346-1-vincent.donnefort@arm.com Signed-off-by: Sasha Levin

sched/fair: Fix detection of per-CPU kthreads waking a task

2022-01-27T11:01:40Z

[ Upstream commit 8b4e74ccb582797f6f0b0a50372ebd9fd2372a27 ] select_idle_sibling() has a special case for tasks woken up by a per-CPU kthread, where the selected CPU is the previous one. However, the current condition for this exit path is incomplete. A task can wake up from an interrupt context (e.g. hrtimer), while a per-CPU kthread is running. A such scenario would spuriously trigger the special case described above. Also, a recent change made the idle task like a regular per-CPU kthread, hence making that situation more likely to happen (is_per_cpu_kthread(swapper) being true now). Checking for task context makes sure select_idle_sibling() will not interpret a wake up from any other context as a wake up by a per-CPU kthread. Fixes: 52262ee567ad ("sched/fair: Allow a per-CPU kthread waking a task to stack on the same CPU, to fix XFS performance regression") Signed-off-by: Vincent Donnefort Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Reviewed-by: Valentin Schneider Link: https://lore.kernel.org/r/20211201143450.479472-1-vincent.donnefort@arm.com Signed-off-by: Sasha Levin

sched/fair: Prevent dead task groups from regaining cfs_rq's

2021-11-11T12:09:33Z

Kevin is reporting crashes which point to a use-after-free of a cfs_rq in update_blocked_averages(). Initial debugging revealed that we've live cfs_rq's (on_list=1) in an about to be kfree()'d task group in free_fair_sched_group(). However, it was unclear how that can happen. His kernel config happened to lead to a layout of struct sched_entity that put the 'my_q' member directly into the middle of the object which makes it incidentally overlap with SLUB's freelist pointer. That, in combination with SLAB_FREELIST_HARDENED's freelist pointer mangling, leads to a reliable access violation in form of a #GP which made the UAF fail fast. Michal seems to have run into the same issue[1]. He already correctly diagnosed that commit a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle") is causing the preconditions for the UAF to happen by re-adding cfs_rq's also to task groups that have no more running tasks, i.e. also to dead ones. His analysis, however, misses the real root cause and it cannot be seen from the crash backtrace only, as the real offender is tg_unthrottle_up() getting called via sched_cfs_period_timer() via the timer interrupt at an inconvenient time. When unregister_fair_sched_group() unlinks all cfs_rq's from the dying task group, it doesn't protect itself from getting interrupted. If the timer interrupt triggers while we iterate over all CPUs or after unregister_fair_sched_group() has finished but prior to unlinking the task group, sched_cfs_period_timer() will execute and walk the list of task groups, trying to unthrottle cfs_rq's, i.e. re-add them to the dying task group. These will later -- in free_fair_sched_group() -- be kfree()'ed while still being linked, leading to the fireworks Kevin and Michal are seeing. To fix this race, ensure the dying task group gets unlinked first. However, simply switching the order of unregistering and unlinking the task group isn't sufficient, as concurrent RCU walkers might still see it, as can be seen below: CPU1: CPU2: : timer IRQ: : do_sched_cfs_period_timer(): : : : distribute_cfs_runtime(): : rcu_read_lock(); : : : unthrottle_cfs_rq(): sched_offline_group(): : : walk_tg_tree_from(…,tg_unthrottle_up,…): list_del_rcu(&tg->list); : (1) : list_for_each_entry_rcu(child, &parent->children, siblings) : : (2) list_del_rcu(&tg->siblings); : : tg_unthrottle_up(): unregister_fair_sched_group(): struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)]; : : list_del_leaf_cfs_rq(tg->cfs_rq[cpu]); : : : : if (!cfs_rq_is_decayed(cfs_rq) || cfs_rq->nr_running) (3) : list_add_leaf_cfs_rq(cfs_rq); : : : : : : : : : : (4) : rcu_read_unlock(); CPU 2 walks the task group list in parallel to sched_offline_group(), specifically, it'll read the soon to be unlinked task group entry at (1). Unlinking it on CPU 1 at (2) therefore won't prevent CPU 2 from still passing it on to tg_unthrottle_up(). CPU 1 now tries to unlink all cfs_rq's via list_del_leaf_cfs_rq() in unregister_fair_sched_group(). Meanwhile CPU 2 will re-add some of these at (3), which is the cause of the UAF later on. To prevent this additional race from happening, we need to wait until walk_tg_tree_from() has finished traversing the task groups, i.e. after the RCU read critical section ends in (4). Afterwards we're safe to call unregister_fair_sched_group(), as each new walk won't see the dying task group any more. On top of that, we need to wait yet another RCU grace period after unregister_fair_sched_group() to ensure print_cfs_stats(), which might run concurrently, always sees valid objects, i.e. not already free'd ones. This patch survives Michal's reproducer[2] for 8h+ now, which used to trigger within minutes before. [1] https://lore.kernel.org/lkml/20211011172236.11223-1-mkoutny@suse.com/ [2] https://lore.kernel.org/lkml/20211102160228.GA57072@blackbody.suse.cz/ Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle") [peterz: shuffle code around a bit] Reported-by: Kevin Tanguy Signed-off-by: Mathias Krause Signed-off-by: Peter Zijlstra (Intel)

sched/fair: Cleanup newidle_balance

2021-10-31T10:11:38Z

update_next_balance() uses sd->last_balance which is not modified by load_balance() so we can merge the 2 calls in one place. No functional change Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Acked-by: Mel Gorman Link: https://lore.kernel.org/r/20211019123537.17146-6-vincent.guittot@linaro.org

sched/fair: Remove sysctl_sched_migration_cost condition

2021-10-31T10:11:38Z

With a default value of 500us, sysctl_sched_migration_cost is significanlty higher than the cost of load_balance. Remove the condition and rely on the sd->max_newidle_lb_cost to abort newidle_balance. Suggested-by: Peter Zijlstra (Intel) Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Acked-by: Mel Gorman Link: https://lore.kernel.org/r/20211019123537.17146-5-vincent.guittot@linaro.org

sched/fair: Wait before decaying max_newidle_lb_cost

2021-10-31T10:11:38Z

Decay max_newidle_lb_cost only when it has not been updated for a while and ensure to not decay a recently changed value. Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Acked-by: Mel Gorman Link: https://lore.kernel.org/r/20211019123537.17146-4-vincent.guittot@linaro.org

sched/fair: Skip update_blocked_averages if we are defering load balance

2021-10-31T10:11:37Z

In newidle_balance(), the scheduler skips load balance to the new idle cpu when the 1st sd of this_rq is: this_rq->avg_idle < sd->max_newidle_lb_cost Doing a costly call to update_blocked_averages() will not be useful and simply adds overhead when this condition is true. Check the condition early in newidle_balance() to skip update_blocked_averages() when possible. Signed-off-by: Vincent Guittot Signed-off-by: Tim Chen Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Acked-by: Mel Gorman Link: https://lore.kernel.org/r/20211019123537.17146-3-vincent.guittot@linaro.org

sched/fair: Account update_blocked_averages in newidle_balance cost

2021-10-31T10:11:37Z

The time spent to update the blocked load can be significant depending of the complexity fo the cgroup hierarchy. Take this time into account in the cost of the 1st load balance of a newly idle cpu. Also reduce the number of call to sched_clock_cpu() and track more actual work. Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dietmar Eggemann Acked-by: Mel Gorman Link: https://lore.kernel.org/r/20211019123537.17146-2-vincent.guittot@linaro.org