From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vincent Guittot Subject: [PATCH v12 1/8] sched/fair: fix unfairness at wakeup Date: Fri, 24 Feb 2023 10:34:47 +0100 Message-ID: <20230224093454.956298-2-vincent.guittot@linaro.org> References: <20230224093454.956298-1-vincent.guittot@linaro.org> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=WRK9CVl+wp5UA9FM0KiXaS6Hv0eR7fUWtXzYt4onYBI=; b=aeaP+8GpSlBrTz0Ck22w37L8jNZXuLY7XzqRiV7TMNqQIraO8lIsG2uI0QxBG6sYSO YaCzS2VTd72to5fFRzTyrxDRqfVmkG/1xLZcqcTROHtnd8QziUlJZ5/GaA2l7chieDrd KfMNbvgA/KZbYLoZm5v/8oLyQRhmDC3fLmX8ke34ZZYBBq3ouVuLw2JMfGK4874hBrwh 5dCJ9xvzrd4k/tpGfHGW/dtUoljWWPlVPVbZkJ0YWxdbO7l3KaAj44H76grIkeXb7Z6/ 0Z0Z3UcS8m81iXcoxjO2GXo0rqHYT6GPKb1TvBOGozY192erQKt1rLmS3DMx/G7T+JYp 2oTQ== In-Reply-To: <20230224093454.956298-1-vincent.guittot@linaro.org> List-ID: Content-Type: text/plain; charset="us-ascii" To: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org, parth@linux.ibm.com, tj@kernel.org, lizefan.x@bytedance.com, hannes@cmpxchg.org, cgroups@vger.kernel.org, corbet@lwn.net, linux-doc@vger.kernel.org Cc: qyousef@layalina.io, chris.hyser@oracle.com, patrick.bellasi@matbug.net, David.Laight@aculab.com, pjt@google.com, pavel@ucw.cz, qperret@google.com, tim.c.chen@linux.intel.com, joshdon@google.com, timj@gnu.org, kprateek.nayak@amd.com, yu.c.chen@intel.com, youssefesmat@chromium.org, joel@joelfernandes.org, Vincent Guittot At wake up, the vruntime of a task is updated to not be more older than a sched_latency period behind the min_vruntime. This prevents long sleeping task to get unlimited credit at wakeup. Such waking task should preempt current one to use its CPU bandwidth but wakeup_gran() can be larger than sched_latency, filter out the wakeup preemption and as a results steals some CPU bandwidth to the waking task. Make sure that a task, which vruntime has been capped, will preempt current task and use its CPU bandwidth even if wakeup_gran() is in the same range as sched_latency. If the waking task failed to preempt current it could to wait up to sysctl_sched_min_granularity before preempting it during next tick. Strictly speaking, we should use cfs->min_vruntime instead of curr->vruntime but it doesn't worth the additional overhead and complexity as the vruntime of current should be close to min_vruntime if not equal. Reported-by: Youssef Esmat Signed-off-by: Vincent Guittot Reviewed-by: Joel Fernandes (Google) Tested-by: K Prateek Nayak --- kernel/sched/fair.c | 46 ++++++++++++++++++++------------------------ kernel/sched/sched.h | 34 +++++++++++++++++++++++++++++++- 2 files changed, 54 insertions(+), 26 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ff4dbbae3b10..81bef11eb660 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4654,33 +4654,17 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) u64 vruntime = cfs_rq->min_vruntime; u64 sleep_time; - /* - * The 'current' period is already promised to the current tasks, - * however the extra weight of the new task will slow them down a - * little, place the new task so that it fits in the slot that - * stays open at the end. - */ - if (initial && sched_feat(START_DEBIT)) - vruntime += sched_vslice(cfs_rq, se); - - /* sleeps up to a single latency don't count. */ - if (!initial) { - unsigned long thresh; - - if (se_is_idle(se)) - thresh = sysctl_sched_min_granularity; - else - thresh = sysctl_sched_latency; - + if (!initial) + /* sleeps up to a single latency don't count. */ + vruntime -= get_sleep_latency(se_is_idle(se)); + else if (sched_feat(START_DEBIT)) /* - * Halve their sleep time's effect, to allow - * for a gentler effect of sleepers: + * The 'current' period is already promised to the current tasks, + * however the extra weight of the new task will slow them down a + * little, place the new task so that it fits in the slot that + * stays open at the end. */ - if (sched_feat(GENTLE_FAIR_SLEEPERS)) - thresh >>= 1; - - vruntime -= thresh; - } + vruntime += sched_vslice(cfs_rq, se); /* * Pull vruntime of the entity being placed to the base level of @@ -7721,6 +7705,18 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se) return -1; gran = wakeup_gran(se); + + /* + * At wake up, the vruntime of a task is capped to not be older than + * a sched_latency period compared to min_vruntime. This prevents long + * sleeping task to get unlimited credit at wakeup. Such waking up task + * has to preempt current in order to not lose its share of CPU + * bandwidth but wakeup_gran() can become higher than scheduling period + * for low priority task. Make sure that long sleeping task will get a + * chance to preempt current. + */ + gran = min_t(s64, gran, get_latency_max()); + if (vdiff > gran) return 1; diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 3e8df6d31c1e..51ba0af7fb27 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -2458,9 +2458,9 @@ extern void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags); extern const_debug unsigned int sysctl_sched_nr_migrate; extern const_debug unsigned int sysctl_sched_migration_cost; -#ifdef CONFIG_SCHED_DEBUG extern unsigned int sysctl_sched_latency; extern unsigned int sysctl_sched_min_granularity; +#ifdef CONFIG_SCHED_DEBUG extern unsigned int sysctl_sched_idle_min_granularity; extern unsigned int sysctl_sched_wakeup_granularity; extern int sysctl_resched_latency_warn_ms; @@ -2475,6 +2475,38 @@ extern unsigned int sysctl_numa_balancing_scan_size; extern unsigned int sysctl_numa_balancing_hot_threshold; #endif +static inline unsigned long get_sleep_latency(bool idle) +{ + unsigned long thresh; + + if (idle) + thresh = sysctl_sched_min_granularity; + else + thresh = sysctl_sched_latency; + + /* + * Halve their sleep time's effect, to allow + * for a gentler effect of sleepers: + */ + if (sched_feat(GENTLE_FAIR_SLEEPERS)) + thresh >>= 1; + + return thresh; +} + +static inline unsigned long get_latency_max(void) +{ + unsigned long thresh = get_sleep_latency(false); + + /* + * If the waking task failed to preempt current it could to wait up to + * sysctl_sched_min_granularity before preempting it during next tick. + */ + thresh -= sysctl_sched_min_granularity; + + return thresh; +} + #ifdef CONFIG_SCHED_HRTICK /* -- 2.34.1