From nobody Wed Apr 8 07:59:16 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 39684C28D13 for ; Mon, 22 Aug 2022 19:05:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237824AbiHVTFT (ORCPT ); Mon, 22 Aug 2022 15:05:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58654 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236695AbiHVTFI (ORCPT ); Mon, 22 Aug 2022 15:05:08 -0400 Received: from mail-pj1-x104a.google.com (mail-pj1-x104a.google.com [IPv6:2607:f8b0:4864:20::104a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7C2A813F92 for ; Mon, 22 Aug 2022 12:05:07 -0700 (PDT) Received: by mail-pj1-x104a.google.com with SMTP id k1-20020a17090a658100b001fb35f86ccdso1944846pjj.9 for ; Mon, 22 Aug 2022 12:05:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:from:to:cc; bh=fRVYXLDIctciukFUKSd2/GooWNYoTVmdQcxMFuUrumY=; b=KKkDGSumaIg53jK3OgpGqth05MPWF/QD24vh55Bcxqs4g+35kL1S/z9Gz4AMcb17V3 laERCP0k0YN7Pz3u29nxktavuY2NpWkqECtzIkpDkvTzfjRJTekym7nuXb8RBzI4sdnu J0aKMoghALFXZ/Lr0o4k5OYB/pshunGPx0dOdAbfbiA0V6VYfb52AfL2UdTgqAFySoyc sR2QbSjqsCy/icBtkaFIqBd6iBqTbVdk669B2HgSDJX0j+wkOfIDfK/iBHB8qKECpNfU vTwxLPMcKTXSMQxPVFmWY9Y37jkW6HMK0TQeOI2PaAv4qNKIg94dXhSdOEb3c3oe5O4/ qXMw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:references:mime-version:message-id:in-reply-to :date:x-gm-message-state:from:to:cc; bh=fRVYXLDIctciukFUKSd2/GooWNYoTVmdQcxMFuUrumY=; b=mktwGMVdR6xzXYtGLzCKhNOAFp9CaMLe6XMX+2zr0GNJ7BpemA39DyTREjflXRfkCa uANqYvdw1x7Np6DobLLxA6VqHpVJxSHUET8tWGogqEe8RoarJ36kBqPQkhAGXWftaimS LCWtKa1gYRw5lAnDtQ1I2hwB0TqVWwWl02qOsN9dvr68ZWXT4RshmvzVnGd9rXlHDugi +9iR1Im1IwZU04nUDls9gaqbvGCGWUx+1qR+C6pooytgWBsjHxTA9E2SfxNcAjMVAzdr Aqqg4ZWR2h3dr1aVh1NSlkKPas5sJ6g6MKed0TeTRcSXvwJoJVC8BaRpht1MykeMlzEB 66AQ== X-Gm-Message-State: ACgBeo1DuRj82tkfOJm0NELTPtP3ZkI5kLdp80PWcAPTzj9akVFtW6AC VCnDmFoSlNgMAgeFwQm+BmZND2qUNSAqA/DwK5cO9N1baztlGpDi9MMrvF1PoLJY/Mok0Yy+DLy X5BKQkB4iSkivQSohYR1YpS9L4Te3WnpzaahI2alRVudJNOtJ0I64RuLpmeKp2aHQ6Q8oACw= X-Google-Smtp-Source: AA6agR4o3ck3mbS34wCnLNWitDB95b+fiEIgKN3Jf0cV/Ttq3uu3wArzsn70bq8B6AqhCGj/CA7pKgwPaYlR X-Received: from jstultz-noogler2.c.googlers.com ([fda3:e722:ac3:cc00:24:72f4:c0a8:600]) (user=jstultz job=sendgmr) by 2002:a17:90a:1b69:b0:1fa:f9de:fbcf with SMTP id q96-20020a17090a1b6900b001faf9defbcfmr14929866pjq.201.1661195106810; Mon, 22 Aug 2022 12:05:06 -0700 (PDT) Date: Mon, 22 Aug 2022 19:05:00 +0000 In-Reply-To: <20220822190501.2171100-1-jstultz@google.com> Message-Id: <20220822190501.2171100-2-jstultz@google.com> Mime-Version: 1.0 References: <20220822190501.2171100-1-jstultz@google.com> X-Mailer: git-send-email 2.37.1.595.g718a3a8f04-goog Subject: [RFC][PATCH v2 1/2] sched: Avoid placing RT threads on cores handling long softirqs From: John Stultz To: LKML Cc: "Connor O'Brien" , John Dias , Rick Yiu , John Kacur , Qais Yousef , Chris Redpath , Abhijeet Dharmapurikar , Peter Zijlstra , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Thomas Gleixner , kernel-team@android.com, "J . Avila" , John Stultz Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Connor O'Brien In certain audio use cases, scheduling RT threads on cores that are handling softirqs can lead to glitches. Prevent this behavior in cases where the softirq is likely to take a long time. To avoid unnecessary migrations, the old behavior is preserved for RCU, SCHED and TIMER irqs which are expected to be relatively quick. This patch reworks and combines two related changes originally by John Dias Cc: John Dias Cc: Connor O'Brien Cc: Rick Yiu Cc: John Kacur Cc: Qais Yousef Cc: Chris Redpath Cc: Abhijeet Dharmapurikar Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Juri Lelli Cc: Vincent Guittot Cc: Dietmar Eggemann Cc: Steven Rostedt Cc: Thomas Gleixner Cc: kernel-team@android.com Signed-off-by: John Dias [elavila: Port to mainline, amend commit text] Signed-off-by: J. Avila [connoro: Reworked, simplified, and merged two patches together] Signed-off-by: Connor O'Brien [jstultz: Further simplified and fixed issues, reworded commit message, removed arm64-isms] Signed-off-by: John Stultz --- v2: * Reformatted Kconfig entry to match coding style (Reported-by: Randy Dunlap ) * Made rt_task_fits_capacity_and_may_preempt static to avoid warnings (Reported-by: kernel test robot ) * Rework to use preempt_count and drop kconfig dependency on ARM64 --- include/linux/interrupt.h | 7 +++++ init/Kconfig | 10 ++++++ kernel/sched/rt.c | 65 +++++++++++++++++++++++++++++++++------ kernel/softirq.c | 9 ++++++ 4 files changed, 82 insertions(+), 9 deletions(-) diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h index a92bce40b04b..bac9da05b9c8 100644 --- a/include/linux/interrupt.h +++ b/include/linux/interrupt.h @@ -571,6 +571,12 @@ enum * _ IRQ_POLL: irq_poll_cpu_dead() migrates the queue */ #define SOFTIRQ_HOTPLUG_SAFE_MASK (BIT(RCU_SOFTIRQ) | BIT(IRQ_POLL_SOFTIRQ= )) +/* Softirq's where the handling might be long: */ +#define LONG_SOFTIRQ_MASK ((1 << NET_TX_SOFTIRQ) | \ + (1 << NET_RX_SOFTIRQ) | \ + (1 << BLOCK_SOFTIRQ) | \ + (1 << IRQ_POLL_SOFTIRQ) | \ + (1 << TASKLET_SOFTIRQ)) =20 /* map softirq index to softirq name. update 'softirq_to_name' in * kernel/softirq.c when adding a new softirq. @@ -606,6 +612,7 @@ extern void raise_softirq_irqoff(unsigned int nr); extern void raise_softirq(unsigned int nr); =20 DECLARE_PER_CPU(struct task_struct *, ksoftirqd); +DECLARE_PER_CPU(u32, active_softirqs); =20 static inline struct task_struct *this_cpu_ksoftirqd(void) { diff --git a/init/Kconfig b/init/Kconfig index 532362fcfe31..8b5add74b6cb 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1284,6 +1284,16 @@ config SCHED_AUTOGROUP desktop applications. Task group autogeneration is currently based upon task session. =20 +config RT_SOFTIRQ_OPTIMIZATION + bool "Improve RT scheduling during long softirq execution" + depends on SMP + default n + help + Enable an optimization which tries to avoid placing RT tasks on CPUs + occupied by nonpreemptible tasks, such as a long softirq or CPUs + which may soon block preemptions, such as a CPU running a ksoftirq + thread which handles slow softirqs. + config SYSFS_DEPRECATED bool "Enable deprecated sysfs features to support old userspace tools" depends on SYSFS diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 55f39c8f4203..5a5cf396d0d2 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1599,12 +1599,50 @@ static void yield_task_rt(struct rq *rq) #ifdef CONFIG_SMP static int find_lowest_rq(struct task_struct *task); =20 +#ifdef CONFIG_RT_SOFTIRQ_OPTIMIZATION +/* + * Return whether the task on the given cpu is currently non-preemptible + * while handling a potentially long softirq, or if the task is likely + * to block preemptions soon because it is a ksoftirq thread that is + * handling slow softirq. + */ +static bool task_may_preempt(struct task_struct *task, int cpu) +{ + u32 softirqs =3D per_cpu(active_softirqs, cpu) | + per_cpu(irq_stat, cpu).__softirq_pending; + + struct task_struct *cpu_ksoftirqd =3D per_cpu(ksoftirqd, cpu); + struct task_struct *curr; + struct rq *rq =3D cpu_rq(cpu); + int ret; + + rcu_read_lock(); + curr =3D READ_ONCE(rq->curr); /* unlocked access */ + ret =3D !((softirqs & LONG_SOFTIRQ_MASK) && + (curr =3D=3D cpu_ksoftirqd || + preempt_count() & SOFTIRQ_MASK)); + rcu_read_unlock(); + return ret; +} +#else +static bool task_may_preempt(struct task_struct *task, int cpu) +{ + return true; +} +#endif /* CONFIG_RT_SOFTIRQ_OPTIMIZATION */ + +static bool rt_task_fits_capacity_and_may_preempt(struct task_struct *p, i= nt cpu) +{ + return task_may_preempt(p, cpu) && rt_task_fits_capacity(p, cpu); +} + static int select_task_rq_rt(struct task_struct *p, int cpu, int flags) { struct task_struct *curr; struct rq *rq; bool test; + bool may_not_preempt; =20 /* For anything but wake ups, just return the task_cpu */ if (!(flags & (WF_TTWU | WF_FORK))) @@ -1616,7 +1654,12 @@ select_task_rq_rt(struct task_struct *p, int cpu, in= t flags) curr =3D READ_ONCE(rq->curr); /* unlocked access */ =20 /* - * If the current task on @p's runqueue is an RT task, then + * If the current task on @p's runqueue is a softirq task, + * it may run without preemption for a time that is + * ill-suited for a waiting RT task. Therefore, try to + * wake this RT task on another runqueue. + * + * Also, if the current task on @p's runqueue is an RT task, then * try to see if we can wake this RT task up on another * runqueue. Otherwise simply start this RT task * on its current runqueue. @@ -1641,9 +1684,10 @@ select_task_rq_rt(struct task_struct *p, int cpu, in= t flags) * requirement of the task - which is only important on heterogeneous * systems like big.LITTLE. */ - test =3D curr && - unlikely(rt_task(curr)) && - (curr->nr_cpus_allowed < 2 || curr->prio <=3D p->prio); + may_not_preempt =3D !task_may_preempt(curr, cpu); + test =3D (curr && (may_not_preempt || + (unlikely(rt_task(curr)) && + (curr->nr_cpus_allowed < 2 || curr->prio <=3D p->prio)))); =20 if (test || !rt_task_fits_capacity(p, cpu)) { int target =3D find_lowest_rq(p); @@ -1656,11 +1700,14 @@ select_task_rq_rt(struct task_struct *p, int cpu, i= nt flags) goto out_unlock; =20 /* - * Don't bother moving it if the destination CPU is + * If cpu is non-preemptible, prefer remote cpu + * even if it's running a higher-prio task. + * Otherwise: Don't bother moving it if the destination CPU is * not running a lower priority task. */ if (target !=3D -1 && - p->prio < cpu_rq(target)->rt.highest_prio.curr) + (may_not_preempt || + p->prio < cpu_rq(target)->rt.highest_prio.curr)) cpu =3D target; } =20 @@ -1901,11 +1948,11 @@ static int find_lowest_rq(struct task_struct *task) =20 ret =3D cpupri_find_fitness(&task_rq(task)->rd->cpupri, task, lowest_mask, - rt_task_fits_capacity); + rt_task_fits_capacity_and_may_preempt); } else { =20 - ret =3D cpupri_find(&task_rq(task)->rd->cpupri, - task, lowest_mask); + ret =3D cpupri_find_fitness(&task_rq(task)->rd->cpupri, + task, lowest_mask, task_may_preempt); } =20 if (!ret) diff --git a/kernel/softirq.c b/kernel/softirq.c index c8a6913c067d..35ee79dd8786 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -60,6 +60,13 @@ static struct softirq_action softirq_vec[NR_SOFTIRQS] __= cacheline_aligned_in_smp =20 DEFINE_PER_CPU(struct task_struct *, ksoftirqd); =20 +/* + * active_softirqs -- per cpu, a mask of softirqs that are being handled, + * with the expectation that approximate answers are acceptable and theref= ore + * no synchronization. + */ +DEFINE_PER_CPU(u32, active_softirqs); + const char * const softirq_to_name[NR_SOFTIRQS] =3D { "HI", "TIMER", "NET_TX", "NET_RX", "BLOCK", "IRQ_POLL", "TASKLET", "SCHED", "HRTIMER", "RCU" @@ -551,6 +558,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(= void) restart: /* Reset the pending bitmask before enabling irqs */ set_softirq_pending(0); + __this_cpu_write(active_softirqs, pending); =20 local_irq_enable(); =20 @@ -580,6 +588,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(= void) pending >>=3D softirq_bit; } =20 + __this_cpu_write(active_softirqs, 0); if (!IS_ENABLED(CONFIG_PREEMPT_RT) && __this_cpu_read(ksoftirqd) =3D=3D current) rcu_softirq_qs(); --=20 2.37.1.595.g718a3a8f04-goog From nobody Wed Apr 8 07:59:16 2026 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4A979C32774 for ; Mon, 22 Aug 2022 19:05:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237892AbiHVTFW (ORCPT ); Mon, 22 Aug 2022 15:05:22 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58660 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237440AbiHVTFJ (ORCPT ); Mon, 22 Aug 2022 15:05:09 -0400 Received: from mail-pl1-x649.google.com (mail-pl1-x649.google.com [IPv6:2607:f8b0:4864:20::649]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E3765DF84 for ; 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Mon, 22 Aug 2022 12:05:08 -0700 (PDT) Date: Mon, 22 Aug 2022 19:05:01 +0000 In-Reply-To: <20220822190501.2171100-1-jstultz@google.com> Message-Id: <20220822190501.2171100-3-jstultz@google.com> Mime-Version: 1.0 References: <20220822190501.2171100-1-jstultz@google.com> X-Mailer: git-send-email 2.37.1.595.g718a3a8f04-goog Subject: [RFC][PATCH v2 2/2] softirq: defer softirq processing to ksoftirqd if CPU is busy with RT From: John Stultz To: LKML Cc: Pavankumar Kondeti , John Dias , "Connor O'Brien" , Rick Yiu , John Kacur , Qais Yousef , Chris Redpath , Abhijeet Dharmapurikar , Peter Zijlstra , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Thomas Gleixner , kernel-team@android.com, Satya Durga Srinivasu Prabhala , "J . Avila" , John Stultz Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Pavankumar Kondeti Defer the softirq processing to ksoftirqd if a RT task is running or queued on the current CPU. This complements the RT task placement algorithm which tries to find a CPU that is not currently busy with softirqs. Currently NET_TX, NET_RX, BLOCK and TASKLET softirqs are only deferred as they can potentially run for long time. Additionally, this patch stubs out ksoftirqd_running() logic, in the CONFIG_RT_SOFTIRQ_OPTIMIZATION case, as deferring potentially long-running softirqs will cause the logic to not process shorter-running softirqs immediately. By stubbing it out the potentially long running softirqs are deferred, but the shorter running ones can still run immediately. This patch includes folded-in fixes by: Lingutla Chandrasekhar Satya Durga Srinivasu Prabhala J. Avila Cc: John Dias Cc: Connor O'Brien Cc: Rick Yiu Cc: John Kacur Cc: Qais Yousef Cc: Chris Redpath Cc: Abhijeet Dharmapurikar Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Juri Lelli Cc: Vincent Guittot Cc: Dietmar Eggemann Cc: Steven Rostedt Cc: Thomas Gleixner Cc: kernel-team@android.com Signed-off-by: Pavankumar Kondeti [satyap@codeaurora.org: trivial merge conflict resolution.] Signed-off-by: Satya Durga Srinivasu Prabhala [elavila: Port to mainline, squash with bugfix] Signed-off-by: J. Avila [jstultz: Rebase to linus/HEAD, minor rearranging of code, included bug fix Reported-by: Qais Yousef ] Signed-off-by: John Stultz --- include/linux/sched.h | 10 ++++++++++ kernel/sched/cpupri.c | 13 +++++++++++++ kernel/softirq.c | 25 +++++++++++++++++++++++-- 3 files changed, 46 insertions(+), 2 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index e7b2f8a5c711..7f76371cbbb0 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1826,6 +1826,16 @@ current_restore_flags(unsigned long orig_flags, unsi= gned long flags) =20 extern int cpuset_cpumask_can_shrink(const struct cpumask *cur, const stru= ct cpumask *trial); extern int task_can_attach(struct task_struct *p, const struct cpumask *cs= _effective_cpus); + +#ifdef CONFIG_RT_SOFTIRQ_OPTIMIZATION +extern bool cpupri_check_rt(void); +#else +static inline bool cpupri_check_rt(void) +{ + return false; +} +#endif + #ifdef CONFIG_SMP extern void do_set_cpus_allowed(struct task_struct *p, const struct cpumas= k *new_mask); extern int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumas= k *new_mask); diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c index fa9ce9d83683..18dc75d16951 100644 --- a/kernel/sched/cpupri.c +++ b/kernel/sched/cpupri.c @@ -64,6 +64,19 @@ static int convert_prio(int prio) return cpupri; } =20 +#ifdef CONFIG_RT_SOFTIRQ_OPTIMIZATION +/* + * cpupri_check_rt - check if CPU has a RT task + * should be called from rcu-sched read section. + */ +bool cpupri_check_rt(void) +{ + int cpu =3D raw_smp_processor_id(); + + return cpu_rq(cpu)->rd->cpupri.cpu_to_pri[cpu] > CPUPRI_NORMAL; +} +#endif + static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p, struct cpumask *lowest_mask, int idx) { diff --git a/kernel/softirq.c b/kernel/softirq.c index 35ee79dd8786..203a70dc9459 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -87,6 +87,7 @@ static void wakeup_softirqd(void) wake_up_process(tsk); } =20 +#ifndef CONFIG_RT_SOFTIRQ_OPTIMIZATION /* * If ksoftirqd is scheduled, we do not want to process pending softirqs * right now. Let ksoftirqd handle this at its own rate, to get fairness, @@ -101,6 +102,9 @@ static bool ksoftirqd_running(unsigned long pending) return false; return tsk && task_is_running(tsk) && !__kthread_should_park(tsk); } +#else +#define ksoftirqd_running(pending) (false) +#endif /* CONFIG_RT_SOFTIRQ_OPTIMIZATION */ =20 #ifdef CONFIG_TRACE_IRQFLAGS DEFINE_PER_CPU(int, hardirqs_enabled); @@ -532,6 +536,17 @@ static inline bool lockdep_softirq_start(void) { retur= n false; } static inline void lockdep_softirq_end(bool in_hardirq) { } #endif =20 +static __u32 softirq_deferred_for_rt(__u32 *pending) +{ + __u32 deferred =3D 0; + + if (cpupri_check_rt()) { + deferred =3D *pending & LONG_SOFTIRQ_MASK; + *pending &=3D ~LONG_SOFTIRQ_MASK; + } + return deferred; +} + asmlinkage __visible void __softirq_entry __do_softirq(void) { unsigned long end =3D jiffies + MAX_SOFTIRQ_TIME; @@ -539,6 +554,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(= void) int max_restart =3D MAX_SOFTIRQ_RESTART; struct softirq_action *h; bool in_hardirq; + __u32 deferred; __u32 pending; int softirq_bit; =20 @@ -551,13 +567,15 @@ asmlinkage __visible void __softirq_entry __do_softir= q(void) =20 pending =3D local_softirq_pending(); =20 + deferred =3D softirq_deferred_for_rt(&pending); softirq_handle_begin(); + in_hardirq =3D lockdep_softirq_start(); account_softirq_enter(current); =20 restart: /* Reset the pending bitmask before enabling irqs */ - set_softirq_pending(0); + set_softirq_pending(deferred); __this_cpu_write(active_softirqs, pending); =20 local_irq_enable(); @@ -596,13 +614,16 @@ asmlinkage __visible void __softirq_entry __do_softir= q(void) local_irq_disable(); =20 pending =3D local_softirq_pending(); + deferred =3D softirq_deferred_for_rt(&pending); + if (pending) { if (time_before(jiffies, end) && !need_resched() && --max_restart) goto restart; + } =20 + if (pending | deferred) wakeup_softirqd(); - } =20 account_softirq_exit(current); lockdep_softirq_end(in_hardirq); --=20 2.37.1.595.g718a3a8f04-goog