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[82.17.115.212]) by smtp.gmail.com with ESMTPSA id i6-20020a05600c480600b003a31f71c5b8sm23376844wmo.27.2022.08.01.16.43.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Aug 2022 16:43:01 -0700 (PDT) From: Aaron Tomlin To: frederic@kernel.org, mtosatti@redhat.com Cc: cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, atomlin@atomlin.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v5 1/2] mm/vmstat: Use per cpu variable to track a vmstat discrepancy Date: Tue, 2 Aug 2022 00:42:57 +0100 Message-Id: <20220801234258.134609-2-atomlin@redhat.com> X-Mailer: git-send-email 2.37.1 In-Reply-To: <20220801234258.134609-1-atomlin@redhat.com> References: <20220801234258.134609-1-atomlin@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" This patch incorporates an idea from Marcelo's patch [1] where a CPU-specific variable namely vmstat_dirty is used to indicate if a vmstat imbalance is present for a given CPU. Therefore, at the appropriate time, we can fold all the remaining differentials. [1]: https://lore.kernel.org/lkml/20220204173554.763888172@fedora.localdoma= in/ Signed-off-by: Aaron Tomlin --- mm/vmstat.c | 46 +++++++++++++++------------------------------- 1 file changed, 15 insertions(+), 31 deletions(-) diff --git a/mm/vmstat.c b/mm/vmstat.c index 373d2730fcf2..51564b7c85fe 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -195,6 +195,12 @@ void fold_vm_numa_events(void) #endif =20 #ifdef CONFIG_SMP +static DEFINE_PER_CPU_ALIGNED(bool, vmstat_dirty); + +static inline void mark_vmstat_dirty(void) +{ + this_cpu_write(vmstat_dirty, true); +} =20 int calculate_pressure_threshold(struct zone *zone) { @@ -367,6 +373,7 @@ void __mod_zone_page_state(struct zone *zone, enum zone= _stat_item item, x =3D 0; } __this_cpu_write(*p, x); + mark_vmstat_dirty(); =20 if (IS_ENABLED(CONFIG_PREEMPT_RT)) preempt_enable(); @@ -405,6 +412,7 @@ void __mod_node_page_state(struct pglist_data *pgdat, e= num node_stat_item item, x =3D 0; } __this_cpu_write(*p, x); + mark_vmstat_dirty(); =20 if (IS_ENABLED(CONFIG_PREEMPT_RT)) preempt_enable(); @@ -603,6 +611,7 @@ static inline void mod_zone_state(struct zone *zone, =20 if (z) zone_page_state_add(z, zone, item); + mark_vmstat_dirty(); } =20 void mod_zone_page_state(struct zone *zone, enum zone_stat_item item, @@ -671,6 +680,7 @@ static inline void mod_node_state(struct pglist_data *p= gdat, =20 if (z) node_page_state_add(z, pgdat, item); + mark_vmstat_dirty(); } =20 void mod_node_page_state(struct pglist_data *pgdat, enum node_stat_item it= em, @@ -1873,6 +1883,7 @@ int sysctl_stat_interval __read_mostly =3D HZ; static void refresh_vm_stats(struct work_struct *work) { refresh_cpu_vm_stats(true); + this_cpu_write(vmstat_dirty, false); } =20 int vmstat_refresh(struct ctl_table *table, int write, @@ -1937,6 +1948,7 @@ int vmstat_refresh(struct ctl_table *table, int write, static void vmstat_update(struct work_struct *w) { if (refresh_cpu_vm_stats(true)) { + this_cpu_write(vmstat_dirty, false); /* * Counters were updated so we expect more updates * to occur in the future. Keep on running the @@ -1948,35 +1960,6 @@ static void vmstat_update(struct work_struct *w) } } =20 -/* - * Check if the diffs for a certain cpu indicate that - * an update is needed. - */ -static bool need_update(int cpu) -{ - pg_data_t *last_pgdat =3D NULL; - struct zone *zone; - - for_each_populated_zone(zone) { - struct per_cpu_zonestat *pzstats =3D per_cpu_ptr(zone->per_cpu_zonestats= , cpu); - struct per_cpu_nodestat *n; - - /* - * The fast way of checking if there are any vmstat diffs. - */ - if (memchr_inv(pzstats->vm_stat_diff, 0, sizeof(pzstats->vm_stat_diff))) - return true; - - if (last_pgdat =3D=3D zone->zone_pgdat) - continue; - last_pgdat =3D zone->zone_pgdat; - n =3D per_cpu_ptr(zone->zone_pgdat->per_cpu_nodestats, cpu); - if (memchr_inv(n->vm_node_stat_diff, 0, sizeof(n->vm_node_stat_diff))) - return true; - } - return false; -} - /* * Switch off vmstat processing and then fold all the remaining differenti= als * until the diffs stay at zero. The function is used by NOHZ and can only= be @@ -1990,7 +1973,7 @@ void quiet_vmstat(void) if (!delayed_work_pending(this_cpu_ptr(&vmstat_work))) return; =20 - if (!need_update(smp_processor_id())) + if (!__this_cpu_read(vmstat_dirty)) return; =20 /* @@ -2000,6 +1983,7 @@ void quiet_vmstat(void) * vmstat_shepherd will take care about that for us. */ refresh_cpu_vm_stats(false); + __this_cpu_write(vmstat_dirty, false); } =20 /* @@ -2021,7 +2005,7 @@ static void vmstat_shepherd(struct work_struct *w) for_each_online_cpu(cpu) { struct delayed_work *dw =3D &per_cpu(vmstat_work, cpu); =20 - if (!delayed_work_pending(dw) && need_update(cpu)) + if (!delayed_work_pending(dw) && per_cpu(vmstat_dirty, cpu)) queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0); =20 cond_resched(); --=20 2.37.1 From nobody Sun Apr 12 02:53:32 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 7CB30C19F29 for ; Mon, 1 Aug 2022 23:43:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235313AbiHAXnR (ORCPT ); Mon, 1 Aug 2022 19:43:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55084 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235276AbiHAXnK (ORCPT ); Mon, 1 Aug 2022 19:43:10 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 37989474E0 for ; Mon, 1 Aug 2022 16:43:09 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1659397388; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bqrH14f6u4rtPBP7mTrKN6TGeXQ06NPlIMwH+7GumHY=; b=M1vbh2kEGPx/eKvcKlcs8tsHK6Phte6uZjXcVAi9MaPaMP8xVGzg5jjeBQt+a8JeLqRpL2 af/dv5OPbux+dZk4vETJFgOAxoduqRe6yILwjZcp7yTNB1rkYIiEm73OJyb9cKyI4tpTkT zlVtWML1sykvo3ulcDel0BkEQaPpePM= Received: from mail-wm1-f71.google.com (mail-wm1-f71.google.com [209.85.128.71]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-331-JI3ZNjb4NRil0bg_j-Wxqg-1; Mon, 01 Aug 2022 19:43:07 -0400 X-MC-Unique: JI3ZNjb4NRil0bg_j-Wxqg-1 Received: by mail-wm1-f71.google.com with SMTP id 131-20020a1c0289000000b003a3497306a8so7427255wmc.9 for ; Mon, 01 Aug 2022 16:43:06 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc; bh=bqrH14f6u4rtPBP7mTrKN6TGeXQ06NPlIMwH+7GumHY=; b=brU5kn1WamUdHmTC/7r6l6aE+Dk0EUz9i7ZgGYZR+aPJuMpxHg/aIX5qkJcC+N96nG oWnTRXtZNKMZT8m/NBp14i0xbY1ESxX+/7rteHq23eCM6TR/qqqftuMzv+6IkTGnWyFI 11ZL/0jYwV83HMZzfLdIQxbs5C9lHJFsItF3YINU84IOiGChd9fqrhGE5jLTT8TKwpmM E9UnMa7oui0AUCyNBFlmhOmMr0UmobGdgdaJATXMFrhjA9Em2daxwSQroDyeDzTwvnd0 qBgjrJccnWwK2QxgqGrnttqws9IctSG+jbHW+tgHZcFiUnISlEfDilSVWSbmMWiFNseM UIPg== X-Gm-Message-State: ACgBeo3xnugJDZ7NZymrpn4C/IlY2hWHuakkZSr+L+AqrflMbc1Gr8lI avMHEkB8fIktKFAms7Kyk3ybxh3Opz/U+hIFTVrORRR304DED3dZ+e1eGxBgmJCz2rrWNk5CwQa 31/Y6lLVZ3LZ1WQ4ZpWxETCk= X-Received: by 2002:a05:6000:1841:b0:21e:fa04:61f4 with SMTP id c1-20020a056000184100b0021efa0461f4mr11433933wri.491.1659397385872; Mon, 01 Aug 2022 16:43:05 -0700 (PDT) X-Google-Smtp-Source: AA6agR62sTUIxjyKmfQTaQnIyR3CaV/L2KTymYkPMJZ/SjfCdT+1VJLxYfrbdThjN3//o4rzt26j2Q== X-Received: by 2002:a05:6000:1841:b0:21e:fa04:61f4 with SMTP id c1-20020a056000184100b0021efa0461f4mr11433927wri.491.1659397385658; Mon, 01 Aug 2022 16:43:05 -0700 (PDT) Received: from localhost (cpc111743-lutn13-2-0-cust979.9-3.cable.virginm.net. [82.17.115.212]) by smtp.gmail.com with ESMTPSA id p15-20020adfce0f000000b0021ee28ff76esm13335489wrn.38.2022.08.01.16.43.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Aug 2022 16:43:04 -0700 (PDT) From: Aaron Tomlin To: frederic@kernel.org, mtosatti@redhat.com Cc: cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, atomlin@atomlin.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v5 2/2] tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too Date: Tue, 2 Aug 2022 00:42:58 +0100 Message-Id: <20220801234258.134609-3-atomlin@redhat.com> X-Mailer: git-send-email 2.37.1 In-Reply-To: <20220801234258.134609-1-atomlin@redhat.com> References: <20220801234258.134609-1-atomlin@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" In the context of the idle task and an adaptive-tick mode/or a nohz_full CPU, quiet_vmstat() can be called: before stopping the idle tick, entering an idle state and on exit. In particular, for the latter case, when the idle task is required to reschedule, the idle tick can remain stopped and the timer expiration time endless i.e., KTIME_MAX. Now, indeed before a nohz_full CPU enters an idle state, CPU-specific vmstat counters should be processed to ensure the respective values have been reset and folded into the zone specific 'vm_stat[]'. That being said, it can only occur when: the idle tick was previously stopped, and reprogramming of the timer is not required. A customer provided some evidence which indicates that the idle tick was stopped; albeit, CPU-specific vmstat counters still remained populated. Thus one can only assume quiet_vmstat() was not invoked on return to the idle loop. If I understand correctly, I suspect this divergence might erroneously prevent a reclaim attempt by kswapd. If the number of zone specific free pages are below their per-cpu drift value then zone_page_state_snapshot() is used to compute a more accurate view of the aforementioned statistic. Thus any task blocked on the NUMA node specific pfmemalloc_wait queue will be unable to make significant progress via direct reclaim unless it is killed after being woken up by kswapd (see throttle_direct_reclaim()). Consider the following theoretical scenario: 1. CPU Y migrated running task A to CPU X that was in an idle state i.e. waiting for an IRQ - not polling; marked the current task on CPU X to need/or require a reschedule i.e., set TIF_NEED_RESCHED and invoked a reschedule IPI to CPU X (see sched_move_task()) 2. CPU X acknowledged the reschedule IPI from CPU Y; generic idle loop code noticed the TIF_NEED_RESCHED flag against the idle task and attempts to exit of the loop and calls the main scheduler function i.e. __schedule(). Since the idle tick was previously stopped no scheduling-clock tick would occur. So, no deferred timers would be handled 3. Post transition to kernel execution Task A running on CPU Y, indirectly released a few pages (e.g. see __free_one_page()); CPU Y's 'vm_stat_diff[NR_FREE_PAGES]' was updated and zone specific 'vm_stat[]' update was deferred as per the CPU-specific stat threshold 4. Task A does invoke exit(2) and the kernel does remove the task from the run-queue; the idle task was selected to execute next since there are no other runnable tasks assigned to the given CPU (see pick_next_task() and pick_next_task_idle()) 5. On return to the idle loop since the idle tick was already stopped and can remain so (see [1] below) e.g. no pending soft IRQs, no attempt is made to zero and fold CPU Y's vmstat counters since reprogramming of the scheduling-clock tick is not required/or needed (see [2]) ... do_idle { __current_set_polling() tick_nohz_idle_enter() while (!need_resched()) { local_irq_disable() ... /* No polling or broadcast event */ cpuidle_idle_call() { if (cpuidle_not_available(drv, dev)) { tick_nohz_idle_stop_tick() __tick_nohz_idle_stop_tick(this_cpu_ptr(&tick_cpu_sched)) { int cpu =3D smp_processor_id() if (ts->timer_expires_base) expires =3D ts->timer_expires else if (can_stop_idle_tick(cpu, ts)) (1) -------> expires =3D tick_nohz_next_event(ts, cpu) else return ts->idle_calls++ if (expires > 0LL) { tick_nohz_stop_tick(ts, cpu) { if (ts->tick_stopped && (expires =3D=3D ts->next_tick)) { (2) -------> if (tick =3D=3D KTIME_MAX || ts->next_tick = =3D=3D hrtimer_get_expires(&ts->sched_timer)) return } ... } So the idea of with this patch is to ensure refresh_cpu_vm_stats(false) is called, when it is appropriate, on return to the idle loop when the idle tick was previously stopped too. Additionally, in the context of nohz_full, when the scheduling-tick is stopped and before exiting to user-mode, ensure no CPU-specific vmstat differentials remain. Signed-off-by: Aaron Tomlin Reported-by: kernel test robot --- include/linux/tick.h | 9 ++------- kernel/time/tick-sched.c | 19 ++++++++++++++++++- 2 files changed, 20 insertions(+), 8 deletions(-) diff --git a/include/linux/tick.h b/include/linux/tick.h index bfd571f18cfd..4c576c9ca0a2 100644 --- a/include/linux/tick.h +++ b/include/linux/tick.h @@ -11,7 +11,6 @@ #include #include #include -#include =20 #ifdef CONFIG_GENERIC_CLOCKEVENTS extern void __init tick_init(void); @@ -123,6 +122,8 @@ enum tick_dep_bits { #define TICK_DEP_MASK_RCU (1 << TICK_DEP_BIT_RCU) #define TICK_DEP_MASK_RCU_EXP (1 << TICK_DEP_BIT_RCU_EXP) =20 +void tick_nohz_user_enter_prepare(void); + #ifdef CONFIG_NO_HZ_COMMON extern bool tick_nohz_enabled; extern bool tick_nohz_tick_stopped(void); @@ -305,10 +306,4 @@ static inline void tick_nohz_task_switch(void) __tick_nohz_task_switch(); } =20 -static inline void tick_nohz_user_enter_prepare(void) -{ - if (tick_nohz_full_cpu(smp_processor_id())) - rcu_nocb_flush_deferred_wakeup(); -} - #endif diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 30049580cd62..c7c69a974414 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -26,6 +26,7 @@ #include #include #include +#include =20 #include =20 @@ -43,6 +44,20 @@ struct tick_sched *tick_get_tick_sched(int cpu) return &per_cpu(tick_cpu_sched, cpu); } =20 +void tick_nohz_user_enter_prepare(void) +{ + struct tick_sched *ts; + + if (tick_nohz_full_cpu(smp_processor_id())) { + ts =3D this_cpu_ptr(&tick_cpu_sched); + + if (ts->tick_stopped) + quiet_vmstat(); + rcu_nocb_flush_deferred_wakeup(); + } +} +EXPORT_SYMBOL(tick_nohz_user_enter_prepare); + #if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS) /* * The time, when the last jiffy update happened. Write access must hold @@ -890,6 +905,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, = int cpu) ts->do_timer_last =3D 0; } =20 + /* Attempt to fold when the idle tick is stopped or not */ + quiet_vmstat(); + /* Skip reprogram of event if its not changed */ if (ts->tick_stopped && (expires =3D=3D ts->next_tick)) { /* Sanity check: make sure clockevent is actually programmed */ @@ -911,7 +929,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, = int cpu) */ if (!ts->tick_stopped) { calc_load_nohz_start(); - quiet_vmstat(); =20 ts->last_tick =3D hrtimer_get_expires(&ts->sched_timer); ts->tick_stopped =3D 1; --=20 2.37.1