From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 38C211922FB for ; Thu, 2 Oct 2025 16:24:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.176.79.56 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759422264; cv=none; b=OUtN/iDEl/rYMPvIgc02Lw7Q+QqZtMljn2fJsnQJllwBnD3v9ShI2MaEuwr9XoqYitEu9Nav/YbDsPY5Vw/HEFRFExVbbLVsaSOMThaE4uVTSJ5qRIZj0n7Xu7BpuBZ+NPrFN2YPxJClHEHaghmnWUqVrq0C0cHI/dR2UObCKtw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759422264; c=relaxed/simple; bh=01pEibkfcOZbELVbTmkEB60bQvbpEiOAuGOZctfVi4M=; h=Date:From:To:CC:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=Krsz+K4XEZzVAkn3AMP32DdAvV8y2NtcGx/NLmtASMUGjQXbMDDAHUFCPmZIDfGCQEy2V7/AOAB3FesI3s8cKY3X6bI4PXqoeH2mrosexNhpUxCgtH22WI1G/JYdnAUdHH/IQMt3joarQXNNk1U+dpkOwhl27xAtDslUR23MA1Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=185.176.79.56 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.18.186.216]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4ccxrp3rrFz6L50J; Fri, 3 Oct 2025 00:22:02 +0800 (CST) Received: from dubpeml100005.china.huawei.com (unknown [7.214.146.113]) by mail.maildlp.com (Postfix) with ESMTPS id E0E4C1400D9; Fri, 3 Oct 2025 00:24:18 +0800 (CST) Received: from localhost (10.203.177.15) by dubpeml100005.china.huawei.com (7.214.146.113) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Thu, 2 Oct 2025 17:24:17 +0100 Date: Thu, 2 Oct 2025 17:24:15 +0100 From: Jonathan Cameron To: Raghavendra K T CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [RFC PATCH V3 07/17] mm: Add throttling of mm scanning using scan_period Message-ID: <20251002172415.00003140@huawei.com> In-Reply-To: <20250814153307.1553061-8-raghavendra.kt@amd.com> References: <20250814153307.1553061-1-raghavendra.kt@amd.com> <20250814153307.1553061-8-raghavendra.kt@amd.com> X-Mailer: Claws Mail 4.3.0 (GTK 3.24.42; x86_64-w64-mingw32) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: lhrpeml100010.china.huawei.com (7.191.174.197) To dubpeml100005.china.huawei.com (7.214.146.113) On Thu, 14 Aug 2025 15:32:57 +0000 Raghavendra K T wrote: > Before this patch, scanning of tasks' mm is done continuously and also > at the same rate. > > Improve that by adding a throttling logic: > 1) If there were useful pages found during last scan and current scan, > decrease the scan_period (to increase scan rate) by TUNE_PERCENT (15%). > > 2) If there were no useful pages found in last scan, and there are > candidate migration pages in the current scan decrease the scan_period > aggressively by 2 power SCAN_CHANGE_SCALE (2^3 = 8 now). Explain why those values were chosen. > > Vice versa is done for the reverse case. > Scan period is clamped between MIN (600ms) and MAX (5sec). > > Signed-off-by: Raghavendra K T Various minor comments inline. Thanks, Jonathan > --- > mm/kscand.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 109 insertions(+), 1 deletion(-) > > diff --git a/mm/kscand.c b/mm/kscand.c > index 5cd2764114df..843069048c61 100644 > --- a/mm/kscand.c > +++ b/mm/kscand.c > @@ -20,6 +20,7 @@ > #include > #include > #include > +#include > > #include > #include "internal.h" > @@ -33,6 +34,16 @@ static DEFINE_MUTEX(kscand_mutex); > #define KSCAND_SCAN_SIZE (1 * 1024 * 1024 * 1024UL) > static unsigned long kscand_scan_size __read_mostly = KSCAND_SCAN_SIZE; > > +/* > + * Scan period for each mm. > + * Min: 600ms default: 2sec Max: 5sec > + */ > +#define KSCAND_SCAN_PERIOD_MAX 5000U > +#define KSCAND_SCAN_PERIOD_MIN 600U > +#define KSCAND_SCAN_PERIOD 2000U > + > +static unsigned int kscand_mm_scan_period_ms __read_mostly = KSCAND_SCAN_PERIOD; > + > /* How long to pause between two scan cycles */ > static unsigned int kscand_scan_sleep_ms __read_mostly = 20; > > @@ -78,6 +89,11 @@ static struct kmem_cache *kmigrated_slot_cache __read_mostly; > /* Per mm information collected to control VMA scanning */ > struct kscand_mm_slot { > struct mm_slot slot; > + /* Unit: ms. Determines how aften mm scan should happen. */ Name it scan_period_ms and you can drop the unit comment + that info is conveyed everywhere it is used. You do that for the global so why not here as well. > + unsigned int scan_period; > + unsigned long next_scan; > + /* Tracks how many useful pages obtained for migration in the last scan */ > + unsigned long scan_delta; > long address; > bool is_scanned; > }; > @@ -715,13 +731,92 @@ static void kmigrated_migrate_folio(void) > } > } > +/* Maintains stability of scan_period by decaying last time accessed pages */ > +#define SCAN_DECAY_SHIFT 4 > +/* > + * X : Number of useful pages in the last scan. > + * Y : Number of useful pages found in current scan. > + * Tuning scan_period: > + * Initial scan_period is 2s. > + * case 1: (X = 0, Y = 0) > + * Increase scan_period by SCAN_PERIOD_TUNE_PERCENT. > + * case 2: (X = 0, Y > 0) > + * Decrease scan_period by (2 << SCAN_PERIOD_CHANGE_SCALE). > + * case 3: (X > 0, Y = 0 ) > + * Increase scan_period by (2 << SCAN_PERIOD_CHANGE_SCALE). > + * case 4: (X > 0, Y > 0) > + * Decrease scan_period by SCAN_PERIOD_TUNE_PERCENT. > + */ > +static inline void kscand_update_mmslot_info(struct kscand_mm_slot *mm_slot, > + unsigned long total) As below. I'd make total a local variable for now. > +{ > + unsigned int scan_period; > + unsigned long now; > + unsigned long old_scan_delta; Might as well combine these two lines. > + > + scan_period = mm_slot->scan_period; > + old_scan_delta = mm_slot->scan_delta; > + > + /* decay old value */ > + total = (old_scan_delta >> SCAN_DECAY_SHIFT) + total; > + > + /* XXX: Hack to get rid of continuously failing/unmigrateable pages */ > + if (total < KSCAND_IGNORE_SCAN_THR) > + total = 0; > + > + /* > + * case 1: old_scan_delta and new delta are similar, (slow) TUNE_PERCENT used. > + * case 2: old_scan_delta and new delta are different. (fast) CHANGE_SCALE used. > + * TBD: > + * 1. Further tune scan_period based on delta between last and current scan delta. > + * 2. Optimize calculation > + */ > + if (!old_scan_delta && !total) { > + scan_period = (100 + SCAN_PERIOD_TUNE_PERCENT) * scan_period; > + scan_period /= 100; > + } else if (old_scan_delta && total) { > + scan_period = (100 - SCAN_PERIOD_TUNE_PERCENT) * scan_period; > + scan_period /= 100; > + } else if (old_scan_delta && !total) { > + scan_period = scan_period << SCAN_PERIOD_CHANGE_SCALE; > + } else { > + scan_period = scan_period >> SCAN_PERIOD_CHANGE_SCALE; > + } > + > + scan_period = clamp(scan_period, KSCAND_SCAN_PERIOD_MIN, KSCAND_SCAN_PERIOD_MAX); > + > + now = jiffies; > + mm_slot->next_scan = now + msecs_to_jiffies(scan_period); > + mm_slot->scan_period = scan_period; > + mm_slot->scan_delta = total; > +} > + > static unsigned long kscand_scan_mm_slot(void) > { > bool next_mm = false; > bool update_mmslot_info = false; > > + unsigned int mm_slot_scan_period; > + unsigned long now; > + unsigned long mm_slot_next_scan; > unsigned long vma_scanned_size = 0; > unsigned long address; > + unsigned long total = 0; Given this never changes in this patch I'd drop the parameter from kscand_update_mmslot_info() and bring it back when you have code that passes in non 0. > > struct mm_slot *slot; > struct mm_struct *mm; > @@ -746,6 +841,8 @@ static unsigned long kscand_scan_mm_slot(void) > > mm = slot->mm; > mm_slot->is_scanned = true; > + mm_slot_next_scan = mm_slot->next_scan; > + mm_slot_scan_period = mm_slot->scan_period; > spin_unlock(&kscand_mm_lock); > > if (unlikely(!mmap_read_trylock(mm))) > @@ -756,6 +853,11 @@ static unsigned long kscand_scan_mm_slot(void) > goto outerloop; > } > > + now = jiffies; > + > + if (mm_slot_next_scan && time_before(now, mm_slot_next_scan)) If now is only used once, seems better just to put jiffies in the call and drop local variable. > + goto outerloop; > + > VMA_ITERATOR(vmi, mm, address); > > kmigrated_mm_slot = kmigrated_get_mm_slot(mm, false); > @@ -786,8 +888,10 @@ static unsigned long kscand_scan_mm_slot(void) > > update_mmslot_info = true; > > - if (update_mmslot_info) > + if (update_mmslot_info) { > mm_slot->address = address; > + kscand_update_mmslot_info(mm_slot, total); > + } > > outerloop: > /* exit_mmap will destroy ptes after this */ > @@ -889,6 +993,10 @@ void __kscand_enter(struct mm_struct *mm) > return; > > kscand_slot->address = 0; > + kscand_slot->scan_period = kscand_mm_scan_period_ms; > + kscand_slot->next_scan = 0; > + kscand_slot->scan_delta = 0; > + > slot = &kscand_slot->slot; > > spin_lock(&kscand_mm_lock);