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Fri, 3 Oct 2025 10:35:57 +0100 Date: Fri, 3 Oct 2025 10:35:56 +0100 From: Jonathan Cameron To: Raghavendra K T CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [RFC PATCH V3 08/17] mm: Add throttling of mm scanning using scan_size Message-ID: <20251003103556.00006e3c@huawei.com> In-Reply-To: <20250814153307.1553061-9-raghavendra.kt@amd.com> References: <20250814153307.1553061-1-raghavendra.kt@amd.com> <20250814153307.1553061-9-raghavendra.kt@amd.com> X-Mailer: Claws Mail 4.3.0 (GTK 3.24.42; x86_64-w64-mingw32) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.203.177.15] X-ClientProxiedBy: lhrpeml100011.china.huawei.com (7.191.174.247) To dubpeml100005.china.huawei.com (7.214.146.113) X-Stat-Signature: p835g3rnaeh16buicz36x3tba9dztifp X-Rspam-User: X-Rspamd-Queue-Id: 561F910000A X-Rspamd-Server: rspam04 X-HE-Tag: 1759484162-145692 X-HE-Meta: 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 4AYe9QdL E0HlnuU7lmSNxOAQwXbgFsO2AeQcv5164al11KfzAGFwR9PQCipVkM//Wt2lTiRgGXehZls3aLuzIT5vs6r+w9lffJtLe6+DSBivjw0qAHGckDVAT9E6b3rO9lLnq+wkaICZvPwwuuBNsl2G8IXJ+HMhHNtHGaI6dM+0eiNAKIHa9XZ2vU6W1HEyQEN/Kr9yDvYOFGDpgygsm+HhRSPkUleh54QnWIKvKBLwdwK233FMyCxAF4gsatFNnqkxvCFjfhDv5gxC7vNMAyi5fS0yaJ6gjEDBg921g3ldsDilAM0YM304M+pxW7djukGbE97YpQDi/d7jrNENe4qyMggdrkycw0e/kiOU0gmRRrqAfRFMrDnAwsmrbmVBTjPv1Hs/+uTxOrZ0U1FHuWCH5Uw+JcnElTQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 14 Aug 2025 15:32:58 +0000 Raghavendra K T wrote: > Before this patch, scanning is done on entire virtual address space > of all the tasks. Now the scan size is shrunk or expanded based on the > useful pages found in the last scan. > > This helps to quickly get out of unnecessary scanning thus burning > lesser CPU. > > Drawback: If a useful chunk is at the other end of the VMA space, it > will delay scanning and migration. > > Shrink/expand algorithm for scan_size: > X : Number of useful pages in the last scan. > Y : Number of useful pages found in current scan. > Initial scan_size is 1GB > case 1: (X = 0, Y = 0) > Decrease scan_size by 2 > case 2: (X = 0, Y > 0) > Aggressively change to MAX (4GB) > case 3: (X > 0, Y = 0 ) > No change > case 4: (X > 0, Y > 0) > Increase scan_size by 2 > > Scan size is clamped between MIN (256MB) and MAX (4GB)). > TBD: Tuning based on real workloads Seems like a reasonable thing to do, but as you say tuning data needed to justify how aggressive this should be and those size limits. Trivial stuff inline. > > Signed-off-by: Raghavendra K T > --- > mm/kscand.c | 29 +++++++++++++++++++++++++++++ > 1 file changed, 29 insertions(+) > > diff --git a/mm/kscand.c b/mm/kscand.c > index 843069048c61..39a7fcef7de8 100644 > --- a/mm/kscand.c > +++ b/mm/kscand.c > @@ -28,10 +28,15 @@ > > static struct task_struct *kscand_thread __read_mostly; > static DEFINE_MUTEX(kscand_mutex); > + Push that into earlier patch to cut down on churn / noise. > /* > * Total VMA size to cover during scan. > + * Min: 256MB default: 1GB max: 4GB > */ > +#define KSCAND_SCAN_SIZE_MIN (256 * 1024 * 1024UL) > +#define KSCAND_SCAN_SIZE_MAX (4 * 1024 * 1024 * 1024UL) > #define KSCAND_SCAN_SIZE (1 * 1024 * 1024 * 1024UL) > + Likewise. > static unsigned long kscand_scan_size __read_mostly = KSCAND_SCAN_SIZE; > > /* > @@ -94,6 +99,8 @@ struct kscand_mm_slot { > unsigned long next_scan; > /* Tracks how many useful pages obtained for migration in the last scan */ > unsigned long scan_delta; > + /* Determines how much VMA address space to be covered in the scanning */ > + unsigned long scan_size; > long address; > bool is_scanned; > }; > static inline void kscand_update_mmslot_info(struct kscand_mm_slot *mm_slot, > unsigned long total) > { > unsigned int scan_period; > unsigned long now; > + unsigned long scan_size; Combining a few of these or assigning at declaration will reduce the code size a bit which is always nice to have if it doesn't hurt readability. > unsigned long old_scan_delta; > > + scan_size = mm_slot->scan_size; > scan_period = mm_slot->scan_period; > old_scan_delta = mm_slot->scan_delta;