From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-188.mta0.migadu.com (out-188.mta0.migadu.com [91.218.175.188]) (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 645BF33DEE1 for ; Thu, 21 May 2026 04:53:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779339184; cv=none; b=FC0NupEncfDSDdLL//KjjUnTk0QUgsGBFtqkEJTIRiSViDwfrme5ZgliosvNWptn06VJaPv/JdBVC+C493rUcjzPp20wmIFy3GDblxLkm15CRL4OUU5YbDY6fpjHOuycgtk9GNnKNlUYQX1M8X+i48TmRHZnOVihD7HX2ZLIQ5s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779339184; c=relaxed/simple; bh=BBIRbu7OFErK/KKz4QUAx+oIApZCly2X+pCMd+7ysNU=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=mhtfcbEDG/Pv1b4ZTJyoSpUTwNTFEQhwtQcBoVwIFjo3OYJFMMmQoboaLqskHbElu/JxgYuE54r/psvK6KHeeHpk0vI4mDz0kJi1Jlhp5WrbmjUz0TyEKBtCA1RAjl1pStDb7dTJOuDBatF5ROL/y1P6ujNt3P1iiJfzcPBTNzU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=nYn1BTHX; arc=none smtp.client-ip=91.218.175.188 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="nYn1BTHX" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779339179; 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=ihrvNaIXgM84SacKpF3s4jv+T6hZY9k+429hTF/wmxo=; b=nYn1BTHXkkqEjDgxljj9bDe0WsfSvO90O7O5eLgE5ADHDmQIk0G1lF9ndyAgb4SBD+BF/0 AWZSrY5kzkRHp1hpEAMdArAoK4YUnIbPi6iUYhVBqHh8mhGwflzQ3Ef9TwvzZumdHX62aM ZKTUcyi7PoolOGHSSVAvyCCdP+DFgtY= From: Jiayuan Chen To: damon@lists.linux.dev Cc: Jiayuan Chen , Jiayuan Chen , SeongJae Park , Andrew Morton , Shu Anzai , Quanmin Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH 1/2] mm/damon/core: split age==0 regions when nr_regions exceeds max/2 Date: Thu, 21 May 2026 12:52:23 +0800 Message-ID: <20260521045236.115749-2-jiayuan.chen@linux.dev> In-Reply-To: <20260521045236.115749-1-jiayuan.chen@linux.dev> References: <20260521045236.115749-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT From: Jiayuan Chen kdamond_split_regions() returns early when nr_regions is above max_nr_regions / 2, leaving internal access variation inside a large region undetected. Such a layout is common with damon-paddr on hugepage workloads or damon-vaddr on processes with a large anonymous mmap. For example, with max_nr_regions == 1500, a target may end up with 799 small alternating-temperature regions plus one large region that absorbed a uniformly-accessed range during an earlier merge: H:hot C:cold r1 r2 r3 r800 HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................| nr_regions = 800 > max_nr_regions / 2 = 750 If a cold subarea later emerges inside r800: r1 r2 r3 r800 HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............| The small regions cannot merge with each other (different access counts), so the budget stays full. r800 cannot be split because nr_regions > max_nr_regions / 2 causes an early return. The cold subarea is never discovered. Split regions whose access pattern has just changed (age == 0) on this path, up to the remaining budget against max_nr_regions. An unnecessary split is reverted by the next kdamond_merge_regions(). Cc: Jiayuan Chen Signed-off-by: Jiayuan Chen --- mm/damon/core.c | 68 ++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 56 insertions(+), 12 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 6b8af7f956b7..442a6c323aeb 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -3452,37 +3452,81 @@ static void damon_split_regions_of(struct damon_ctx *ctx, } /* - * Split every target region into randomly-sized small regions + * Split each region whose access pattern has just changed (age == 0) + * into two, until @budget new regions have been produced or no eligible + * region remains. + */ +static void damon_split_zero_age_regions(struct damon_ctx *ctx, + unsigned long budget) +{ + struct damon_target *t; + struct damon_region *r, *next; + + damon_for_each_target(t, ctx) { + damon_for_each_region_safe(r, next, t) { + unsigned long sz_region, sz_sub; + + if (!budget) + return; + if (r->age != 0) + continue; + sz_region = damon_sz_region(r); + if (sz_region < 2 * ctx->min_region_sz) + continue; + + sz_sub = ALIGN_DOWN(damon_rand(ctx, 1, 10) * + sz_region / 10, ctx->min_region_sz); + /* Do not allow blank region */ + if (sz_sub == 0 || sz_sub >= sz_region) + continue; + + damon_split_region_at(t, r, sz_sub); + budget--; + } + } +} + +/* + * Split target regions to refine the monitoring resolution under + * dynamically changing access patterns. * - * This function splits every target region into random-sized small regions if - * current total number of the regions is equal or smaller than half of the - * user-specified maximum number of regions. This is for maximizing the - * monitoring accuracy under the dynamically changeable access patterns. If a - * split was unnecessarily made, later 'kdamond_merge_regions()' will revert - * it. + * When the total region count leaves room for a blanket doubling + * (nr_regions <= max_nr_regions / 2), every region is randomly split. + * Otherwise, only regions whose access pattern has just changed + * (age == 0) are split, up to the remaining budget against + * max_nr_regions. + * + * Unnecessary splits are reverted by a later kdamond_merge_regions(). */ static void kdamond_split_regions(struct damon_ctx *ctx) { struct damon_target *t; - unsigned int nr_regions = 0; - static unsigned int last_nr_regions; + unsigned long nr_regions = 0; + unsigned long max_nr_regions = ctx->attrs.max_nr_regions; + static unsigned long last_nr_regions; int nr_subregions = 2; damon_for_each_target(t, ctx) nr_regions += damon_nr_regions(t); - if (nr_regions > ctx->attrs.max_nr_regions / 2) - return; + if (nr_regions >= max_nr_regions) + goto done; + + if (nr_regions > max_nr_regions / 2) { + damon_split_zero_age_regions(ctx, max_nr_regions - nr_regions); + goto done; + } /* Maybe the middle of the region has different access frequency */ if (last_nr_regions == nr_regions && - nr_regions < ctx->attrs.max_nr_regions / 3) + nr_regions < max_nr_regions / 3) nr_subregions = 3; damon_for_each_target(t, ctx) damon_split_regions_of(ctx, t, nr_subregions, ctx->min_region_sz); +done: last_nr_regions = nr_regions; } -- 2.43.0