From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta1.migadu.com (out-189.mta1.migadu.com [95.215.58.189]) (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 C9C763E835F for ; Fri, 26 Jun 2026 08:59:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782464398; cv=none; b=KI8CUQ0PE3tE0ldxQNt+g1P8LhN9QmcNjOGFrK+euntXza28myjr6+NLa7HhaloQQNJPWQVMsFoEz7Iv4Co4cHn5keDREUpn3DMHJm8233tigK6yZ1fwqgTu8cjrpSXx/Bw6TSPQ2w2UwOBNa8GlYTN4GTQWF8U5B21xIXZSrG8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782464398; c=relaxed/simple; bh=SPCcEpGfkluEqaIPY1rO0mYTGBVaaXe+Br7oVTeZiFQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=QPe1sWUNNDc6rbUQLWSO0RAzP716JLhkHOxSSPOcnpgvggAlOO//GJar5Dna823Y/N6CvYJGAPH5HXtup9w0+S+gE4/SwsLlfmr2V5TY6iXPwogAKKiyaR7qUk0kzrovlmYnIgbuOlO4X+z2PmiQaJOEDFVEwin4CXEShNoxmjs= 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=TWTPuoHN; arc=none smtp.client-ip=95.215.58.189 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="TWTPuoHN" 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=1782464393; 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=o/AKESdZLDH+aWzHoqnqaRdmen2KGUt9vPtou3JWZ00=; b=TWTPuoHNMpUrutD9Nj5/mAVZStDcfAmZBTehgtlcu4Pw1KjmDMv6X8cIENgSdluXtb2DiO rDcUEbC/8L/P7nXqzUCERKrX1CMhLuZ9eCdUtfE2U6eDRoiN4Arr85sfdy+2e/dFL205// v6ZGeIiQGRf1NGv6mLwnvxX3zOCHK3w= From: Jiayuan Chen To: damon@lists.linux.dev Cc: jiayuan.chen@shopee.com, Jiayuan Chen , SeongJae Park , Andrew Morton , Shu Anzai , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH v2 1/2] mm/damon/core: split a fraction of regions when nr_regions exceeds max/2 Date: Fri, 26 Jun 2026 16:58:37 +0800 Message-ID: <20260626085851.70754-2-jiayuan.chen@linux.dev> In-Reply-To: <20260626085851.70754-1-jiayuan.chen@linux.dev> References: <20260626085851.70754-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org 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. When nr_regions is above max_nr_regions / 2 but still under the maximum, split only a fraction of the regions instead of returning. One region in every 'max_nr_regions / budget' regions is split, where budget is the remaining room (max_nr_regions - nr_regions), starting from a rotating offset so different regions get picked over time. The fraction shrinks as the budget shrinks, so the region count keeps refining while approaching max_nr_regions smoothly rather than overshooting it. An unnecessary split is reverted by the next kdamond_merge_regions(). Cc: Jiayuan Chen Signed-off-by: Jiayuan Chen --- mm/damon/core.c | 49 ++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 44 insertions(+), 5 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index d0cbfb6d1e4c..26a2cdb935f5 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -3712,6 +3712,37 @@ static void damon_split_regions_of(struct damon_ctx *ctx, } } +/* Split one in every @split_step regions into two, from a rotating offset */ +static void damon_split_some_regions(struct damon_ctx *ctx, + unsigned long split_step) +{ + static unsigned long rotation; + struct damon_target *t; + struct damon_region *r, *next; + unsigned long offset = rotation++ % split_step; + unsigned long idx = 0; + + damon_for_each_target(t, ctx) { + damon_for_each_region_safe(r, next, t) { + unsigned long sz_region, sz_sub; + + if (idx++ % split_step != offset) + 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); + } + } +} + /* * Split every target region into randomly-sized small regions * @@ -3725,25 +3756,33 @@ static void damon_split_regions_of(struct damon_ctx *ctx, 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_some_regions(ctx, + max_nr_regions / (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