From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 37706314B9D; Sat, 25 Apr 2026 15:59:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777132797; cv=none; b=uz129guiP4CPgG/Y2peb6PJk0Xqyx39PGGHtLSz7CO2ZfqifANdrxgoahYQikeKuqmzPGeRoZs5yyUrKwacfPuEC7Ywpq/wcLmBvG60YsMezbqOY5Sd8wMPZ8gpp4IwJsdr4gP0asrfQUl32zsUtJEy17LbW/6fO5jeme835+IU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777132797; c=relaxed/simple; bh=DEqdJy/28z3gGC1txmGyukKotmF5okcgnV0GsqgHyVw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=b9ynmORz7hXUjtKhM+llqy+Fsz8xtvnuVZ4uh3TOaamPQrFIoTqyp8A4ZOyVsDejbKQL9JnNiOu/PeRCVS8ulfH8deeTl0YnCpjvA8be9SsxAI7BvB0CQ5D7eaMaTZ3oNypdUzlMpW2icy2xziyxopJX4O814D7C4NqOQ+EhkQw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=i1aHgkr9; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="i1aHgkr9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7988EC2BCB0; Sat, 25 Apr 2026 15:59:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777132796; bh=DEqdJy/28z3gGC1txmGyukKotmF5okcgnV0GsqgHyVw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=i1aHgkr96M93PgifDMLXkU27xgWWi/FWy0PrfWaWaromyXpgy7Jns+UHZghLKuAmN 14QDE8vGqOaWDhnuzV0RJEPAcjYVHlS08O4VmBBBSWKYeVUOATdQdmsxD5rmfAa7rO 6ZeJMzcnyYkD1L+FWtr+J2fcahu+BtvsiLzrjlnfUPRhVCXWTvVhGlGF+8VE1434tZ NBciTL27oBraNsdVaJ5vl8vCRpTjntkInHiROpNH0VH3DNnIda5Wz2gb8/kz1GBA+D b8u8qj6yk/UeyEAL7f3hUhx54rLSaq44M5RXrbhjv6pHa6A3zHcYXaCGwtZ5EdllHg AxovvLoaEjdsg== From: SeongJae Park To: Jiayuan Chen Cc: SeongJae Park , damon@lists.linux.dev, Jiayuan Chen , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] mm/damon: introduce damon_rand_fast() for per-ctx PRNG Date: Sat, 25 Apr 2026 08:59:52 -0700 Message-ID: <20260425155953.89674-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: 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=UTF-8 Content-Transfer-Encoding: 8bit On Sat, 25 Apr 2026 11:36:02 +0800 Jiayuan Chen wrote: > > On 4/24/26 11:11 PM, SeongJae Park wrote: > > On Fri, 24 Apr 2026 10:29:58 +0800 Jiayuan Chen wrote: > > > >> Hello SJ, > >> > >> Thank you for the review. > >> > >> On 4/24/26 9:36 AM, SeongJae Park wrote: > >>> Hello Jiayuan, > >>> > >>> > >>> Thank you for sharing this patch with us! > >>> > >>> On Thu, 23 Apr 2026 20:23:36 +0800 Jiayuan Chen wrote: > >>> > >>>> From: Jiayuan Chen > >>>> > >>>> damon_rand() on the sampling_addr hot path calls get_random_u32_below(), > >>>> which takes a local_lock_irqsave() around a per-CPU batched entropy pool > >>>> and periodically refills it with ChaCha20. On workloads with large > >>>> nr_regions (20k+), this shows up as a large fraction of kdamond CPU > >>>> time: the lock_acquire / local_lock pair plus __get_random_u32_below() > >>>> dominate perf profiles. > >>> Could you please share more details about the use case? I'm particularly > >>> curious how you ended up setting 'nr_regiions' that high, while the upper limit > >>> of nr_regions is set to 1,0000 by default. > >> We use DAMON paddr on a 2 TiB host for per-cgroup hot/cold page > >> classification.  Target cgroups can be as small as 1-2% of total > >> memory (tens of GiB). > > Thank you for sharing this detail! > > > > Could you further share me what do you do with the hot/cold information? We > > found some people want to use DAMON for proactive cold pages reclamation but > > they watned to use it for only file-backed pages, and therefore developed page > > level DAMOS filter. In a similar way, if we know your detailed use case, we > > might be able to serve you better by finding not well advertised features, or > > developing a new features. > > > > Hello SJ, > > Thanks. > > Use case: mixed-workload Kubernetes host (latency-sensitive + batch > pods on the same machine).  We need continuous per-pod cold/hot > signal for two things -- k8s scheduling decisions (which pod can > give up memory under host pressure) and sizing the eventual > reclamation through memory.reclaim.  Both anon and file pages > matter (swap is configured); reclamation itself goes through > mainline.  What we add is the per-pod cold attribution feeding the > scheduler. Thank you for sharing this detail! > > DAMON_RECLAIM alone doesn't fit -- it is reclaim-only with > aggregate stats, while we need continuous per-pod visibility even > when reclamation isn't firing. > > Let me re-frame the region-size point, since I don't think I > explained it well last time.  We are NOT trying to make region > size match cgroup size; you're right that pages are scattered and > that mapping doesn't work.  The high nr_regions is so that DAMON's > adaptive split has enough resolution to identify cold sub-areas of > physical memory at all -- regardless of cgroup. Thank you, this helps me betetr understand your concern. > With ~2 GiB > default regions on a 2 TiB host, a small pod's pages are averaged > with thousands of non-pod pages in the same region, and the > region never reaches nr_accesses=0 even when the pod is genuinely > idle. But, the adaptive regions adjustment mechanism dynamically change size of regions, down to 4 KiB. If the small pod's page is really cold while its surrounding pages are not, DAMON should down-size the region to capture only the page and show you nr_accesses=0. > The cold signal is gone before any cgroup attribution > happens.  Cgroup attribution itself is done at sample granularity > (folio_memcg per sampled page), not at region granularity -- the > regions just need to be fine enough that there *is* a cold signal > to attribute. Could you please share more details about what is the cgroup attribution, and how it is done? I guess that is the way to map DAMON's monitoring regions to each cgroup to determine if each cgroup is hot or cold. I'm unsure how it is really be done. > > >> With the default max_nr_regions=1000, each region covers ~2 GiB on > >> average, which is often larger than the entire target cgroup. > >> Adaptive split cannot carve out cgroup-homogeneous regions in that > >> regime: each region's nr_accesses averages the (small) cgroup > >> fraction with the surrounding non-cgroup pages, and the cgroup's > >> access signal gets washed out. > > But, pages of cgroups would be scattered around on the physical address space. > > So even if DAMON finds a hot or cold region on physical address space that > > having a size smaller than a cgroup's memory, it is hard to say if it is for a > > specific cgroup. How do you know if the region is for which cgroup? Do you > > have a way to make sure pages of same cgroup are gathered on the physical > > address space? If not, I'm not sure how ensuring size of region to be equal or > > smaller than each cgroup size helps you. > > > > We are supporting page level monitoring [1] for such cgroup-aware use case. > > Are you using it? But again, if you use it, I'm not sure why you need to > > ensure DAMON region size smaller than the cgroup size. > This is also why DAMOS memcg filters do not bypass the > nr_regions constraint -- the scheme's access-pattern matcher > operates on the already-averaged region, so by the time any > filter sees pages the signal is already lost.  Whatever > attribution mechanism we use afterwards (custom callback, memcg > filter, page-level reporting), the region count needs to be high > enough first. But, I'm not understanding why the regions adjustment mechanism cannot work here. Your answer to my above question will be helpful. > > Fyi, I'm also working [2] on making the cgroup-aware monitoring much more > > lightweight. > > > Looking forward to it -- hopefully it'll cover our case. 😁 I hope so, too! :) > > > >> Raising max_nr_regions to 10k-20k gives the adaptive split enough > >> spatial resolution that cgroup-majority regions can form at cgroup > >> boundaries (allocations have enough physical locality in practice for > >> this to work -- THP, per-node allocation, etc.).  At that region > >> count, damon_rand() starts showing up at the top of kdamond profiles, > >> which is what motivated this patch. > > Thank you for sharing how this patch has developed. I'm still curious if there > > is yet more ways to make DAMON better for your use case. But this patch makes > > sense in general to me. > > > >> > >>> I know some people worry if the limit is too low and it could result in poor > >>> monitoring accuracy. Therefore we developed [1] monitoring intervals > >>> auto-tuning. From multiple tests on real environment it showed somewhat > >>> convincing results, and therefore I nowadays suggest DAMON users to try that if > >>> they didn't try. > >>> > >>> I'm bit concerned if this is over-engineering. It would be helpful to know if > >>> it is, if you could share the more detailed use case. > >> > >> Thanks for pointing to intervals auto-tuning - we do use it.  But it > >> > >> trades sampling frequency against monitoring overhead; it cannot > >> change the spatial resolution.  With N=1000 regions on a 2 TiB host, > >> a 20 GiB cgroup cannot be resolved no matter how we tune sample_us / > >> aggr_us, because the region boundary itself averages cgroup and > >> > >> non-cgroup pages together. > >> > >> So raising max_nr_regions and making the per-region overhead cheap > >> are complementary to interval auto-tuning, not redundant with it. > > Makes sense. I'm still not sure how it helps for cgroup. But, if you do want > > it and if it helps you, you are of course ok to use it in your way, and I will > > help you. :) > > > [...] > >> Sashiko is correct that (u32)(r - l) truncates spans greater than 4 GiB. > >> > >> This is a pre-existing limitation of damon_rand() itself, which > >> passes r - l to get_random_u32_below() (a u32 parameter) and > >> truncates the same way.  My patch makes that truncation > >> explicit but does not introduce a new bug. > > You are 100% correct and I agree. Thank you for making this clear. I should > > also mentioned and underlined this in my previous reply, but I forgot it. > > > > So I think this patch is ok to be merged as is (after addressing my nit trivial > > comments about coding styles), but we may still want to fix it in future. So I > damon_rand() is now only called by damon_split_regions_of() with > the constant range (1, 10).  may by we can rename it to > damon_rand_u32() to make the u32 constraint explicit in the API > name; that closes out the truncation concern at the legacy helper > without needing a separate series. Good point. I'm wondering if we have a reason to keep using damon_rand() at all. I find no such reason. If you also find no real reason, how about simply removing existing damon_rand() and renaming damon_rand_fast() to damon_rand()? > > was wondering if such future fix for this new, shiny and efficient function is > > easy. > > > Will do. > > v2 is a single patch with the style fixes (drop 'likely', 'r' on the > first line for paddr.c, ctx at the end for > > vaddr.c) and 64-bit handling: > >       static inline unsigned long damon_rand_fast(struct damon_ctx *ctx, >                       unsigned long l, unsigned long r) >       { >               unsigned long span = r - l; >               u64 rnd; > >               if (span <= U32_MAX) { >                       rnd = prandom_u32_state(&ctx->rnd_state); >                       return l + (unsigned long)((rnd * span) >> 32); >               } >               rnd = ((u64)prandom_u32_state(&ctx->rnd_state) << 32) | >                     prandom_u32_state(&ctx->rnd_state); >               return l + mul_u64_u64_shr(rnd, span, 64); >       } > > I will also drop Prefetch patch. Sounds good, looking forward to v2! Thanks, SJ [...]