From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 51B55442B15 for ; Tue, 15 Sep 2026 07:44:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789458243; cv=none; b=m8coBJcdxBu3B1uYUDTtpv9NDL6Q2/aLd8jvsXwqFT8WrAJMQqE1SLm6TAlqBn9Fcc1REeV9WVX9hrscbwastEqd3lw+GD0DhxkdN71ZREwHAkXIHfKaj28dTo0LzYAPh2kna7v/h+riMVyBRIiN9frbdtv9SHIfO2GK9o6YGwA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789458243; c=relaxed/simple; bh=1cuSGOQ8+2+u1lYjudnVqpZ5hI2Gp0gr+Hn4Pp0ngX0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Cpw//MdMm7uycQ01Pmaidge8isgYKbM8f8md8WAOKpVUs43Ysi2K3OYLhjP3pc1DaBYOLNothvqqhctDq+GvsEujfELC97feZlrekpVrcBHbkrM5mN7F8wWTcgRiZSg+dbbcfm0Z+/fwYsxhPuet4G34weI4tc65+c/rgmgC32w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hjzh+yDj; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hjzh+yDj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AE3EE1F000FF; Tue, 15 Sep 2026 07:43:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789458241; bh=a1sJzvgOgJNCxfS0oOMKOm7WAbUXXUX12nNClSaMftY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=hjzh+yDj79TLeMx7ShM85hvPYiywM9kcZkYof4XTdh4b99k0T1ojSwF3T56j0s2aL sLU2fx5xNHP62zrgDTlOa7FVZhD9PaOklfQlYWekCnK6Qin6+QjxBQE0XQ6aJxUYnC 7UnLfoxPoNTw1H62XE+SyxEF7MsZY8DSX94Z7Reh0SY0tABTtQDgouRBroBre8173s eWhE+t+GQhncz4UNo/GOp8+rWBy8RDjJmHLZCigdwP5qr1Q5jqefMmp7vvFkXVj246 GPtBju1qfQde0MEOm7AiU+FZCl61I0IR2GdFkDkWxSlw3YUpdWp4+qbh3Tbc+DJ0CW Ji+u4rWP9RMmg== Message-ID: <819fbc70-4c6b-4202-ada0-c3ef8ace1408@kernel.org> Date: Tue, 15 Sep 2026 09:43:58 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 2/2] mm/slub: introduce slab parking to reduce list_lock contention Content-Language: en-US To: Hao Li , Pedro Falcato Cc: harry@kernel.org, akpm@linux-foundation.org, cl@gentwo.org, rientjes@google.com, roman.gushchin@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260824122004.3652-1-hao.li@linux.dev> <20260824122513.3829-1-hao.li@linux.dev> <20260824122513.3829-2-hao.li@linux.dev> <8d73f087-42e2-454c-8e5f-93c1b64cb969@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/11/26 15:06, Hao Li wrote: > On Mon, Sep 07, 2026 at 03:38:22PM +0200, Vlastimil Babka (SUSE) wrote: >> On 8/24/26 14:25, Hao Li wrote: >> > Introduce a mechanism called parking to mitigate lock contention in the >> > free slowpath. >> >> Interesting! > > Thanks! > >> >> > In the free slowpath, when __slab_free() transitions a full slab into a >> > partial/empty slab through a free operation, it must acquire the list >> > lock to add these newly freed partial/empty slabs to the partial list. >> > >> > Why must partial and empty slabs converted from full slabs be added to >> > the partial list? Because only by doing so can the sheaf refill or alloc >> > slowpath see these partial slabs and allocate from them. Therefore, the >> > list insertion must be performed, which requires acquiring the lock and >> > leads to heavy lock contention under high concurrency. >> > >> > Analysis of profiling data from the will-it-scale mmap1 benchmark shows >> > that full -> partial transitions account for a large proportion, second >> > only to partial -> partial. >> > >> > With extra instrumentation added to __slab_free(), the following data >> > was collected for the maple_node cache (in counts): >> > >> > partial->partial 843017414 >> > full->partial 550719384 >> > partial->empty 17459564 >> > full->empty 2 >> >> That's a lot inded. I'd be careful if it's some specific aspect of the test, >> i.e. lots of parallel allocations followed by lots of frees, that wouldn't >> be that common in realistic workloads. But worth looking into at least. >> If it's this kind of pathologic behavior, then I expect changing sheaf size >> as suggested by Pedro wouldn't help much. > > Thanks for pointing this out. I looked into this bursty alloc-and-free behavior > a bit deeper and ran some further experiments. > > The core question we want to answer is: why does such a massive volume of > object allocations and frees fall straight through to the node partial list > layer, rather than being caught and handled at the barn/sheaf layer? In SLUB's > current design, the per-CPU main/spare sheaves act as the L1 cache, the barn as > L2, and the node partial list as L3. For the mmap1 benchmark (which heavily > stresses the maple tree), the allocation path uses kmem_cache_prefill_sheaf() > rather than the generic allocation APIs, and the frees go through kfree_rcu(). > > Then, here is what happens during allocation: kmem_cache_prefill_sheaf() > normally borrows the spare sheaf directly. If the sheaf holds fewer objects > than requested, it refills it to capacity from the node partial list layer and > this completely bypasses the barn layer. Once the maple tree finishes > allocating a batch of objects, it returns the sheaf back to pcs->spare via > kmem_cache_return_sheaf(). So in essence, this prefill path is just funneling > objects directly from the node partial list into the maple tree through the > spare sheaf. It skips the barn layer. > > Then on the free side: these objects are freed via kfree_rcu, and then > rcu_free_sheaf() checks if there is still room on the barn's full list. But > since the allocation path never actually pulled from the barn, the full list > stays permanently saturated. As a result, rcu_free_sheaf() always falls back to > sheaf_flush_unused(), flushing objects straight into the node partial list > layer. It skips the barn layer too. > > So looking at this behavior, the benchmark does seem to reveal a gap in this > allocation path, where a huge amount of traffic ends up bypassing the barn > layer entirely. Great find! Indeed that's a big gap for prefilled sheaf users, doh. > To see if we can address this, I draft an experimental patch. It introduces a > new field, barn->sheaf_partial, which is a single sheaf rather than a list. > > [The patch code is included at the end of this email.] > > Whenever kmem_cache_prefill_sheaf() runs, it detaches pcs->spare and checks > whether it holds enough objects for the request. > > If so, it returns it right away as in the original code. > > If not, call __prefill_sheaf_pfmemalloc() and then go into > barn_replace_partial_sheaf() to swap the non-full spare sheaf with a full sheaf > from the barn. The full sheaf is handed to the caller, while the non-full sheaf > is temporarily stashed into barn->sheaf_partial. This largely avoids falling > back to the node partial list. If barn->sheaf_partial already has a sheaf, we > merge them together, and any resulting full or empty sheaves are placed back > into the barn accordingly. Makes sense to me! > The key idea here is simply to let __prefill_sheaf_pfmemalloc() pull a sheaf > from the barn's full list, which makes room on the list for future > rcu_free_sheaf() calls. > > Here are the numbers with just this experimental patch applied (without the > parking patch): > > baseline: 28779879 > after experimental patch: 35550211 (+23.5%) > > metric before after delta change > ============================================================================================= > aliases 0 0 0 +0.00% > align 256 256 0 +0.00% > alloc_fastpath 23,259 59,287 36,028 +154.90% > alloc_node_mismatch 0 0 0 +0.00% > alloc_slab 10,171,378 4,796,346 -5,375,032 -52.84% > alloc_slowpath 0 0 0 +0.00% > barn_get 441 193,807,528 193,807,087 +43947185.26% > barn_get_fail 0 377 377 new > barn_put 441 181,694,607 181,694,166 +41200491.16% > barn_put_fail 272,868,220 156,335,632 -116,532,588 -42.71% > cache_dma 0 0 0 +0.00% > cmpxchg_double_fail 744,975 357,255 -387,720 -52.04% > cpu_partial 0 0 0 +0.00% > cpu_slabs 0 0 0 +0.00% > destroy_by_rcu 0 0 0 +0.00% > free_add_partial 337,390,126 162,178,648 -175,211,478 -51.93% > free_fastpath 5,204 14,081 8,877 +170.58% > free_rcu_sheaf 8,731,794,372 10,816,953,777 2,085,159,405 +23.88% > free_rcu_sheaf_fail 0 0 0 +0.00% > free_remove_partial 10,170,229 4,794,785 -5,375,444 -52.85% > free_slab 10,170,229 4,794,785 -5,375,444 -52.85% > free_slowpath 18,697,056 11,563,127 -7,133,929 -38.16% > hwcache_align 0 0 0 +0.00% > min_partial 5 5 0 +0.00% > object_size 256 256 0 +0.00% > objects 14,774 14,596 -178 -1.20% > objects_partial 14,774 14,596 -178 -1.20% > objs_per_slab 64 64 0 +0.00% > order 2 2 0 +0.00% > order_fallback 0 0 0 +0.00% > partial 1,913 2,544 631 +32.98% > poison 0 0 0 +0.00% > reclaim_account 0 0 0 +0.00% > red_zone 0 0 0 +0.00% > remote_node_defrag_ratio 100 100 0 +0.00% > sanity_checks 0 0 0 +0.00% > sheaf_alloc 145,870,437 151,536,660 5,666,223 +3.88% > sheaf_capacity 32 32 0 +0.00% > sheaf_flush 8,731,787,311 5,002,740,743 -3,729,046,568 -42.71% > sheaf_free 145,870,431 151,536,655 5,666,224 +3.88% > sheaf_prefill_fast 3,500,187,266 4,331,383,393 831,196,127 +23.75% > sheaf_prefill_oversize 0 0 0 +0.00% > sheaf_prefill_slow 322 646 324 +100.62% > sheaf_refill 8,750,484,664 5,014,304,944 -3,736,179,720 -42.70% > sheaf_return_fast 3,500,187,348 4,331,383,596 831,196,248 +23.75% > sheaf_return_slow 240 443 203 +84.58% > slab_size 256 256 0 +0.00% > slabs 1,913 2,544 631 +32.98% > slabs_cpu_partial 0 0 0 +0.00% > store_user 0 0 0 +0.00% > total_objects 122,432 162,816 40,384 +32.98% > trace 0 0 0 +0.00% > usersize 0 0 0 +0.00% > > derived before after change > ============================================================================================= > page allocator churn (alloc_slab + free_slab) 20,341,607 9,591,131 -52.85% Very nice! > > As we can see from the data, barn_get and barn_put spike significantly, which > shows a large part of the traffic is redirected into the barn. This eases slab > alloc/free churn and cuts page allocator allocations/frees by 52.85%. > > Additionally, NUMA performance also seems to see some improvement. Under the > maple tree benchmark, the free_slowpath metric likely reflects objects that > enter add_ptr_to_bulk_krc_lock() due to nid mismatches and are eventually freed > via kfree_bulk(). This metric also shows a noticeable drop. > > Metrics like alloc_fastpath did improve, but their absolute numbers are small > and likely unrelated to the maple tree test. > > The tradeoff is a increase in slab fragmentation, with total_objects and slabs > growing by 32.98%. I suspect this happens because as more traffic gets routed > to the barn layer, objects end up being more scattered, which ends up pinning > more slabs. Maybe it's partially also due to the fact that the test can run faster (as we discussed earlier), thus have e.g. more kfree_rcu() objects in flight (free_rcu_sheaf above increased a lot), etc. So I wouldn't worry too much. > For comparison: the parking mechanism reduces lock contention at the node > partial list layer, while this experimental patch absorb the traffic earlier at > the barn layer. They are independent in mechanism. Interestingly, both > approaches deliver very comparable performance improvements. A bit Great. > frustratingly, combining the two only squeezes out an extra ~1% gain, I'm still > investigating why that is. I don't think it would be bad if this change rendered the parking approach unnecessary. I suspect Pedro would be very happy :) > Phew, that turned out to be quite a long write-up! Thanks for that :) > All in all, I feel we could probably focus on evaluating and pursuing this > experimental patch first. For maple tree performance specifically, it seems > like it might be the better fit compared to the parking mechanism (which is > probably better suited for generic allocation pressure outside of maple tree). Agreed! I'd try to look at the code ASAP. For now we can probably... eh... park the parking patch :) and its possible improvements. Thanks again!