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([2a02:6b67:d751:7400:c2b:f323:d172:e42a]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43186c1eccesm24571635e9.46.2024.10.23.12.47.00 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Oct 2024 12:47:01 -0700 (PDT) Message-ID: Date: Wed, 23 Oct 2024 20:47:00 +0100 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 0/4] mm: zswap: add support for zswapin of large folios To: Barry Song <21cnbao@gmail.com> Cc: Yosry Ahmed , senozhatsky@chromium.org, minchan@kernel.org, hanchuanhua@oppo.com, v-songbaohua@oppo.com, akpm@linux-foundation.org, linux-mm@kvack.org, hannes@cmpxchg.org, david@redhat.com, willy@infradead.org, kanchana.p.sridhar@intel.com, nphamcs@gmail.com, chengming.zhou@linux.dev, ryan.roberts@arm.com, ying.huang@intel.com, riel@surriel.com, shakeel.butt@linux.dev, kernel-team@meta.com, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org References: <20241018105026.2521366-1-usamaarif642@gmail.com> <5313c721-9cf1-4ecd-ac23-1eeddabd691f@gmail.com> <4c30cc30-0f7c-4ca7-a933-c8edfadaee5c@gmail.com> <7a14c332-3001-4b9a-ada3-f4d6799be555@gmail.com> <3dca2498-363c-4ba5-a7e6-80c5e5532db5@gmail.com> Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 23/10/2024 19:52, Barry Song wrote: > On Thu, Oct 24, 2024 at 7:31 AM Usama Arif wrote: >> >> >> >> On 23/10/2024 19:02, Yosry Ahmed wrote: >>> [..] >>>>>> I suspect the regression occurs because you're running an edge case >>>>>> where the memory cgroup stays nearly full most of the time (this isn't >>>>>> an inherent issue with large folio swap-in). As a result, swapping in >>>>>> mTHP quickly triggers a memcg overflow, causing a swap-out. The >>>>>> next swap-in then recreates the overflow, leading to a repeating >>>>>> cycle. >>>>>> >>>>> >>>>> Yes, agreed! Looking at the swap counters, I think this is what is going >>>>> on as well. >>>>> >>>>>> We need a way to stop the cup from repeatedly filling to the brim and >>>>>> overflowing. While not a definitive fix, the following change might help >>>>>> improve the situation: >>>>>> >>>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>>>>> >>>>>> index 17af08367c68..f2fa0eeb2d9a 100644 >>>>>> --- a/mm/memcontrol.c >>>>>> +++ b/mm/memcontrol.c >>>>>> >>>>>> @@ -4559,7 +4559,10 @@ int mem_cgroup_swapin_charge_folio(struct folio >>>>>> *folio, struct mm_struct *mm, >>>>>> memcg = get_mem_cgroup_from_mm(mm); >>>>>> rcu_read_unlock(); >>>>>> >>>>>> - ret = charge_memcg(folio, memcg, gfp); >>>>>> + if (folio_test_large(folio) && mem_cgroup_margin(memcg) < >>>>>> MEMCG_CHARGE_BATCH) >>>>>> + ret = -ENOMEM; >>>>>> + else >>>>>> + ret = charge_memcg(folio, memcg, gfp); >>>>>> >>>>>> css_put(&memcg->css); >>>>>> return ret; >>>>>> } >>>>>> >>>>> >>>>> The diff makes sense to me. Let me test later today and get back to you. >>>>> >>>>> Thanks! >>>>> >>>>>> Please confirm if it makes the kernel build with memcg limitation >>>>>> faster. If so, let's >>>>>> work together to figure out an official patch :-) The above code hasn't consider >>>>>> the parent memcg's overflow, so not an ideal fix. >>>>>> >>>> >>>> Thanks Barry, I think this fixes the regression, and even gives an improvement! >>>> I think the below might be better to do: >>>> >>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>>> index c098fd7f5c5e..0a1ec55cc079 100644 >>>> --- a/mm/memcontrol.c >>>> +++ b/mm/memcontrol.c >>>> @@ -4550,7 +4550,11 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm, >>>> memcg = get_mem_cgroup_from_mm(mm); >>>> rcu_read_unlock(); >>>> >>>> - ret = charge_memcg(folio, memcg, gfp); >>>> + if (folio_test_large(folio) && >>>> + mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH, folio_nr_pages(folio))) >>>> + ret = -ENOMEM; >>>> + else >>>> + ret = charge_memcg(folio, memcg, gfp); >>>> >>>> css_put(&memcg->css); >>>> return ret; >>>> >>>> >>>> AMD 16K+32K THP=always >>>> metric mm-unstable mm-unstable + large folio zswapin series mm-unstable + large folio zswapin + no swap thrashing fix >>>> real 1m23.038s 1m23.050s 1m22.704s >>>> user 53m57.210s 53m53.437s 53m52.577s >>>> sys 7m24.592s 7m48.843s 7m22.519s >>>> zswpin 612070 999244 815934 >>>> zswpout 2226403 2347979 2054980 >>>> pgfault 20667366 20481728 20478690 >>>> pgmajfault 385887 269117 309702 >>>> >>>> AMD 16K+32K+64K THP=always >>>> metric mm-unstable mm-unstable + large folio zswapin series mm-unstable + large folio zswapin + no swap thrashing fix >>>> real 1m22.975s 1m23.266s 1m22.549s >>>> user 53m51.302s 53m51.069s 53m46.471s >>>> sys 7m40.168s 7m57.104s 7m25.012s >>>> zswpin 676492 1258573 1225703 >>>> zswpout 2449839 2714767 2899178 >>>> pgfault 17540746 17296555 17234663 >>>> pgmajfault 429629 307495 287859 >>>> >>> >>> Thanks Usama and Barry for looking into this. It seems like this would >>> fix a regression with large folio swapin regardless of zswap. Can the >>> same result be reproduced on zram without this series? >> >> >> Yes, its a regression in large folio swapin support regardless of zswap/zram. >> >> Need to do 3 tests, one with probably the below diff to remove large folio support, >> one with current upstream and one with upstream + swap thrashing fix. >> >> We only use zswap and dont have a zram setup (and I am a bit lazy to create one :)). >> Any zram volunteers to try this? > > Hi Usama, > > I tried a quick experiment: > > echo 1 > /sys/module/zswap/parameters/enabled > echo 0 > /sys/module/zswap/parameters/enabled > > This was to test the zRAM scenario. Enabling zswap even > once disables mTHP swap-in. :) > > I noticed a similar regression with zRAM alone, but the change resolved > the issue and even sped up the kernel build compared to the setup without > mTHP swap-in. Thanks for trying, this is amazing! > > However, I’m still working on a proper patch to address this. The current > approach: > > mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH, folio_nr_pages(folio)) > > isn’t sufficient, as it doesn’t cover cases where group A contains group B, and > we’re operating within group B. The problem occurs not at the boundary of > group B but at the boundary of group A. I am not sure I completely followed this. As MEMCG_CHARGE_BATCH=64, if we are trying to swapin a 16kB page, we basically check if atleast 64/4 = 16 folios can be charged to cgroup, which is reasonable. If we try to swapin a 1M folio, we just check if we can charge atleast 1 folio. Are you saying that checking just 1 folio is not enough in this case and can still cause thrashing, i.e we should check more? If we want to maintain consitency for all folios another option is mem_cgroup_margin(memcg) < MEMCG_CHARGE_BATCH * folio_nr_pages(folio) but I think this is too extreme, we would be checking if 64M can be charged to cgroup just to swapin 1M. > > I believe there’s still room for improvement. For example, if a 64KB charge > attempt fails, there’s no need to waste time trying 32KB or 16KB. We can > directly fall back to 4KB, as 32KB and 16KB will also fail based on our > margin detection logic. > Yes that makes sense. Would something like below work to fix that: diff --git a/mm/memcontrol.c b/mm/memcontrol.c index c098fd7f5c5e..0a1ec55cc079 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4550,7 +4550,11 @@ int mem_cgroup_swapin_charge_folio(struct folio *folio, struct mm_struct *mm, memcg = get_mem_cgroup_from_mm(mm); rcu_read_unlock(); - ret = charge_memcg(folio, memcg, gfp); + if (folio_test_large(folio) && + mem_cgroup_margin(memcg) < max(MEMCG_CHARGE_BATCH, folio_nr_pages(folio))) + ret = -ENOMEM; + else + ret = charge_memcg(folio, memcg, gfp); css_put(&memcg->css); return ret; diff --git a/mm/memory.c b/mm/memory.c index fecdd044bc0b..b6ce6605dc63 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4123,6 +4123,7 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf) pte_t *pte; gfp_t gfp; int order; + int ret; /* * If uffd is active for the vma we need per-page fault fidelity to @@ -4170,9 +4171,13 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf) addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order); folio = vma_alloc_folio(gfp, order, vma, addr, true); if (folio) { - if (!mem_cgroup_swapin_charge_folio(folio, vma->vm_mm, - gfp, entry)) + ret = mem_cgroup_swapin_charge_folio(folio, vma->vm_mm, gfp, entry); + if (!ret) { return folio; + } else if (ret == -ENOMEM) { + folio_put(folio); + goto fallback; + } folio_put(folio); } order = next_order(&orders, order);