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Tue, 14 Oct 2025 01:08:24 -0700 (PDT) From: Barry Song <21cnbao@gmail.com> To: mhocko@suse.com Cc: 21cnbao@gmail.com, alexei.starovoitov@gmail.com, corbet@lwn.net, davem@davemloft.net, david@redhat.com, edumazet@google.com, hannes@cmpxchg.org, harry.yoo@oracle.com, horms@kernel.org, jackmanb@google.com, kuba@kernel.org, kuniyu@google.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linyunsheng@huawei.com, netdev@vger.kernel.org, pabeni@redhat.com, roman.gushchin@linux.dev, surenb@google.com, v-songbaohua@oppo.com, vbabka@suse.cz, willemb@google.com, willy@infradead.org, zhouhuacai@oppo.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com Subject: Re: [RFC PATCH] mm: net: disable kswapd for high-order network buffer allocation Date: Tue, 14 Oct 2025 16:08:12 +0800 Message-Id: <20251014080812.2985-1-21cnbao@gmail.com> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: References: Precedence: bulk X-Mailing-List: netdev@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 Tue, Oct 14, 2025 at 3:26 PM Michal Hocko wrote: > > On Mon 13-10-25 20:30:13, Vlastimil Babka wrote: > > On 10/13/25 12:16, Barry Song wrote: > > > From: Barry Song > [...] > > I wonder if we should either: > > > > 1) sacrifice a new __GFP flag specifically for "!allow_spin" case to > > determine it precisely. > > As said in other reply I do not think this is a good fit for this > specific case as it is all or nothing approach. Soon enough we discover > that "no effort to reclaim/compact" hurts other usecases. So I do not > think we need a dedicated flag for this specific case. We need a way to > tell kswapd/kcompactd how much to try instead. +Baolin, who may have observed the same issue. An issue with vmscan is that kcompactd is woken up very late, only after reclaiming a large number of order-0 pages to satisfy an order-3 application. static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx) { ... balanced = pgdat_balanced(pgdat, sc.order, highest_zoneidx); if (!balanced && nr_boost_reclaim) { nr_boost_reclaim = 0; goto restart; } /* * If boosting is not active then only reclaim if there are no * eligible zones. Note that sc.reclaim_idx is not used as * buffer_heads_over_limit may have adjusted it. */ if (!nr_boost_reclaim && balanced) goto out; ... if (kswapd_shrink_node(pgdat, &sc)) raise_priority = false; ... out: ... /* * As there is now likely space, wakeup kcompact to defragment * pageblocks. */ wakeup_kcompactd(pgdat, pageblock_order, highest_zoneidx); } As pgdat_balanced() needs at least one 3-order pages to return true: bool __zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark, int highest_zoneidx, unsigned int alloc_flags, long free_pages) { ... if (free_pages <= min + z->lowmem_reserve[highest_zoneidx]) return false; /* If this is an order-0 request then the watermark is fine */ if (!order) return true; /* For a high-order request, check at least one suitable page is free */ for (o = order; o < NR_PAGE_ORDERS; o++) { struct free_area *area = &z->free_area[o]; int mt; if (!area->nr_free) continue; for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) { if (!free_area_empty(area, mt)) return true; } #ifdef CONFIG_CMA if ((alloc_flags & ALLOC_CMA) && !free_area_empty(area, MIGRATE_CMA)) { return true; } #endif if ((alloc_flags & (ALLOC_HIGHATOMIC|ALLOC_OOM)) && !free_area_empty(area, MIGRATE_HIGHATOMIC)) { return true; } } This appears to be incorrect and will always lead to over-reclamation in order0 to satisfy high-order applications. I wonder if we should "goto out" earlier to wake up kcompactd when there is plenty of memory available, even if no order-3 pages exist. Conceptually, what I mean is: diff --git a/mm/vmscan.c b/mm/vmscan.c index c80fcae7f2a1..d0e03066bbaa 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -7057,9 +7057,8 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx) * eligible zones. Note that sc.reclaim_idx is not used as * buffer_heads_over_limit may have adjusted it. */ - if (!nr_boost_reclaim && balanced) + if (!nr_boost_reclaim && (balanced || we_have_plenty_memory_to_compact())) goto out; /* Limit the priority of boosting to avoid reclaim writeback */ if (nr_boost_reclaim && sc.priority == DEF_PRIORITY - 2) raise_priority = false; Thanks Barry