From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta1.migadu.com (out-180.mta1.migadu.com [95.215.58.180]) (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 46A4029B79B for ; Tue, 19 May 2026 14:03:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779199407; cv=none; b=LHJdL3Bt9O3MWIqdUcLuTJdsLIBUTAXEigtkQubD30SRO0jK7SlPVhOto8BVLhxWrAAXnDyjunYsdVo6jmWy8yATUSHkLMfXlknV2UIRMOpi8gI85CB6dO2iKXSshusL5tddtAh6v/v5lZRf5OIpldz89FCUuJPNKdOMIk8yeOg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779199407; c=relaxed/simple; bh=OxYKVT+8P4ybN/wzn3h6yUrqje4l9mbayRiLtNAFPWU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ppFjHi/l+qCgnWlmRSmZ5YVB8KnSVePLZPK9QJosOcenl89cwfMdBWZCmCqQaJKEE+ZDIF1RvqfxGrV5UJuHIdX+gBrW9mbOVZDzviTFihXZNpL9OQix4R01OKjwDP90nSZcipxQsJtRCK9TtymDz6B/jVYApt61ao/aThJDj2k= 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=k/Sf/ctN; arc=none smtp.client-ip=95.215.58.180 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="k/Sf/ctN" Date: Tue, 19 May 2026 07:02:47 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779199391; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=RLkJJxHnitGVDPhMID677Jv5DkOQCt8pQUC50Rx5KFQ=; b=k/Sf/ctN+0g0y8K/IpdbvTp1cYEfoUAApm7k/DJj6iPyQHQkBLM9gQ6ylpR0M2yakt6epr jI0VSIqp37yHfaJPa46xMDp3LZg9yTfFjrirJEbUePonoLShekskAYNYqheBsfSPWAFsG9 Ukp0H+D0/xyn1dk/GrfPw8zgWZRKFFs= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Shakeel Butt To: Harry Yoo Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Alexandre Ghiti , Joshua Hahn , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Subject: Re: [PATCH v3] memcg: cache obj_stock by memcg, not by objcg pointer Message-ID: References: <20260518222827.110696-1-shakeel.butt@linux.dev> <4e296262-fbbf-4ac7-aecc-3ef831583704@kernel.org> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4e296262-fbbf-4ac7-aecc-3ef831583704@kernel.org> X-Migadu-Flow: FLOW_OUT On Tue, May 19, 2026 at 03:46:51PM +0900, Harry Yoo wrote: > > > On 5/19/26 8:41 AM, Shakeel Butt wrote: > > On Mon, May 18, 2026 at 03:28:27PM -0700, Shakeel Butt wrote: > > > Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg > > > per-node type") split a memcg's single obj_cgroup into one per NUMA > > > node, but the per-CPU obj_stock_pcp still keys cached_objcg by > > > pointer. Cross-NUMA workloads now see a drain on every refill and a > > > miss on every consume that targets a sibling per-node objcg of the > > > same memcg, producing the 67.7% stress-ng switch-mq regression > > > reported by LKP. > > > > > > stock->nr_bytes are fungible across per-node objcgs of one memcg. > > > Treat the cache as keyed by memcg in __consume_obj_stock() and > > > __refill_obj_stock() so siblings share the reserve. Compare via > > > READ_ONCE(objcg->memcg) directly: pointer-compare only, no deref, so > > > the rcu_read_lock contract on obj_cgroup_memcg() does not apply. > > > > > > Sharing the reserve without re-caching means bytes funded by one > > > per-node objcg's slow path can be consumed/freed under a different > > > sibling, leaving sub-page residue on whichever sibling was cached at > > > drain time. The pre-existing obj_cgroup_release() path would WARN and > > > silently drop that residue, leaking up to nr_node_ids * (PAGE_SIZE - 1) > > > bytes per memcg lifecycle from the page_counter. Forward the residue > > > into a per-node objcg of the same (post-reparent) memcg at release time > > > instead, so it can be reconciled later via a refill atomic_xchg or > > > another release; the chain terminates at root_mem_cgroup, whose > > > page_counter has no enforced limit. > > > > > > Please note that this is temporary fix and will be reverted when > > > per-node kmem accounting is introduced. > > ... because once per-node kmem accounting is introduced, > "stock->nr_bytes are fungible across per-node objcgs of one memcg" > no longer holds? Yes > > And the follow-up plain is to revert this and address it with a multi-objcg > percpu stock [1], similar to a multi-memcg percpu charge cache we have now, > right? (regardless of per-node kmem accounting's progress) > Yes > If this temporary fix imposes other potential correctness issues, would it > make sense to land [1] in mainline before the next LTS release and skip this > temporary fix? > > [1] https://lore.kernel.org/oe-lkp/agtPMpQK2jXdQAY4@linux.dev > The full clean solution might take one more cycle and I think we can not just ignore 67% regression on 7.1.