From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) (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 7C54638A716 for ; Mon, 18 May 2026 18:32:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779129169; cv=none; b=TLl0BO1shNBwA003v4l0ak8wckXXQ/3nkPBb/yp16tYIiFBsNJjuU3wVlXAU1lyiSS0bjMznp2jZ2s5QNipwfPR3t3hc1x4gYwiJav1oRrro8KPic9FFK3LtY+yKAkLf1JudY4tT3HheWrxuHmQwgqpIreifhryw8McxuCz714c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779129169; c=relaxed/simple; bh=BuwAlBQNUrzN0SwgsF4mfLwvIdFbOc/B84lH4N4VHpY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=N4ZGShJC1+Jd7Tq+tJFSu4gjOrFFVWvr000B2JSAuYJrHFKzGW1lgKMNyc4NPib9Bj8Gjlj8ehpNmTk+R9baJpON8EIyvzyUpywvPP7xeXYXMJXRnFqu36NjlRaNsr5feBiUF9UWRvhzC8/DCeuWazKgLU+0+4OSNKGF8V+z/Yw= 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=oMay0ixZ; arc=none smtp.client-ip=91.218.175.189 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="oMay0ixZ" Date: Mon, 18 May 2026 11:32:30 -0700 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1779129155; 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=FXHUYRMce7ARIc5s4WnL47A3d01wGcNanUodNfVijnI=; b=oMay0ixZ8rUB5vFEuLI8LeuR0E7rii2KoCUvRIRwi9ahQcInm6XJsG/kmSCF3JIPGhEOij df40rL6/I/w4/poq/oF9Ydo/WfNP9U/69c7BkPH3Fv0HnP/eYUg0ym+ycUfYXVIsfFdFva 1GYmVy1PqokvefTdam4LyFdtnr1wlOY= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Shakeel Butt To: Andrew Morton Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Qi Zheng , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot , alex@ghiti.fr, joshua.hahnjy@gmail.com Subject: Re: [PATCH v2] memcg: cache obj_stock by memcg, not by objcg pointer Message-ID: References: <20260517194308.952655-1-shakeel.butt@linux.dev> 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: X-Migadu-Flow: FLOW_OUT On Mon, May 18, 2026 at 09:46:04AM -0700, Shakeel Butt wrote: > Cc Alex, Joshua (since they are working on making per-num kmem accounting work) > > On Sun, May 17, 2026 at 12:43:08PM -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. > > > > In the same-memcg refill path also fold the incoming objcg's > > nr_charged_bytes into the stock; otherwise sub-page residue > > accumulates on whichever sibling was cached at drain time and > > obj_cgroup_release() silently drops it, leaking up to nr_node_ids * > > (PAGE_SIZE - 1) bytes per memcg lifecycle from the page_counter. > > This issue was reported by Sashiko. > > > > Update the now-stale invariant comment on __account_obj_stock(). > > > > Qi Zheng built a specialized reproducer [1] for the corner case and > > confirmed the fix. > > > > Reported-by: kernel test robot > > Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com > > Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type") > > Link: https://lore.kernel.org/19693be6-7132-446e-b3fc-b7e9f56e5949@linux.dev/ [1] > > Signed-off-by: Shakeel Butt > > Debugged-by: Qi Zheng > > Tested-by: Qi Zheng > > Sashiko [1] reported two issues. First one seems benign but the second one is > real. However I think we need to take a step back and rethink on how to solve > this issue in more future proof way. > > It seems like Alex and Joshua are working on enabling per-node kmem accounting > and that would need accurate per-numa association for each per-node objcg. > So, checking objcg->memcg in consume and refill, would go against the per-node > kmem accounting. > > One way to fix the regression and be future proof is to follow the approach we > have for memcg_stock_pcp which is multiple per-cpu objcg stocks. We will need to > test it more and depending on the additional code complexity, we will need to > decide to backport it to 7.2 or not. > > > [1] https://sashiko.dev/#/patchset/20260517194308.952655-1-shakeel.butt@linux.dev?part=1 > Previously I had prototyped the multiple per-cpu objcg stocks (when I worked on multi-memcg percpu stock) which I just rebased on latest linux-next and sent [1]. That patch is additional 100 LOC. For upstreaming, I will break it up into at least 4 patches. However I am questioning about backporting them to 7.1. One thing I can do is fix whatever sashiko is asking for and send v3 which can be ported to 7.1 and then later for 7.2+, revert this short term fix and send out the multiple objcg patch series. Any concerns? [1] http://lore.kernel.org/agtPMpQK2jXdQAY4@linux.dev