From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7A8B3FF8860 for ; Mon, 27 Apr 2026 15:11:29 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id E0F286B008A; Mon, 27 Apr 2026 11:11:28 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id DE6316B0092; Mon, 27 Apr 2026 11:11:28 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D23816B009E; Mon, 27 Apr 2026 11:11:28 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id BEE956B008A for ; Mon, 27 Apr 2026 11:11:28 -0400 (EDT) Received: from smtpin01.hostedemail.com (lb01b-stub [10.200.18.250]) by unirelay01.hostedemail.com (Postfix) with ESMTP id B76A81C0D9D for ; Mon, 27 Apr 2026 15:08:54 +0000 (UTC) X-FDA: 84704668188.01.9ACBFF7 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf28.hostedemail.com (Postfix) with ESMTP id C37C1C0007 for ; Mon, 27 Apr 2026 15:08:52 +0000 (UTC) Authentication-Results: imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=uR9XjItI; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf28.hostedemail.com: domain of vbabka@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=vbabka@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1777302533; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=J8PlT6fj0xMyuWjkW+CGfI8o224FBRVwpmBUb7a7EoM=; b=GCt5yEn0xZF8JmY0Apk1VCbA1mBmhvzT79Qnd2DPpg8ME2zc8jboJqg+/XgtE2voR6J6e+ p1nkwjlQj4KmoqNo5icwozavF42/FMS/yIUPLdf5I9SonbThFrg6tR1DmG0NYhBJA9MGlQ 8ixcwy5U9Mn3s3GF2wfyyTcKZSCqk+0= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1777302533; a=rsa-sha256; cv=none; b=ZMzYvvcqCZLx+co0l+TU2WN3VBzTPsKkkJYtneZkDYyB9HeHflLsqOH7r+yCpLbgb/KMQv DpGpj0GGqF3nJ7CtAu9+GWDix7earsJ5ShTzQZw95pbFBPlgHvvHCbNFSndB0JGrHH3Ntg gEMs8WqS8dKpFTEEGouavCBeblSFuyY= ARC-Authentication-Results: i=1; imf28.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=uR9XjItI; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf28.hostedemail.com: domain of vbabka@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=vbabka@kernel.org Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id BF7D1404C4; Mon, 27 Apr 2026 15:08:51 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2BE68C19425; Mon, 27 Apr 2026 15:08:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777302531; bh=DGNTE2ObghVxBxWjipSbc5ELG8n2Tma9SvBr8kxeivk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=uR9XjItIqVDi8Bce3rum/9uj8K9FNsIxp6xDK74Xg+XygXLaqqDTVmbBPlU04KYCE lnQTeWG2aJzRkPLAJqaE1KG1fiKn+DRVpaLAS2JcwqhywMmT8xEUk6g9wXhE4gQFzI oVZ3M4IcLXyZ/dg1Y/ci1u6j49nMsQPFlPhlUBfaPc2mK9SEp7ZyQHqQDz3tqwMTXT gQyHKDQWPNGwg7qKmc+9LHpyb2ud6GUn7Zrboj0IAKWc9iy3U8CbbCKpHRjqVWTi7v LcoC3/F9Rx8Wl6tnTPLSLBA4kI67ZBSKP9M0GN7YrkrnUj7W1iwuwdsTGpImCjBlXW 1CZNfSio813pA== Message-ID: <6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org> Date: Mon, 27 Apr 2026 17:08:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 5/8] mm/slab: make kfree_rcu_nolock() work with sheaves Content-Language: en-US To: Alexei Starovoitov Cc: "Harry Yoo (Oracle)" , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Hao Li , Alexei Starovoitov , Uladzislau Rezki , "Paul E . McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Zqiang , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , rcu@vger.kernel.org, linux-mm References: <20260416091022.36823-1-harry@kernel.org> <20260416091022.36823-6-harry@kernel.org> <56381493-214b-406d-9121-f80be7658445@kernel.org> <7101d55e-ba43-4df9-a345-2af579f8e940@kernel.org> From: "Vlastimil Babka (SUSE)" In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: C37C1C0007 X-Rspamd-Server: rspam04 X-Stat-Signature: ms8gmd3f4he1yscp6xm4wfuqd6mz3c67 X-HE-Tag: 1777302532-747971 X-HE-Meta: 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 FVNwGZ+m xW+WO0vEhV0gLaYNgXwFaxCbcSR2vMj3VxIh5M3RDrSnLNv1atEKeS5RkJl+fM8R430nUtNn1QnXfiBpyXiRPXyMFzVYJCvVXNdsAp3MmF2Dgl2fHPh+SziZtmz0WKZY8uWJY9u25pr8CkU3zKR1aaBXCDJTsl3N+9RV81nENZJJTfFNq/h7ZdCabBzIj+YMfkz2uRpJv5o4NLXM+IpFP+6EqOf5Zd16aJky5ajO5XQ8CFk/zauGC/GElTxlvPIoHvMe5RShDMQMmT+c7+ws0/cpP35uN7RQV0J+WOZJ8IZWQLWrxD0wGvjR+lNsW9Vtzd48dpEKVH+M8DHCv56fGwWLxvzHG3epXx1WkjKE0NKzKeb3E/O8vnI0+DClwvBstQeXrtxKqvseKnI59LpZf0F8cQYVzBtodpM2GWZF0KfjSAvI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/27/26 16:45, Alexei Starovoitov wrote: > On Mon, Apr 27, 2026 at 2:53 PM Vlastimil Babka (SUSE) > wrote: >> >> @@ -5973,6 +5977,12 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj) >> >> if (likely(rcu_sheaf->size < s->sheaf_capacity)) { >> >> rcu_sheaf = NULL; >> >> } else { >> >> + if (unlikely(!allow_spin)) { >> >> + /* call_rcu() cannot be called in an unknown context */ >> >> + rcu_sheaf->size--; >> >> + local_unlock(&s->cpu_sheaves->lock); >> >> + goto fail; >> >> + } >> >> ... this part, so an unconditional allow_spin=false on RT would only fill >> the rcu_sheaf once and then stop working. >> >> However, can we condition this on in_nmi() or something, and pretend the >> other cases of "unknown context" exist? Installing BPF hooks >> allocating/freeing memory into call_rcu() internals sounds crazy, no? > > Unfortunately no. People attach to tracepoint lock contention > which can be inside call_rcu. > As discussed call_rcu_nolock() is only way forward. Alright. But we assume those tracepoint-attached callers know to use kfree_rcu_nolock(), while kfree_rcu() callers under PREEMPT_RT can still assume it's not called in NMI or tracepoint-attached context, right? So I think what __kfree_rcu_sheaf() could do is - with CONFIG_PREEMPT_RT, treat allow_spin == true as allow_spin == false for purposes of barn_get_empty_sheaf() and to bail out before trying alloc_empty_sheaf() - this is to avoid any lock activity that's not a trylock. - allow_spin controls ability to do call_rcu() as above, regardless of PREEMPT_RT, to avoid it from kfree_rcu_nolock()