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 64F78CD98C7 for ; Fri, 12 Jun 2026 01:11:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id A6E606B0088; Thu, 11 Jun 2026 21:11:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 9D0E86B008C; Thu, 11 Jun 2026 21:11:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8BF436B0092; Thu, 11 Jun 2026 21:11:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id 764036B0088 for ; Thu, 11 Jun 2026 21:11:00 -0400 (EDT) Received: from smtpin30.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay04.hostedemail.com (Postfix) with ESMTP id 0B3FA1A02E8 for ; Fri, 12 Jun 2026 01:11:00 +0000 (UTC) X-FDA: 84869481480.30.D4EE708 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf24.hostedemail.com (Postfix) with ESMTP id 7E512180003 for ; Fri, 12 Jun 2026 01:10:58 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="K/ebhH3u"; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf24.hostedemail.com: domain of sj@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=sj@kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1781226658; 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-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=8Nt7zTy7OwkHO+yy23DmbB2qHh++8NKVSmgy/TvQ9cw=; b=yvlWBGkhJW3kREtCpTvjYEd3WlgqosFQNDNraHwgf8biP1L3ah8qkQMPRtwQenKKngN/Hx bTlfJL1xU0Ko5H1PxnfvYPnAeTjcbfxS9+HXrxA1eb8EIgARs6Vh6ozxHSvReB9FdTdefr d5fADprkGBohTDw9Jon0gtVBshNsuIQ= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="K/ebhH3u"; dmarc=pass (policy=quarantine) header.from=kernel.org; spf=pass (imf24.hostedemail.com: domain of sj@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=sj@kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1781226658; b=VxKWrxBiEaDnAmbkjgl0e9+2kUCWbpb8NgL7bJszMkoJEGULQYHIMw0gV7cPJsflCSjbcR 7VASPXMgcBXa3eXrCcBbZ+S1wttY134iGJowQ1cTPH2EEzr3u8AnxwFgo4vN7QrwmcU3HO Bi85PtFR71/y2g44eJfnQ2DNfLxZJD4= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id CE14C600FC; Fri, 12 Jun 2026 01:10:57 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 029811F000E9; Fri, 12 Jun 2026 01:10:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781226657; bh=8Nt7zTy7OwkHO+yy23DmbB2qHh++8NKVSmgy/TvQ9cw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=K/ebhH3uC6E2Exq+61ZEWztMIPk4YRRJLlxdF0tImKaO7Pulx8fUcCw5EAjS5dWwh CrWmAh9m7eAliMCSb4PmrSoykALflrAEc2CohDRTdeG3Jjf5Zt+moL1/4i8BYeFtty eIk9wHnCOLR1AB/7SyBbjq0X92u3+y/mD2alOLNCVNlLcvMiE3iT5BLE/6JMdcGan+ DcoRRlqbxoefSGjIZsUbdbfSBmhqVo6Zwm7nplyHcjle6GLcRhKobLZM4NSrCNNZzh PyzIRxxql7Jcs89uniKkOKVvXaLWyBH4+WkE/fra00bR9W6HQQT4JXoTZsAUkUnM1/ WgVpovFv6TNew== From: SeongJae Park To: Breno Leitao Cc: SeongJae Park , Catalin Marinas , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH RFC] mm/kmemleak: avoid soft lockup when scanning task stacks Date: Thu, 11 Jun 2026 18:10:48 -0700 Message-ID: <20260612011049.84146-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260611-kmemleak-stack-resched-v1-1-d6248ade5f4a@debian.org> References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: brt5zc6sqrjzhp5yhhw8nx8of8mb6agp X-Rspamd-Queue-Id: 7E512180003 X-HE-Tag: 1781226658-892399 X-HE-Meta: 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 xg+7V8SR IgbEwFxiJoWokicZYrobEiQMRNzIq7qz2ogOxsYUpaOKYv9eCQC67DVUsXBiT/ombKzsZvLcmGJatwQo6LeuVcYl/92b1QQr0npV7RrFjYbvumUphN6CQSMMSlR1paJVsYdgOaaw3ufT1oF4/JNKDYa0cium8jrg/AseT+xaM5fux8LY0lxqOZMJxHoWTygI+8VlyNPYYiCCz6SmNcU1wn+6vehH4G+gfq0y9AjqSvf/F8SOGrgIx4if6XZ3tiAM76vsCFM4IS6/g5cg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, 11 Jun 2026 05:45:00 -0700 Breno Leitao wrote: > kmemleak_scan() walks every thread and scans its kernel stack under a > single rcu_read_lock() with no reschedule point. On a host with very > many threads -- amplified by KASAN/lockdep in debug builds -- this loop > can hog a CPU long enough to trip the soft lockup watchdog: > > watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [kmemleak:537] > scan_block > kmemleak_scan > kmemleak_scan_thread > kthread > > A cond_resched() cannot be added directly: the loop runs inside an RCU > read-side critical section. > > Split the scan in two parts: > > 1) get the list of tasks (with RCU read lock) in an array > 2) run scan_block() for the tasks (with cond_reschd()). > > Is it a sane approach? > > Signed-off-by: Breno Leitao > --- > mm/kmemleak.c | 26 ++++++++++++++++++++++---- > 1 file changed, 22 insertions(+), 4 deletions(-) > > diff --git a/mm/kmemleak.c b/mm/kmemleak.c > index 7c7ba17ce7af0..9f8a35ecbb50c 100644 > --- a/mm/kmemleak.c > +++ b/mm/kmemleak.c > @@ -62,6 +62,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -1885,17 +1886,34 @@ static void kmemleak_scan(void) > * Scanning the task stacks (may introduce false negatives). > */ > if (kmemleak_stack_scan) { > - struct task_struct *p, *g; > + struct task_struct **tasks, *p, *g; > + unsigned int nr = 0, max, i; > > + max = nr_threads + 64; > + tasks = kvmalloc_array(max, sizeof(*tasks), GFP_KERNEL); > + > + /* Snapshot the threads under RCU */ > rcu_read_lock(); > for_each_process_thread(g, p) { > - void *stack = try_get_task_stack(p); > + if (!tasks || nr >= max) > + break; Why don't you check !tasks right after the allocation? > + get_task_struct(p); > + tasks[nr++] = p; > + } > + rcu_read_unlock(); > + > + /* now scan_block for the tasks above with cond_resched() */ > + for (i = 0; i < nr; i++) { > + void *stack = try_get_task_stack(tasks[i]); > + > if (stack) { > scan_block(stack, stack + THREAD_SIZE, NULL); > - put_task_stack(p); > + put_task_stack(tasks[i]); > } > + put_task_struct(tasks[i]); > + cond_resched(); > } > - rcu_read_unlock(); > + kvfree(tasks); > } Thanks, SJ [...]