From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 E098625A646; Fri, 25 Jul 2025 18:57:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753469855; cv=none; b=pAdGpecnYMXfwkmIKVVN4yZ/5hMKPhaCoaWwQlcatrJ9/9OcP8O6tv6YHVzuNny0vRNEAb2FtZQ+OSwPNt+u11xKZS2Xm4PMA8M2vGsdDCk62KqEj1q3K/T8FHtoIzHVavWf2p0fF0ng1FTak5ODYeiL6Cg+l+WiE3U6FQPRH7k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753469855; c=relaxed/simple; bh=yP5vORM2KBmYRwfhcpa2pP/ppHXLVEurCzyq34xBP5s=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=pBqF2wNh3PXQlqk84VN6DRJf3ruMxVgGaZ9yLtMva1LRBsJ4Yei/JzVlkAMC12bDF6KXNZ5LjApXIh0WpBQeUQcmXb8uKn9kYfyr2s6UF1BK6CXj4TJDyRlKFS+1DuXQQ1+wmlc0oXtb+wESJn0EObY5XImKBMd3JuR5KdFj0R0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Jk+s5a+4; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Jk+s5a+4" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B7F74C4AF09; Fri, 25 Jul 2025 18:57:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1753469854; bh=yP5vORM2KBmYRwfhcpa2pP/ppHXLVEurCzyq34xBP5s=; h=Date:From:To:Cc:Subject:References:From; b=Jk+s5a+4jFgLFWrrAL+8mke+c0XqWoB164dSK8VoAtb/6YfjQYJW14QJo6UTB23O0 dUyuAZt25zEkFoP+w01vkyhcC29hVd8cUwFTn0vAEvW9w7+7GI4iAePP8RlxXBCGE0 UKMxXJvDWtxSPqK+ukRz6g9qeHM38iD17PBkaOf0SPQI0wtB4EYny8NdRZDo+lo0nN hJaWsZrLBD4Yk9wxv1PbNSNIGLggiQ4uk2PsuXZv4ORrg4VH5gaIGXCst9UNedUR4O A6LpYf06aYcsrAG1fNsjAGywXrF3/p1aV8fzuSm2GrAbjh426xrAynWZJYdoV3XTxp tYQ2p2LiGuuZg== Received: from rostedt by gandalf with local (Exim 4.98.2) (envelope-from ) id 1ufNbw-00000001N7R-33Kg; Fri, 25 Jul 2025 14:57:40 -0400 Message-ID: <20250725185740.581435592@kernel.org> User-Agent: quilt/0.68 Date: Fri, 25 Jul 2025 14:55:21 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org, x86@kernel.org Cc: Masami Hiramatsu , Mathieu Desnoyers , Josh Poimboeuf , Peter Zijlstra , Ingo Molnar , Jiri Olsa , Arnaldo Carvalho de Melo , Namhyung Kim , Thomas Gleixner , Andrii Nakryiko , Indu Bhagat , "Jose E. Marchesi" , Beau Belgrave , Jens Remus , Linus Torvalds , Andrew Morton , Jens Axboe , Florian Weimer , Sam James , "Paul E. McKenney" Subject: [PATCH v15 09/10] unwind deferred: Use SRCU unwind_deferred_task_work() References: <20250725185512.673587297@kernel.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 From: Steven Rostedt Instead of using the callback_mutex to protect the link list of callbacks in unwind_deferred_task_work(), use SRCU instead. This gets called every time a task exits that has to record a stack trace that was requested. This can happen for many tasks on several CPUs at the same time. A mutex is a bottleneck and can cause a bit of contention and slow down performance. As the callbacks themselves are allowed to sleep, regular RCU cannot be used to protect the list. Instead use SRCU, as that still allows the callbacks to sleep and the list can be read without needing to hold the callback_mutex. Link: https://lore.kernel.org/all/ca9bd83a-6c80-4ee0-a83c-224b9d60b755@efficios.com/ Cc: "Paul E. McKenney" Suggested-by: Mathieu Desnoyers Signed-off-by: Steven Rostedt (Google) --- Changes since v14: https://lore.kernel.org/20250717004957.918908732@kernel.org - Use normal SRCU instead of srcu_lite as srcu_lite is being deprecated. May later replace the normal SRCU with src_fast. (Paul McKenney) kernel/unwind/deferred.c | 27 +++++++++++++++++++++------ 1 file changed, 21 insertions(+), 6 deletions(-) diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c index 2311b725d691..a5ef1c1f915e 100644 --- a/kernel/unwind/deferred.c +++ b/kernel/unwind/deferred.c @@ -41,7 +41,7 @@ static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt) #define UNWIND_MAX_ENTRIES \ ((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long)) -/* Guards adding to and reading the list of callbacks */ +/* Guards adding to or removing from the list of callbacks */ static DEFINE_MUTEX(callback_mutex); static LIST_HEAD(callbacks); @@ -49,6 +49,7 @@ static LIST_HEAD(callbacks); /* Zero'd bits are available for assigning callback users */ static unsigned long unwind_mask = RESERVED_BITS; +DEFINE_STATIC_SRCU(unwind_srcu); static inline bool unwind_pending(struct unwind_task_info *info) { @@ -174,8 +175,9 @@ static void unwind_deferred_task_work(struct callback_head *head) cookie = info->id.id; - guard(mutex)(&callback_mutex); - list_for_each_entry(work, &callbacks, list) { + guard(srcu)(&unwind_srcu); + list_for_each_entry_srcu(work, &callbacks, list, + srcu_read_lock_held(&unwind_srcu)) { if (test_bit(work->bit, &bits)) { work->func(work, &trace, cookie); if (info->cache) @@ -213,7 +215,7 @@ int unwind_deferred_request(struct unwind_work *work, u64 *cookie) { struct unwind_task_info *info = ¤t->unwind_info; unsigned long old, bits; - unsigned long bit = BIT(work->bit); + unsigned long bit; int ret; *cookie = 0; @@ -230,6 +232,14 @@ int unwind_deferred_request(struct unwind_work *work, u64 *cookie) if (WARN_ON_ONCE(!CAN_USE_IN_NMI && in_nmi())) return -EINVAL; + /* Do not allow cancelled works to request again */ + bit = READ_ONCE(work->bit); + if (WARN_ON_ONCE(bit < 0)) + return -EINVAL; + + /* Only need the mask now */ + bit = BIT(bit); + guard(irqsave)(); *cookie = get_cookie(info); @@ -281,10 +291,15 @@ void unwind_deferred_cancel(struct unwind_work *work) return; guard(mutex)(&callback_mutex); - list_del(&work->list); + list_del_rcu(&work->list); + + /* Do not allow any more requests and prevent callbacks */ + work->bit = -1; __clear_bit(bit, &unwind_mask); + synchronize_srcu(&unwind_srcu); + guard(rcu)(); /* Clear this bit from all threads */ for_each_process_thread(g, t) { @@ -307,7 +322,7 @@ int unwind_deferred_init(struct unwind_work *work, unwind_callback_t func) work->bit = ffz(unwind_mask); __set_bit(work->bit, &unwind_mask); - list_add(&work->list, &callbacks); + list_add_rcu(&work->list, &callbacks); work->func = func; return 0; } -- 2.47.2