From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 8B4511D5CEA for ; Tue, 5 Aug 2025 12:44:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754397891; cv=none; b=BQrJGrrL/HHIzXQZ2tIlNJS2pghv4/sLWr45FEzyhGPiIQ3VIWpEWYjohO+BY2qyyo1YSXVgC4TGbOjF/c/z12xj8uBZ1WRRMJp9/JyiI+wx6WQK3HgNNj2CwvQZgW8qVoFtTlwuOK27j78AHqPllOAyk+6gkbrg9rH+K45Vli8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754397891; c=relaxed/simple; bh=Si6MReIrTk1/rh8ENRbKAplQGUzXxcK5ZPZD845rZrs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=RpJMuTvU3KWhoUMOEpI5wiXDsjbn4TZKj852qz17NGPeXHF+vNCDKfVmFJpGuBGRk73M0Py0QJIoW67B9eFMnU9wdgdBDHQtUH9KNaiXASyG/W51lrj+VL7+TiIbrDIjQ5JVCIStnIxEILN5LZomXz1lPv0VPhnMG/9AnrjNOIM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=B1PJmNfC; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="B1PJmNfC" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1754397888; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=BF1RUyoME7iGL8y4OTbb+xtfAVb+8hSzDJ3hc266uHI=; b=B1PJmNfCK0vDF3OkBrejja+GpQe+n+s6+jJRzjO349/ewFG+Oig+gRDuKrx+8s1yO2tehh sP73Lhs5q/S0/PlM1WLBkgu+CgqpsEGG7SBRRX1YFQ0JvTs08vG/1U87PtkW9tLu93DP4f +/gAmPM9OOeRe5A9CUdMyS66q4k5Tus= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-110-Fz0r2K1QNyyX1ACUJqYXKw-1; Tue, 05 Aug 2025 08:44:45 -0400 X-MC-Unique: Fz0r2K1QNyyX1ACUJqYXKw-1 X-Mimecast-MFC-AGG-ID: Fz0r2K1QNyyX1ACUJqYXKw_1754397884 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 808BC195D020; Tue, 5 Aug 2025 12:44:43 +0000 (UTC) Received: from fedora (unknown [10.72.116.20]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 880E81954B0C; Tue, 5 Aug 2025 12:44:34 +0000 (UTC) Date: Tue, 5 Aug 2025 20:44:24 +0800 From: Ming Lei To: Nilay Shroff Cc: linux-block@vger.kernel.org, axboe@kernel.dk, kch@nvidia.com, shinichiro.kawasaki@wdc.com, hch@lst.de, gjoyce@ibm.com Subject: Re: [PATCH 0/2] block: blk-rq-qos: replace static key with atomic bitop Message-ID: References: <20250804122125.3271397-1-nilay@linux.ibm.com> <682f0f43-733a-4c04-91ed-5665815128bc@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <682f0f43-733a-4c04-91ed-5665815128bc@linux.ibm.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On Tue, Aug 05, 2025 at 10:28:14AM +0530, Nilay Shroff wrote: > > > On 8/4/25 7:12 PM, Ming Lei wrote: > > On Mon, Aug 04, 2025 at 05:51:09PM +0530, Nilay Shroff wrote: > >> This patchset replaces the use of a static key in the I/O path (rq_qos_ > >> xxx()) with an atomic queue flag (QUEUE_FLAG_QOS_ENABLED). This change > >> is made to eliminate a potential deadlock introduced by the use of static > >> keys in the blk-rq-qos infrastructure, as reported by lockdep during > >> blktests block/005[1]. > >> > >> The original static key approach was introduced to avoid unnecessary > >> dereferencing of q->rq_qos when no blk-rq-qos module (e.g., blk-wbt or > >> blk-iolatency) is configured. While efficient, enabling a static key at > >> runtime requires taking cpu_hotplug_lock and jump_label_mutex, which > >> becomes problematic if the queue is already frozen — causing a reverse > >> dependency on ->freeze_lock. This results in a lockdep splat indicating > >> a potential deadlock. > >> > >> To resolve this, we now gate q->rq_qos access with a q->queue_flags > >> bitop (QUEUE_FLAG_QOS_ENABLED), avoiding the static key and the associated > >> locking altogether. > >> > >> I compared both static key and atomic bitop implementations using ftrace > >> function graph tracer over ~50 invocations of rq_qos_issue() while ensuring > >> blk-wbt/blk-iolatency were disabled (i.e., no QoS functionality). For > >> easy comparision, I made rq_qos_issue() noinline. The comparision was > >> made on PowerPC machine. > >> > >> Static Key (disabled : QoS is not configured): > >> 5d0: 00 00 00 60 nop # patched in by static key framework (not taken) > >> 5d4: 20 00 80 4e blr # return (branch to link register) > >> > >> Only a nop and blr (branch to link register) are executed — very lightweight. > >> > >> atomic bitop (QoS is not configured): > >> 5d0: 20 00 23 e9 ld r9,32(r3) # load q->queue_flags > >> 5d4: 00 80 29 71 andi. r9,r9,32768 # check QUEUE_FLAG_QOS_ENABLED (bit 15) > >> 5d8: 20 00 82 4d beqlr # return if bit not set > >> > >> This performs an ld and and andi. before returning. Slightly more work, > >> but q->queue_flags is typically hot in cache during I/O submission. > >> > >> With Static Key (disabled): > >> Duration (us): min=0.668 max=0.816 avg≈0.750 > >> > >> With atomic bitop QUEUE_FLAG_QOS_ENABLED (bit not set): > >> Duration (us): min=0.684 max=0.834 avg≈0.759 > >> > >> As expected, both versions are almost similar in cost. The added latency > >> from an extra ld and andi. is in the range of ~9ns. > >> > >> There're two patches in the series. The first patch replaces static key > >> with QUEUE_FLAG_QOS_ENABLED. The second patch ensures that we disable > >> the QUEUE_FLAG_QOS_ENABLED when the queue no longer has any associated > >> rq_qos policies. > >> > >> As usual, feedback and review comments are welcome! > >> > >> [1] https://lore.kernel.org/linux-block/4fdm37so3o4xricdgfosgmohn63aa7wj3ua4e5vpihoamwg3ui@fq42f5q5t5ic/ > > > > > > Another approach is to call memalloc_noio_save() in cpu hotplug code... > > > Yes that would help fix this. However per the general usage of GFP_NOIO scope in > kernel, it is used when we're performing memory allocations in a context where I/O > must not be initiated, because doing so could cause deadlocks or recursion. > > So we typically, use GFP_NOIO in a code path that is already doing I/O, such as: > - In block layer context: during request submission > - Filesystem writeback, or swap-out. > - Memory reclaim or writeback triggered by memory pressure. If you grep blk_mq_freeze_queue, you will see the above list is far from enough, :-) > > The cpu hotplug code may not be running in any of the above context. So > IMO, adding memalloc_noio_save() in the cpu hotplug code would not be > a good idea, isn't it? The reasoning(A -> B) looks correct, but the condition A is obviously not. Thanks, Ming