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 085CB2522BE for ; Mon, 4 Aug 2025 13:42:30 +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=1754314952; cv=none; b=T07msepRbLJ1NbIB5H/wTQAXUIOlD293BX4vP6N+FthdEi82HroRaw9oPvO+nn380nV9gl0+u3vJloMsVzUeZcbXDtmNz98BM+dgVbdplVqLy3amBy0UXQqxetWPKtDPEpJwPQW/HtRnPv9IE8cEqYZf6LSRxNlWJpfludLoaR8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754314952; c=relaxed/simple; bh=r+35R3cMW5I905QXPJ68CBu9Hx4eTADCxXxLBeTKgiE=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=HaOFT+WRqHafM9QlAkXLpvY17NFXoWKfa9aQYqZBh8yMXD7b3xeoTaqeikD8ZydDQ+piA3YB7KvwBFN3R14KRmptZsIRqLaACgH0GAi2Sv13eRNlWvL6VcPhheVboKJaiQqqcoeEma+vFMIMDpB+pnvSn5jPq3q6frZygIrIUys= 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=fiDpbEoG; 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="fiDpbEoG" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1754314949; 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=EZvrclNS6/cHIwXDV7qrig8vc0YOkD6KFqwoaeUmEdw=; b=fiDpbEoG6O/4Zk0jIsopoHg4fzP6ybjl/9Uk3WQ1IPFOhvebGE5At4qqOsBRp8XL8tDJdj ebhPBFjfY+XqbGe6QloTQ0fZ2vjL3Vbh6tfCAqGY6pLwgPqSvSCMHV/+U5IcqGeokFNwmF iO+4JSemfp9HkgVCqWBBveDbXPXOL3s= 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-628-OAezRv-XPxmFqWyH6aXRDw-1; Mon, 04 Aug 2025 09:42:25 -0400 X-MC-Unique: OAezRv-XPxmFqWyH6aXRDw-1 X-Mimecast-MFC-AGG-ID: OAezRv-XPxmFqWyH6aXRDw_1754314944 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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 A4EC3195D018; Mon, 4 Aug 2025 13:42:23 +0000 (UTC) Received: from fedora (unknown [10.72.116.20]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6CFFE1956094; Mon, 4 Aug 2025 13:42:18 +0000 (UTC) Date: Mon, 4 Aug 2025 21:42:12 +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> 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: <20250804122125.3271397-1-nilay@linux.ibm.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 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... Thanks, Ming