From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 2B7E63A1685 for ; Fri, 5 Jun 2026 18:05:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780682732; cv=none; b=DYZhw/9PO91uD/I/u/+bSBXmObQIgo2+hIRPIC+5GW3I5HCfa5hZD+0JCgKy0PZUMMrOrXa525wJiqk/EWSnBYtEwnmWwuZRLlPEJQEa/j83e2jOJFmyoDvchtg8Ve1a553XESr4sb+3j0pMAKR67Y+dvzXjQPyVThEQLrq4jgU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780682732; c=relaxed/simple; bh=MWQpVz6mI0I32LN+oi4vCe1pzKjUOWpiarigsce9YzQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=AHkodUgt9rEZTBpoFHXEJq2kYv5+moqCTTXWwaj9cqHMkCurAgKXhu9ANv0YzeobavNiOKpXwBA8QEDUG2Hpnk8DhUrZiiFTho0sl5Kw94t/9DDcio6hF7p2mTyCmwPeazDs+OMbUDV+QhXdG9RRe/vulRDTo6Hd+01QrxMcDIA= 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=fmIAlIa0; arc=none smtp.client-ip=170.10.129.124 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="fmIAlIa0" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1780682729; 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=hAOg9orqrzODNnWCYrZ2iDbkCKOJXEC8T7V/aprMX4g=; b=fmIAlIa02usViv+PCaVYZ96Je5KHzCCxNrdbUexemntbc38DLkZOLXqvYurhOHmdZ8EMsO 2p2/rrJ9klukaXBW0cizgBiueO369SGWVbikeP5LTNXMVA/U4vksJI5e70e0Sxh6Ei/RWG k7On5DbEJDqu7qBiXaR8YX6awZDjYQ0= 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-253-mrtGH3t3MziWxKa5ynLtnA-1; Fri, 05 Jun 2026 14:05:23 -0400 X-MC-Unique: mrtGH3t3MziWxKa5ynLtnA-1 X-Mimecast-MFC-AGG-ID: mrtGH3t3MziWxKa5ynLtnA_1780682722 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (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 3CB3019560AE; Fri, 5 Jun 2026 18:05:22 +0000 (UTC) Received: from [10.22.88.138] (unknown [10.22.88.138]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id B89BE180049F; Fri, 5 Jun 2026 18:05:20 +0000 (UTC) Message-ID: <6789ae0a-3c87-4d62-970f-2662e0050062@redhat.com> Date: Fri, 5 Jun 2026 14:05:19 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] usb: hub: Make usb_hub_wq type depend on isolcpus/nohz_full setting To: Frederic Weisbecker Cc: Greg Kroah-Hartman , Mathias Nyman , Alan Stern , Kuen-Han Tsai , linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, Vratislav Bendel References: <20260521170659.984284-1-longman@redhat.com> Content-Language: en-US From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 On 5/27/26 10:21 AM, Frederic Weisbecker wrote: > Le Thu, May 21, 2026 at 01:06:59PM -0400, Waiman Long a écrit : >> A Red Hat customer reports a kernel stability problem where hung tasks >> are reported with occasional kernel panics. Analysis of the core dump >> indicates that USB work items are running on isolcpus+nohz_full cores >> competing with RT-class tasks running on those core while holding >> usb_hub device mutex transitively blocking other kworkers waiting for >> the same mutex leading to hung_task reports. >> >> As the usb_hub_wq uses the WQ_PERCPU flag, it will run the work items on >> the same CPU that queues them. For many use cases, it is a more efficient >> setup leading to higher throughput as it reduces cacheline bouncing. >> >> It is a different story if the system needs to run latency sensitive RT >> workload on dedicated isolated CPUs. Having the kworkers processing work >> items on the same set of isolated CPUs will likely break the low latency >> requirements of the RT tasks. As the RT tasks have higher priority, >> not much CPU time will be left running the kworkers to process work >> items which, in turn, will block other tasks that have dependency on >> the completion of those work items. In this case, using a WQ_UNBOUND >> workqueue to avoid running on isolated CPUs will be more beneficial. >> >> One solution to get the best of both worlds is to make the workqueue >> type depending on whether the "isolcpus" or "nohz_full" boot command >> line options have been specified. If at least one of those options are >> present, usb_hub_wq will be created as an unbound workqueue. Otherwise, >> it will remain as a percpu workqueue. >> >> Signed-off-by: Waiman Long >> --- >> drivers/usb/core/hub.c | 14 +++++++++++++- >> 1 file changed, 13 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c >> index 24960ba9caa9..f79e5edd627a 100644 >> --- a/drivers/usb/core/hub.c >> +++ b/drivers/usb/core/hub.c >> @@ -33,6 +33,7 @@ >> #include >> #include >> #include >> +#include >> >> #include >> #include >> @@ -6066,6 +6067,8 @@ static struct usb_driver hub_driver = { >> >> int usb_hub_init(void) >> { >> + unsigned int wq_flags; >> + >> if (usb_register(&hub_driver) < 0) { >> printk(KERN_ERR "%s: can't register hub driver\n", >> usbcore_name); >> @@ -6077,8 +6080,17 @@ int usb_hub_init(void) >> * USB-PERSIST port handover. Otherwise it might see that a full-speed >> * device was gone before the EHCI controller had handed its port >> * over to the companion full-speed controller. >> + * >> + * Create WQ_UNBOUND workqueue instead of WQ_PERCPU if either isolcpus >> + * or nohz_full boot option is specified. >> */ >> - hub_wq = alloc_workqueue("usb_hub_wq", WQ_FREEZABLE | WQ_PERCPU, 0); >> + if (housekeeping_enabled(HK_TYPE_DOMAIN) || >> + housekeeping_enabled(HK_TYPE_KERNEL_NOISE)) > HK_TYPE_DOMAIN is supposed to be a subset of HK_TYPE_KERNEL_NOISE anyway so > the first should be enough. Yes, that is the ideal case. However, that is not currently what some users are doing as they haven't changed their setup yet. > >> + wq_flags = WQ_UNBOUND; >> + else >> + wq_flags = WQ_PERCPU; >> + >> + hub_wq = alloc_workqueue("usb_hub_wq", WQ_FREEZABLE | wq_flags, 0); > But then what happens if no isolcpus= is passed but later cpuset creates > an isolated partition? > > Tejun and Marco thought about introducing a WQ_PREFER_PERCPU flag that would > do what you want above. And the workqueue code should also handle dynamic > isolation, that is switch from per-cpu workqueues to unbound ones or vice-versa > dynamically. > > Marco is working on it. I was thinking that it will be done with a later patch to make it works with dynamically isolated CPUs. Good to hear that Tejun and Macro are working with a new WQ flag. Will switch to the new flag once it is available. However, the current patch will still be useful especially for backporting to older distro kernels. Cheers, Longman > Thanks. > >> if (hub_wq) >> return 0; >> >> -- >> 2.54.0 >> >>