From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AB8JxZoB9Hz+ywWYgSekr2ztsXKsfBXHztSL8Lpxv+qNtHOrjTTxiW52l3HExjcsRp4kwpzTnRas ARC-Seal: i=1; a=rsa-sha256; t=1525714664; cv=none; d=google.com; s=arc-20160816; b=Sx79ZEhFeP7WdcZEO6m8V/Bve4WlC5orOabqnQGVRZFOYz72G5kB4onDTFRfd9oohc 7vwDDAZFkI5KhcyOxwVrUDVjefJjlyF3T+CUDR2bn92SQ7vFB+tNZs+ItCItebz+5g9G RQTyNsMaNs6RMFCLdZGk4TUl/JJppigjfLOcV6HfCG9tBrUyBZUZJQnuORas840Uvofw 7bARvKm+phk3yGdcVVPmg+WmBKeu9LZ5Tj1cB5qCP1r9D1KESOr2RXlG63rf0mBHXftZ 5GYcXC/6GIOd0fj72DQaxyN+ps3N3eSPniNYOXsqtqKnVYxytJT8SntQjnQVl1EVe9+/ /MQw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=message-id:content-transfer-encoding:content-language:in-reply-to :mime-version:user-agent:date:from:references:cc:to:subject :arc-authentication-results; bh=qVbVXkmcxEV4v9zJ+GiUVp7rlLCLgcPut7oSBdW5oTs=; b=vrC3mtNR0l+OyhwPwNiGq8f/sj2DN9x2prFib+3tvdMbPycR+o33R2LAA5QiJdsiVk isGWSFdm14/3niOEFMwI+M+U9/RW3HhH+eAMMSBDMs0JQ5PzLzVVABsmbb9a1esjIDCt R+rHH2gV4peb/wrVPVrZ2wXITDyaa4p6PpqZD9oJ/YD6aGx2lPyu4uWefu+e97Fw6f9v AhbmaGaaifopCWZU3Ip3/1bnTtqnwT592YbRepSjdq24fQ2HxqHqA8O9WXFTqsVfIynw Oeao8px3E56KhTyLvnHScaN7Jg3XYIl6c5wq/2wsxIGN+4qtupkDfTL2ejMb0hpNt9ck +YBA== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of fbarrat@linux.ibm.com designates 148.163.158.5 as permitted sender) smtp.mailfrom=fbarrat@linux.ibm.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=ibm.com Authentication-Results: mx.google.com; spf=pass (google.com: domain of fbarrat@linux.ibm.com designates 148.163.158.5 as permitted sender) smtp.mailfrom=fbarrat@linux.ibm.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=ibm.com Subject: Re: [PATCH v2 3/7] powerpc: use task_pid_nr() for TID allocation To: "Alastair D'Silva" , linuxppc-dev@lists.ozlabs.org Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, mikey@neuling.org, vaibhav@linux.vnet.ibm.com, aneesh.kumar@linux.vnet.ibm.com, malat@debian.org, felix@linux.vnet.ibm.com, pombredanne@nexb.com, sukadev@linux.vnet.ibm.com, npiggin@gmail.com, gregkh@linuxfoundation.org, arnd@arndb.de, andrew.donnellan@au1.ibm.com, fbarrat@linux.vnet.ibm.com, corbet@lwn.net, "Alastair D'Silva" References: <20180417020950.21446-1-alastair@au1.ibm.com> <20180418010810.30937-1-alastair@au1.ibm.com> <20180418010810.30937-4-alastair@au1.ibm.com> From: Frederic Barrat Date: Mon, 7 May 2018 19:37:33 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180418010810.30937-4-alastair@au1.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 x-cbid: 18050717-0020-0000-0000-0000041A0C3C X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18050717-0021-0000-0000-000042AF40E1 Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-07_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1011 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805070177 X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1597957550726280592?= X-GMAIL-MSGID: =?utf-8?q?1599827779662609793?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: Le 18/04/2018 à 03:08, Alastair D'Silva a écrit : > From: Alastair D'Silva > > The current implementation of TID allocation, using a global IDR, may > result in an errant process starving the system of available TIDs. > Instead, use task_pid_nr(), as mentioned by the original author. The > scenario described which prevented it's use is not applicable, as > set_thread_tidr can only be called after the task struct has been > populated. Here is how I understand what's going to happen if 2 threads are using the same TIDR value, which is possible with this patch (if unlikely): 1. waking up the wrong thread is not really a problem, as threads have to handle spurious wake up from the 'wait' instruction anyway, and must be using some other condition to know when to loop around the 'wait' instruction. 2. missing the right thread: if the wrong thread is on a CPU, and a wake_host_thread/as_notify is sent, the core will see a matching thread and will accept the command. The (open)capi adapter won't send an interrupt. The wrong thread is awaken, which is not a problem as discussed above. As the right thread to notify is not running, no harm is done either: as soon as the thread runs, it's supposed to check its condition (which will be met) or call 'wait', but 'wait' immediately returns when called the first time after a thread is scheduled. So I believe we are ok. But I think it requires a huge comment with the above (at the minimum) :-) With a comment: Reviewed-by: Frederic Barrat Fred > Signed-off-by: Alastair D'Silva > --- > arch/powerpc/include/asm/switch_to.h | 1 - > arch/powerpc/kernel/process.c | 97 +----------------------------------- > 2 files changed, 1 insertion(+), 97 deletions(-) > > diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h > index be8c9fa23983..5b03d8a82409 100644 > --- a/arch/powerpc/include/asm/switch_to.h > +++ b/arch/powerpc/include/asm/switch_to.h > @@ -94,6 +94,5 @@ static inline void clear_task_ebb(struct task_struct *t) > extern int set_thread_uses_vas(void); > > extern int set_thread_tidr(struct task_struct *t); > -extern void clear_thread_tidr(struct task_struct *t); > > #endif /* _ASM_POWERPC_SWITCH_TO_H */ > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c > index 3b00da47699b..87f047fd2762 100644 > --- a/arch/powerpc/kernel/process.c > +++ b/arch/powerpc/kernel/process.c > @@ -1496,103 +1496,12 @@ int set_thread_uses_vas(void) > } > > #ifdef CONFIG_PPC64 > -static DEFINE_SPINLOCK(vas_thread_id_lock); > -static DEFINE_IDA(vas_thread_ida); > - > -/* > - * We need to assign a unique thread id to each thread in a process. > - * > - * This thread id, referred to as TIDR, and separate from the Linux's tgid, > - * is intended to be used to direct an ASB_Notify from the hardware to the > - * thread, when a suitable event occurs in the system. > - * > - * One such event is a "paste" instruction in the context of Fast Thread > - * Wakeup (aka Core-to-core wake up in the Virtual Accelerator Switchboard > - * (VAS) in POWER9. > - * > - * To get a unique TIDR per process we could simply reuse task_pid_nr() but > - * the problem is that task_pid_nr() is not yet available copy_thread() is > - * called. Fixing that would require changing more intrusive arch-neutral > - * code in code path in copy_process()?. > - * > - * Further, to assign unique TIDRs within each process, we need an atomic > - * field (or an IDR) in task_struct, which again intrudes into the arch- > - * neutral code. So try to assign globally unique TIDRs for now. > - * > - * NOTE: TIDR 0 indicates that the thread does not need a TIDR value. > - * For now, only threads that expect to be notified by the VAS > - * hardware need a TIDR value and we assign values > 0 for those. > - */ > -#define MAX_THREAD_CONTEXT ((1 << 16) - 1) > -static int assign_thread_tidr(void) > -{ > - int index; > - int err; > - unsigned long flags; > - > -again: > - if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL)) > - return -ENOMEM; > - > - spin_lock_irqsave(&vas_thread_id_lock, flags); > - err = ida_get_new_above(&vas_thread_ida, 1, &index); > - spin_unlock_irqrestore(&vas_thread_id_lock, flags); > - > - if (err == -EAGAIN) > - goto again; > - else if (err) > - return err; > - > - if (index > MAX_THREAD_CONTEXT) { > - spin_lock_irqsave(&vas_thread_id_lock, flags); > - ida_remove(&vas_thread_ida, index); > - spin_unlock_irqrestore(&vas_thread_id_lock, flags); > - return -ENOMEM; > - } > - > - return index; > -} > - > -static void free_thread_tidr(int id) > -{ > - unsigned long flags; > - > - spin_lock_irqsave(&vas_thread_id_lock, flags); > - ida_remove(&vas_thread_ida, id); > - spin_unlock_irqrestore(&vas_thread_id_lock, flags); > -} > - > -/* > - * Clear any TIDR value assigned to this thread. > - */ > -void clear_thread_tidr(struct task_struct *t) > -{ > - if (!t->thread.tidr) > - return; > - > - if (!cpu_has_feature(CPU_FTR_P9_TIDR)) { > - WARN_ON_ONCE(1); > - return; > - } > - > - mtspr(SPRN_TIDR, 0); > - free_thread_tidr(t->thread.tidr); > - t->thread.tidr = 0; > -} > - > -void arch_release_task_struct(struct task_struct *t) > -{ > - clear_thread_tidr(t); > -} > - > /* > * Assign a unique TIDR (thread id) for task @t and set it in the thread > * structure. For now, we only support setting TIDR for 'current' task. > */ > int set_thread_tidr(struct task_struct *t) > { > - int rc; > - > if (!cpu_has_feature(CPU_FTR_P9_TIDR)) > return -EINVAL; > > @@ -1602,11 +1511,7 @@ int set_thread_tidr(struct task_struct *t) > if (t->thread.tidr) > return 0; > > - rc = assign_thread_tidr(); > - if (rc < 0) > - return rc; > - > - t->thread.tidr = rc; > + t->thread.tidr = (u16)task_pid_nr(t); > mtspr(SPRN_TIDR, t->thread.tidr); > > return 0; > From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 832537DE74 for ; Mon, 7 May 2018 17:37:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752721AbeEGRho (ORCPT ); Mon, 7 May 2018 13:37:44 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:50228 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752708AbeEGRho (ORCPT ); Mon, 7 May 2018 13:37:44 -0400 Received: from pps.filterd (m0098417.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w47HYRce044765 for ; Mon, 7 May 2018 13:37:43 -0400 Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) by mx0a-001b2d01.pphosted.com with ESMTP id 2htsmfcjpg-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Mon, 07 May 2018 13:37:42 -0400 Received: from localhost by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; Mon, 7 May 2018 18:37:37 +0100 Received: from d06av22.portsmouth.uk.ibm.com (d06av22.portsmouth.uk.ibm.com [9.149.105.58]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id w47HbaJf8192400; Mon, 7 May 2018 17:37:36 GMT Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B52024C04E; Mon, 7 May 2018 18:29:38 +0100 (BST) Received: from d06av22.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 92DE94C044; Mon, 7 May 2018 18:29:36 +0100 (BST) Received: from [9.167.245.55] (unknown [9.167.245.55]) by d06av22.portsmouth.uk.ibm.com (Postfix) with ESMTP; Mon, 7 May 2018 18:29:36 +0100 (BST) Subject: Re: [PATCH v2 3/7] powerpc: use task_pid_nr() for TID allocation To: "Alastair D'Silva" , linuxppc-dev@lists.ozlabs.org Cc: linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, mikey@neuling.org, vaibhav@linux.vnet.ibm.com, aneesh.kumar@linux.vnet.ibm.com, malat@debian.org, felix@linux.vnet.ibm.com, pombredanne@nexb.com, sukadev@linux.vnet.ibm.com, npiggin@gmail.com, gregkh@linuxfoundation.org, arnd@arndb.de, andrew.donnellan@au1.ibm.com, fbarrat@linux.vnet.ibm.com, corbet@lwn.net, "Alastair D'Silva" References: <20180417020950.21446-1-alastair@au1.ibm.com> <20180418010810.30937-1-alastair@au1.ibm.com> <20180418010810.30937-4-alastair@au1.ibm.com> From: Frederic Barrat Date: Mon, 7 May 2018 19:37:33 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180418010810.30937-4-alastair@au1.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 x-cbid: 18050717-0020-0000-0000-0000041A0C3C X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18050717-0021-0000-0000-000042AF40E1 Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-07_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805070177 Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org Le 18/04/2018 à 03:08, Alastair D'Silva a écrit : > From: Alastair D'Silva > > The current implementation of TID allocation, using a global IDR, may > result in an errant process starving the system of available TIDs. > Instead, use task_pid_nr(), as mentioned by the original author. The > scenario described which prevented it's use is not applicable, as > set_thread_tidr can only be called after the task struct has been > populated. Here is how I understand what's going to happen if 2 threads are using the same TIDR value, which is possible with this patch (if unlikely): 1. waking up the wrong thread is not really a problem, as threads have to handle spurious wake up from the 'wait' instruction anyway, and must be using some other condition to know when to loop around the 'wait' instruction. 2. missing the right thread: if the wrong thread is on a CPU, and a wake_host_thread/as_notify is sent, the core will see a matching thread and will accept the command. The (open)capi adapter won't send an interrupt. The wrong thread is awaken, which is not a problem as discussed above. As the right thread to notify is not running, no harm is done either: as soon as the thread runs, it's supposed to check its condition (which will be met) or call 'wait', but 'wait' immediately returns when called the first time after a thread is scheduled. So I believe we are ok. But I think it requires a huge comment with the above (at the minimum) :-) With a comment: Reviewed-by: Frederic Barrat Fred > Signed-off-by: Alastair D'Silva > --- > arch/powerpc/include/asm/switch_to.h | 1 - > arch/powerpc/kernel/process.c | 97 +----------------------------------- > 2 files changed, 1 insertion(+), 97 deletions(-) > > diff --git a/arch/powerpc/include/asm/switch_to.h b/arch/powerpc/include/asm/switch_to.h > index be8c9fa23983..5b03d8a82409 100644 > --- a/arch/powerpc/include/asm/switch_to.h > +++ b/arch/powerpc/include/asm/switch_to.h > @@ -94,6 +94,5 @@ static inline void clear_task_ebb(struct task_struct *t) > extern int set_thread_uses_vas(void); > > extern int set_thread_tidr(struct task_struct *t); > -extern void clear_thread_tidr(struct task_struct *t); > > #endif /* _ASM_POWERPC_SWITCH_TO_H */ > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c > index 3b00da47699b..87f047fd2762 100644 > --- a/arch/powerpc/kernel/process.c > +++ b/arch/powerpc/kernel/process.c > @@ -1496,103 +1496,12 @@ int set_thread_uses_vas(void) > } > > #ifdef CONFIG_PPC64 > -static DEFINE_SPINLOCK(vas_thread_id_lock); > -static DEFINE_IDA(vas_thread_ida); > - > -/* > - * We need to assign a unique thread id to each thread in a process. > - * > - * This thread id, referred to as TIDR, and separate from the Linux's tgid, > - * is intended to be used to direct an ASB_Notify from the hardware to the > - * thread, when a suitable event occurs in the system. > - * > - * One such event is a "paste" instruction in the context of Fast Thread > - * Wakeup (aka Core-to-core wake up in the Virtual Accelerator Switchboard > - * (VAS) in POWER9. > - * > - * To get a unique TIDR per process we could simply reuse task_pid_nr() but > - * the problem is that task_pid_nr() is not yet available copy_thread() is > - * called. Fixing that would require changing more intrusive arch-neutral > - * code in code path in copy_process()?. > - * > - * Further, to assign unique TIDRs within each process, we need an atomic > - * field (or an IDR) in task_struct, which again intrudes into the arch- > - * neutral code. So try to assign globally unique TIDRs for now. > - * > - * NOTE: TIDR 0 indicates that the thread does not need a TIDR value. > - * For now, only threads that expect to be notified by the VAS > - * hardware need a TIDR value and we assign values > 0 for those. > - */ > -#define MAX_THREAD_CONTEXT ((1 << 16) - 1) > -static int assign_thread_tidr(void) > -{ > - int index; > - int err; > - unsigned long flags; > - > -again: > - if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL)) > - return -ENOMEM; > - > - spin_lock_irqsave(&vas_thread_id_lock, flags); > - err = ida_get_new_above(&vas_thread_ida, 1, &index); > - spin_unlock_irqrestore(&vas_thread_id_lock, flags); > - > - if (err == -EAGAIN) > - goto again; > - else if (err) > - return err; > - > - if (index > MAX_THREAD_CONTEXT) { > - spin_lock_irqsave(&vas_thread_id_lock, flags); > - ida_remove(&vas_thread_ida, index); > - spin_unlock_irqrestore(&vas_thread_id_lock, flags); > - return -ENOMEM; > - } > - > - return index; > -} > - > -static void free_thread_tidr(int id) > -{ > - unsigned long flags; > - > - spin_lock_irqsave(&vas_thread_id_lock, flags); > - ida_remove(&vas_thread_ida, id); > - spin_unlock_irqrestore(&vas_thread_id_lock, flags); > -} > - > -/* > - * Clear any TIDR value assigned to this thread. > - */ > -void clear_thread_tidr(struct task_struct *t) > -{ > - if (!t->thread.tidr) > - return; > - > - if (!cpu_has_feature(CPU_FTR_P9_TIDR)) { > - WARN_ON_ONCE(1); > - return; > - } > - > - mtspr(SPRN_TIDR, 0); > - free_thread_tidr(t->thread.tidr); > - t->thread.tidr = 0; > -} > - > -void arch_release_task_struct(struct task_struct *t) > -{ > - clear_thread_tidr(t); > -} > - > /* > * Assign a unique TIDR (thread id) for task @t and set it in the thread > * structure. For now, we only support setting TIDR for 'current' task. > */ > int set_thread_tidr(struct task_struct *t) > { > - int rc; > - > if (!cpu_has_feature(CPU_FTR_P9_TIDR)) > return -EINVAL; > > @@ -1602,11 +1511,7 @@ int set_thread_tidr(struct task_struct *t) > if (t->thread.tidr) > return 0; > > - rc = assign_thread_tidr(); > - if (rc < 0) > - return rc; > - > - t->thread.tidr = rc; > + t->thread.tidr = (u16)task_pid_nr(t); > mtspr(SPRN_TIDR, t->thread.tidr); > > return 0; > -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html