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Wed, 11 Dec 2024 10:45:52 +0000 (GMT) Received: from nasanex01c.na.qualcomm.com (nasanex01c.na.qualcomm.com [10.45.79.139]) by NASANPPMTA01.qualcomm.com (8.18.1.2/8.18.1.2) with ESMTPS id 4BBAjpkv004326 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Wed, 11 Dec 2024 10:45:51 GMT Received: from [10.239.133.66] (10.80.80.8) by nasanex01c.na.qualcomm.com (10.45.79.139) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.9; Wed, 11 Dec 2024 02:45:48 -0800 Message-ID: <1b9b0482-9a6c-4182-ae87-9202f15725ca@quicinc.com> Date: Wed, 11 Dec 2024 18:45:39 +0800 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: [RFC PATCH] sched/core: Enhanced debug logs in do_task_dead() To: Peter Zijlstra CC: , , , , , , , , , Zhongqiu Han References: <20241210134513.2179786-1-quic_zhonhan@quicinc.com> <20241210141823.GU35539@noisy.programming.kicks-ass.net> Content-Language: en-US From: Zhongqiu Han In-Reply-To: <20241210141823.GU35539@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nasanex01c.na.qualcomm.com (10.45.79.139) X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-ORIG-GUID: m60oAYt6exWg6lJHcuMaaOfL0G4_KvyQ X-Proofpoint-GUID: m60oAYt6exWg6lJHcuMaaOfL0G4_KvyQ X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1039,Hydra:6.0.680,FMLib:17.12.60.29 definitions=2024-09-06_09,2024-09-06_01,2024-09-02_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 bulkscore=0 suspectscore=0 priorityscore=1501 adultscore=0 mlxlogscore=999 clxscore=1015 spamscore=0 mlxscore=0 lowpriorityscore=0 phishscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2411120000 definitions=main-2412110081 On 12/10/2024 10:18 PM, Peter Zijlstra wrote: > On Tue, Dec 10, 2024 at 09:45:13PM +0800, Zhongqiu Han wrote: >> If BUG() is a NOP, dump the problematic stack for debugging purposes. >> >> Signed-off-by: Zhongqiu Han >> --- >> If BUG() is a NOP, it should make sense for debugging purposes. However, >> just arising the patch with RFC, because at least for now, I haven't found >> a definition of BUG() as NOP in various architectures. Thanks~ > > Yeah, this don't make sense. If you want a stack-trace you shouldn't > have killed BUG. > > And yeah, having done a quick peek, I don't see how you can kill BUG > these days other than explicitly modyfing your source, in which case you > get to keep the pieces. Hi Peter， Thanks a lot for your review and reply! My initial thought was that if BUG() is a NOP, we should dump the stack for debugging purposes before executing cpu_relax(), because according to the current do_task_dead() code comments, BUG() might be a NOP in some cases. void __noreturn do_task_dead(void) { /* Causes final put_task_struct in finish_task_switch(): */ set_special_state(TASK_DEAD); /* Tell freezer to ignore us: */ current->flags |= PF_NOFREEZE; __schedule(SM_NONE); BUG(); /* Avoid "noreturn function does return" - but don't continue if BUG() is a NOP: */-------------------> This comments for (;;) cpu_relax(); } For example, in the code _before_ 7/4/2014 commit a4b5d580e078 ('bug: Make BUG() always stop the machine')., BUG() is indeed a NOP if ( !CONFIG_BUG && !HAVE_ARCH_BUG) #else /* !CONFIG_BUG */ #ifndef HAVE_ARCH_BUG -#define BUG() do {} while (0) +#define BUG() do {} while (1) #endif However, since 10/4/2024 commit 5284984a4fba ('bug: Fix no-return statement warning with !CONFIG_BUG'), with !CONFIG_BUG, the default BUG() is implemented to call unreachable() to ensure it does not return, and if all architecture-specific implementations of BUG() also do not return, I now think my thought should be meaningless. commit 5284984a4fba ('bug: Fix no-return-statement warning with !CONFIG_BUG') #else /* !CONFIG_BUG */ #ifndef HAVE_ARCH_BUG -#define BUG() do {} while (1) +#define BUG() do { \ + do {} while (1); \ + unreachable(); \ +} while (0) #endif By the way, do you think it is reasonable to remove the cpu_relax code by evaluating whether BUG() does not return in all architectures and configurations? Additionally, _if_ some architecture-specific implementations of BUG() do not have the capability to output the thread stack, should we consider using the panic() function as a replacement? Thanks for your time. -- Thx and BRs, Zhongqiu Han