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crash_prepare_elf64_headers() to for_each_possible_cpu() Content-Language: en-US To: Sourabh Jain , linux-kernel@vger.kernel.org, x86@kernel.org, kexec@lists.infradead.org, ebiederm@xmission.com, dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com Cc: tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, nramas@linux.microsoft.com, thomas.lendacky@amd.com, robh@kernel.org, efault@gmx.de, rppt@kernel.org, david@redhat.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com References: <20230306162228.8277-1-eric.devolder@oracle.com> <20230306162228.8277-7-eric.devolder@oracle.com> From: Eric DeVolder In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: DS7PR03CA0278.namprd03.prod.outlook.com (2603:10b6:5:3ad::13) To CO1PR10MB4531.namprd10.prod.outlook.com (2603:10b6:303:6c::22) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: 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X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 151kEJsQ31aEGkwl61RuM8dWqQU3oD0jfIeBmcegjWnVG1XV63h3aJCTQoc6aec6QU4YzDHNGAhP4i5PlI06uQhlZMzjWB28Ox7jTuPt1zs= X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW4PR10MB6535 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.254,Aquarius:18.0.942,Hydra:6.0.573,FMLib:17.11.170.22 definitions=2023-03-17_14,2023-03-16_02,2023-02-09_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 bulkscore=0 adultscore=0 phishscore=0 mlxscore=0 malwarescore=0 mlxlogscore=999 spamscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2303150002 definitions=main-2303170132 X-Proofpoint-GUID: YhnVoMhfPVPYpoDwQrM4FBquCCrYZz_a X-Proofpoint-ORIG-GUID: YhnVoMhfPVPYpoDwQrM4FBquCCrYZz_a Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/7/23 02:48, Sourabh Jain wrote: > Hello Eric, > > On 06/03/23 21:52, Eric DeVolder wrote: >> The function crash_prepare_elf64_headers() generates the elfcorehdr >> which describes the cpus and memory in the system for the crash kernel. >> In particular, it writes out ELF PT_NOTEs for memory regions and the >> cpus in the system. >> >> With respect to the cpus, the current implementation utilizes >> for_each_present_cpu() which means that as cpus are added and removed, >> the elfcorehdr must again be updated to reflect the new set of cpus. >> >> The reasoning behind the change to use for_each_possible_cpu(), is: >> >> - At kernel boot time, all percpu crash_notes are allocated for all >>    possible cpus; that is, crash_notes are not allocated dynamically >>    when cpus are plugged/unplugged. Thus the crash_notes for each >>    possible cpu are always available. >> >> - The crash_prepare_elf64_headers() creates an ELF PT_NOTE per cpu. >>    Changing to for_each_possible_cpu() is valid as the crash_notes >>    pointed to by each cpu PT_NOTE are present and always valid. >> >> Furthermore, examining a common crash processing path of: >> >>   kernel panic -> crash kernel -> makedumpfile -> 'crash' analyzer >>             elfcorehdr      /proc/vmcore     vmcore >> >> reveals how the ELF cpu PT_NOTEs are utilized: >> >> - Upon panic, each cpu is sent an IPI and shuts itself down, recording >>   its state in its crash_notes. When all cpus are shutdown, the >>   crash kernel is launched with a pointer to the elfcorehdr. >> >> - The crash kernel via linux/fs/proc/vmcore.c does not examine or >>   use the contents of the PT_NOTEs, it exposes them via /proc/vmcore. >> >> - The makedumpfile utility uses /proc/vmcore and reads the cpu >>   PT_NOTEs to craft a nr_cpus variable, which is reported in a >>   header but otherwise generally unused. Makedumpfile creates the >>   vmcore. >> >> - The 'crash' dump analyzer does not appear to reference the cpu >>   PT_NOTEs. Instead it looks-up the cpu_[possible|present|onlin]_mask >>   symbols and directly examines those structure contents from vmcore >>   memory. From that information it is able to determine which cpus >>   are present and online, and locate the corresponding crash_notes. >>   Said differently, it appears that 'crash' analyzer does not rely >>   on the ELF PT_NOTEs for cpus; rather it obtains the information >>   directly via kernel symbols and the memory within the vmcore. >> >> (There maybe other vmcore generating and analysis tools that do use >> these PT_NOTEs, but 'makedumpfile' and 'crash' seems to be the most >> common solution.) >> >> This change results in the benefit of having all cpus described in >> the elfcorehdr, and therefore reducing the need to re-generate the >> elfcorehdr on cpu changes, at the small expense of an additional >> 56 bytes per PT_NOTE for not-present-but-possible cpus. >> >> On systems where kexec_file_load() syscall is utilized, all the above >> is valid. On systems where kexec_load() syscall is utilized, there >> may be the need for the elfcorehdr to be regenerated once. The reason >> being that some archs only populate the 'present' cpus in the >> /sys/devices/system/cpus entries, which the userspace 'kexec' utility >> uses to generate the userspace-supplied elfcorehdr. In this situation, >> one memory or cpu change will rewrite the elfcorehdr via the >> crash_prepare_elf64_headers() function and now all possible cpus will >> be described, just as with kexec_file_load() syscall. >> >> Suggested-by: Sourabh Jain >> Signed-off-by: Eric DeVolder >> --- >>   kernel/crash_core.c | 2 +- >>   1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kernel/crash_core.c b/kernel/crash_core.c >> index dba4b75f7541..537b199a8774 100644 >> --- a/kernel/crash_core.c >> +++ b/kernel/crash_core.c >> @@ -365,7 +365,7 @@ int crash_prepare_elf64_headers(struct crash_mem *mem, int need_kernel_map, >>       ehdr->e_phentsize = sizeof(Elf64_Phdr); >>       /* Prepare one phdr of type PT_NOTE for each present CPU */ > We need to change this comment as well. Done, thanks! eric >> -    for_each_present_cpu(cpu) { >> +    for_each_possible_cpu(cpu) { > - Sourabh Jain