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Thu, 27 Apr 2023 07:08:46 +0000 (GMT) Message-ID: <2bc7d559-d827-bae9-33e0-4b7ca172ce5d@linux.ibm.com> Date: Thu, 27 Apr 2023 12:38:45 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: [PATCH v21 0/7] crash: Kernel handling of CPU and memory hot un/plug Content-Language: en-US To: Eric DeVolder , 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, sourabhjain@linux.ibm.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com References: <20230404180326.6890-1-eric.devolder@oracle.com> From: Hari Bathini In-Reply-To: <20230404180326.6890-1-eric.devolder@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: LjrBv1jfrpPeAR5_o7oQjwI0FVfREAjz X-Proofpoint-GUID: YlfV2DA4ospcZQkqGsyhCpEdHhEnnNId Content-Transfer-Encoding: 7bit X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 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=2023-04-27_05,2023-04-26_03,2023-02-09_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 adultscore=0 priorityscore=1501 bulkscore=0 clxscore=1011 mlxscore=0 lowpriorityscore=0 mlxlogscore=999 phishscore=0 spamscore=0 suspectscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2303200000 definitions=main-2304270061 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Eric, On 04/04/23 11:33 pm, Eric DeVolder wrote: > Once the kdump service is loaded, if changes to CPUs or memory occur, > either by hot un/plug or off/onlining, the crash elfcorehdr must also > be updated. > > The elfcorehdr describes to kdump the CPUs and memory in the system, > and any inaccuracies can result in a vmcore with missing CPU context > or memory regions. > > The current solution utilizes udev to initiate an unload-then-reload > of the kdump image (eg. kernel, initrd, boot_params, purgatory and > elfcorehdr) by the userspace kexec utility. In the original post I > outlined the significant performance problems related to offloading > this activity to userspace. > > This patchset introduces a generic crash handler that registers with > the CPU and memory notifiers. Upon CPU or memory changes, from either > hot un/plug or off/onlining, this generic handler is invoked and > performs important housekeeping, for example obtaining the appropriate > lock, and then invokes an architecture specific handler to do the > appropriate elfcorehdr update. > > Note the description in patch 'crash: change crash_prepare_elf64_headers() > to for_each_possible_cpu()' and 'x86/crash: optimize CPU changes' that > enables further optimizations related to CPU plug/unplug/online/offline > performance of elfcorehdr updates. > > In the case of x86_64, the arch specific handler generates a new > elfcorehdr, and overwrites the old one in memory; thus no involvement > with userspace needed. > > To realize the benefits/test this patchset, one must make a couple > of minor changes to userspace: > > - Prevent udev from updating kdump crash kernel on hot un/plug changes. > Add the following as the first lines to the RHEL udev rule file > /usr/lib/udev/rules.d/98-kexec.rules: > > # The kernel updates the crash elfcorehdr for CPU and memory changes > SUBSYSTEM=="cpu", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end" > SUBSYSTEM=="memory", ATTRS{crash_hotplug}=="1", GOTO="kdump_reload_end" > > With this changeset applied, the two rules evaluate to false for > CPU and memory change events and thus skip the userspace > unload-then-reload of kdump. > > - Change to the kexec_file_load for loading the kdump kernel: > Eg. on RHEL: in /usr/bin/kdumpctl, change to: > standard_kexec_args="-p -d -s" > which adds the -s to select kexec_file_load() syscall. > > This kernel patchset also supports kexec_load() with a modified kexec > userspace utility. A working changeset to the kexec userspace utility > is posted to the kexec-tools mailing list here: > > http://lists.infradead.org/pipermail/kexec/2022-October/026032.html With the in-kernel update, the size is anyway getting calculated in kernel for kexec_file_load case, how about passing the recommended size for elfcorehdr segment and any other segment that needs an in-kernel update as a sysfs attribute instead of kexec-tools having to do the calculation again in the userspace (get_elfcorehdrsz()) for kexec_load case. That makes segment size calculation less error prone. Thanks Hari