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Wed, 27 Aug 2025 07:08:53 -0700 (PDT) Message-ID: <98afe1bd-99d2-4b5d-866a-e9541390fab4@linaro.org> Date: Wed, 27 Aug 2025 17:08:51 +0300 Precedence: bulk X-Mailing-List: linux-arm-msm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC][PATCH v2 22/29] mm/numa: Register information into Kmemdump To: David Hildenbrand , Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, tglx@linutronix.de, andersson@kernel.org, pmladek@suse.com, linux-arm-kernel@lists.infradead.org, linux-hardening@vger.kernel.org, corbet@lwn.net, mojha@qti.qualcomm.com, rostedt@goodmis.org, jonechou@google.com, tudor.ambarus@linaro.org, Christoph Hellwig , Sergey Senozhatsky References: <20250724135512.518487-1-eugen.hristev@linaro.org> <20250724135512.518487-23-eugen.hristev@linaro.org> <751514db-9e03-4cf3-bd3e-124b201bdb94@redhat.com> <23e7ec80-622e-4d33-a766-312c1213e56b@redhat.com> <77d17dbf-1609-41b1-9244-488d2ce75b33@redhat.com> <9f13df6f-3b76-4d02-aa74-40b913f37a8a@redhat.com> <64a93c4a-5619-4208-9e9f-83848206d42b@linaro.org> <01c67173-818c-48cf-8515-060751074c37@linaro.org> <1b52419c-101b-487e-a961-97bd405c5c33@linaro.org> <99d2cc96-03ea-4026-883e-1ee083a96c39@redhat.com> From: Eugen Hristev Content-Language: en-US In-Reply-To: <99d2cc96-03ea-4026-883e-1ee083a96c39@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/27/25 15:18, David Hildenbrand wrote: > On 27.08.25 13:59, Eugen Hristev wrote: >> >> >> On 8/25/25 16:58, David Hildenbrand wrote: >>> On 25.08.25 15:36, Eugen Hristev wrote: >>>> >>>> >>>> On 8/25/25 16:20, David Hildenbrand wrote: >>>>> >>>>>>> >>>>>>> IIRC, kernel/vmcore_info.c is never built as a module, as it also >>>>>>> accesses non-exported symbols. >>>>>> >>>>>> Hello David, >>>>>> >>>>>> I am looking again into this, and there are some things which in my >>>>>> opinion would be difficult to achieve. >>>>>> For example I looked into my patch #11 , which adds the `runqueues` into >>>>>> kmemdump. >>>>>> >>>>>> The runqueues is a variable of `struct rq` which is defined in >>>>>> kernel/sched/sched.h , which is not supposed to be included outside of >>>>>> sched. >>>>>> Now moving all the struct definition outside of sched.h into another >>>>>> public header would be rather painful and I don't think it's a really >>>>>> good option (The struct would be needed to compute the sizeof inside >>>>>> vmcoreinfo). Secondly, it would also imply moving all the nested struct >>>>>> definitions outside as well. I doubt this is something that we want for >>>>>> the sched subsys. How the subsys is designed, out of my understanding, >>>>>> is to keep these internal structs opaque outside of it. >>>>> >>>>> All the kmemdump module needs is a start and a length, correct? So the >>>>> only tricky part is getting the length. >>>> >>>> I also have in mind the kernel user case. How would a kernel programmer >>>> want to add some kernel structs/info/buffers into kmemdump such that the >>>> dump would contain their data ? Having "KMEMDUMP_VAR(...)" looks simple >>>> enough. >>> >>> The other way around, why should anybody have a saying in adding their >>> data to kmemdump? Why do we have that all over the kernel? >>> >>> Is your mechanism really so special? >>> >>> A single composer should take care of that, and it's really just start + >>> len of physical memory areas. >>> >>>> Otherwise maybe the programmer has to write helpers to compute lengths >>>> etc, and stitch them into kmemdump core. >>>> I am not saying it's impossible, but just tiresome perhaps. >>> >>> In your patch set, how many of these instances did you encounter where >>> that was a problem? >>> >>>>> >>>>> One could just add a const variable that holds this information, or even >>>>> better, a simple helper function to calculate that. >>>>> >>>>> Maybe someone else reading along has a better idea. >>>> >>>> This could work, but it requires again adding some code into the >>>> specific subsystem. E.g. struct_rq_get_size() >>>> I am open to ideas , and thank you very much for your thoughts. >>>> >>>>> >>>>> Interestingly, runqueues is a percpu variable, which makes me wonder if >>>>> what you had would work as intended (maybe it does, not sure). >>>> >>>> I would not really need to dump the runqueues. But the crash tool which >>>> I am using for testing, requires it. Without the runqueues it will not >>>> progress further to load the kernel dump. >>>> So I am not really sure what it does with the runqueues, but it works. >>>> Perhaps using crash/gdb more, to actually do something with this data, >>>> would give more insight about its utility. >>>> For me, it is a prerequisite to run crash, and then to be able to >>>> extract the log buffer from the dump. >>> >>> I have the faint recollection that percpu vars might not be stored in a >>> single contiguous physical memory area, but maybe my memory is just >>> wrong, that's why I was raising it. >>> >>>> >>>>> >>>>>> >>>>>> From my perspective it's much simpler and cleaner to just add the >>>>>> kmemdump annotation macro inside the sched/core.c as it's done in my >>>>>> patch. This macro translates to a noop if kmemdump is not selected. >>>>> >>>>> I really don't like how we are spreading kmemdump all over the kernel, >>>>> and adding complexity with __section when really, all we need is a place >>>>> to obtain a start and a length. >>>>> >>>> >>>> I understand. The section idea was suggested by Thomas. Initially I was >>>> skeptic, but I like how it turned out. >>> >>> Yeah, I don't like it. Taste differs ;) >>> >>> I am in particular unhappy about custom memblock wrappers. >>> >>> [...] >>> >>>>>> >>>>>> To have this working outside of printk, it would be required to walk >>>>>> through all the printk structs/allocations and select the required info. >>>>>> Is this something that we want to do outside of printk ? >>>>> >>>>> I don't follow, please elaborate. >>>>> >>>>> How is e.g., log_buf_len_get() + log_buf_addr_get() not sufficient, >>>>> given that you run your initialization after setup_log_buf() ? >>>>> >>>>> >>>> >>>> My initial thought was the same. However I got some feedback from Petr >>>> Mladek here : >>>> >>>> https://lore.kernel.org/lkml/aBm5QH2p6p9Wxe_M@localhost.localdomain/ >>>> >>>> Where he explained how to register the structs correctly. >>>> It can be that setup_log_buf is called again at a later time perhaps. >>>> >>> >>> setup_log_buf() is a __init function, so there is only a certain time >>> frame where it can be called. >>> >>> In particular, once the buddy is up, memblock allocations are impossible >>> and it would be deeply flawed to call this function again. >>> >>> Let's not over-engineer this. >>> >>> Peter is on CC, so hopefully he can share his thoughts. >>> >> >> Hello David, >> >> I tested out this snippet (on top of my series, so you can see what I >> changed): >> >> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c >> index 18ba6c1e174f..7ac4248a00e5 100644 >> --- a/kernel/sched/core.c >> +++ b/kernel/sched/core.c >> @@ -67,7 +67,6 @@ >> #include >> #include >> #include >> -#include >> >> #ifdef CONFIG_PREEMPT_DYNAMIC >> # ifdef CONFIG_GENERIC_IRQ_ENTRY >> @@ -120,7 +119,12 @@ >> EXPORT_TRACEPOINT_SYMBOL_GPL(sched_update_nr_running_tp); >> EXPORT_TRACEPOINT_SYMBOL_GPL(sched_compute_energy_tp); >> >> DEFINE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues); >> -KMEMDUMP_VAR_CORE(runqueues, sizeof(runqueues)); >> + >> +size_t runqueues_get_size(void); >> +size_t runqueues_get_size(void) >> +{ >> + return sizeof(runqueues); >> +} >> >> #ifdef CONFIG_SCHED_PROXY_EXEC >> DEFINE_STATIC_KEY_TRUE(__sched_proxy_exec); >> diff --git a/kernel/vmcore_info.c b/kernel/vmcore_info.c >> index d808c5e67f35..c6dd2d6e96dd 100644 >> --- a/kernel/vmcore_info.c >> +++ b/kernel/vmcore_info.c >> @@ -24,6 +24,12 @@ >> #include "kallsyms_internal.h" >> #include "kexec_internal.h" >> >> +typedef void* kmemdump_opaque_t; >> + >> +size_t runqueues_get_size(void); >> + >> +extern kmemdump_opaque_t runqueues; > > I would have tried that through: > > struct rq; > extern struct rq runqueues; > > But the whole PER_CPU_SHARED_ALIGNED makes this all weird, and likely > not the way we would want to handle that. > >> /* vmcoreinfo stuff */ >> unsigned char *vmcoreinfo_data; >> size_t vmcoreinfo_size; >> @@ -230,6 +236,9 @@ static int __init crash_save_vmcoreinfo_init(void) >> >> kmemdump_register_id(KMEMDUMP_ID_COREIMAGE_VMCOREINFO, >> (void *)vmcoreinfo_data, vmcoreinfo_size); >> + kmemdump_register_id(KMEMDUMP_ID_COREIMAGE_runqueues, >> + (void *)&runqueues, runqueues_get_size()); >> + >> return 0; >> } >> >> With this, no more .section, no kmemdump code into sched, however, there >> are few things : > > I would really just do here something like the following: > > /** > * sched_get_runqueues_area - obtain the runqueues area for dumping > * @start: ... > * @size: ... > * > * The obtained area is only to be used for dumping purposes. > */ > void sched_get_runqueues_area(void *start, size_t size) > { > start = &runqueues; > size = sizeof(runqueues); > } > > might be cleaner. > How about this in the header: #define DECLARE_DUMP_AREA_FUNC(subsys, symbol) \ void subsys ## _get_ ## symbol ##_area(void **start, size_t *size); #define DEFINE_DUMP_AREA_FUNC(subsys, symbol) \ void subsys ## _get_ ## symbol ##_area(void **start, size_t *size)\ {\ *start = &symbol;\ *size = sizeof(symbol);\ } then, in sched just DECLARE_DUMP_AREA_FUNC(sched, runqueues); DEFINE_DUMP_AREA_FUNC(sched, runqueues); or a single macro that wraps both. would make it shorter and neater. What do you think ? > > Having said that, if you realize that there is a fundamental issue with > what I propose, please speak up. > > So far, I feel like there are only limited number of "suboptimal" cases > of this kind, but I might be wrong of course. >