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From: David Hildenbrand <david@redhat.com>
To: Baoquan He <bhe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	andreyknvl@google.com, christian.brauner@ubuntu.com,
	colin.king@canonical.com, corbet@lwn.net, dyoung@redhat.com,
	frederic@kernel.org, gpiccoli@canonical.com,
	john.p.donnelly@oracle.com, jpoimboe@redhat.com,
	keescook@chromium.org, linux-mm@kvack.org, masahiroy@kernel.org,
	mchehab+huawei@kernel.org, mike.kravetz@oracle.com,
	mingo@kernel.org, mm-commits@vger.kernel.org, paulmck@kernel.org,
	peterz@infradead.org, rdunlap@infradead.org, rostedt@goodmis.org,
	rppt@kernel.org, saeed.mirzamohammadi@oracle.com,
	samitolvanen@google.com, sboyd@kernel.org, tglx@linutronix.de,
	torvalds@linux-foundation.org, vgoyal@redhat.com,
	yifeifz2@illinois.edu, Michal Hocko <mhocko@kernel.org>
Subject: Re: [patch 48/91] kernel/crash_core: add crashkernel=auto for vmcore creation
Date: Mon, 10 May 2021 13:01:08 +0200	[thread overview]
Message-ID: <e9332e4e-57bd-9c7c-b3ba-7e3b29fad7c8@redhat.com> (raw)
In-Reply-To: <20210510104359.GC2946@localhost.localdomain>


>> I can understand the reasoning of "using a fraction of the memory size" when
>> booting up just to be on the safe side as we don't know", and that
>> motivation is much better than what I read so far. But then I wonder if we
>> cannot handle that any better? Because this feels very suboptimal to me and
>> I feel like there can be cases where the heuristic is just wrong.
> 
> Yes, I understand what you said. Our headache is mainly from bare metal
> system worrying the reservation is not enough becuase of many devices.
> 
> On VM, it is truly different. With much less devices, it does waste some
> memory. Usually a fixed minimal size can cover 99.9% of system unless
> too many devices attached/added to VM, I am not sure what's the
> probability it could happen. While, by the help of /sys/kernel/kexec_crash_size,
> you can shrink it to an small enough but available size. Just you may
> need to reload kdump kernel because the loaded kernel should have been
> erazed and out of control. The shrinking should be done at early stage of
> kernel running, I would say, lest crash may happen during that period.
> 
> We ever tried several different ways to enlarge the crashkernel size
> dynamically, but didn't think of a good way.

Yes, enlarging it at runtime much more difficult than shrinking.

[...]

>> A kernel early during boot can only guess. A kernel late during boot knows.
>> Please correct me if I'm wrong.
> 
> Well, I would not say it's guess, and would like to call them experical
> values from statistical data. With a priori vlaue given by 'auto',
> basically normal users of kdump don't need to care about the setting.
> E.g on Fedora, 'auto' can cover all systems, assume nobody would deploy
> it on a high end server. Everything we do is to make thing simple enough.
> If you don't know how to set, just add 'crashkernel=auto' to cmdline,
> then everything is done. I believe you agree that not everybody would
> like to dig into kexec/kdump just for getting how big crashkernel size
> need be set when they want to use kdump functionality.

Oh absolutely. But OTOH, most users will leave the value untouched if it 
works -- and complain at least in the VM environment to me about 
surpises waste of system RAM with "crashkernel=auto".

[...]

>> I know how helpful "crashkernel=auto" was so far, but I am also aware that
>> there was strong pushback in the past, and I remember for the reasons I
>> gave. IMHO we should refine that approach instead of trying to push the same
>> thing upstream every couple of years.
>>
>> I ran into the "512MB crashkernel" on a 64G VM with memory ballooning issue
>> already but didn't report a BZ, because so far, I was under the impression
>> that more memory means more crashkernel. But you explained to me that I was
>> just running into a (for my use case) bad heuristic.
> 
> I re-read the old posts, didn't see strong push-back. People just gave
> some different ideas instead. When we were silent, we tried different
> way, e.g the enlarging crashkernel at run time as told at above, but
> failed. Reusing free pages and user space pages of 1st kernel in kdump
> kernel, also failed. We also talked with people to consult if it's

Thanks for an insight into the history.

> doable to remove 'auto' support, nobody would like to give an affirmative
> answer. I know SUSE is using the way you mentioned to get a recommended
> size for long time, but it needs severeal more steps and need reboot. We
> prefer to take that way too as an improvement. The simpler, the better.

At least I'm happy to hear that other people had the same idea as me ;)

I can understand the desire for simplicity. it would be great to hear 
SUSEs perception of the problem and how they would ideally want to move 
forward with this.

[...]

>> I'll be happy to help looking into dynamic shrinking of the crashkernel size
>> if that approach makes sense. We could even let user space trigger that
>> resizing -- without a reboot.
> 
> Don't reply each inline comment since I believe they have been covered
> by the earlier reply. Thanks for looking to this and telling your
> thought, to let us know that in fact you really care about the extra
> memory on VMs which we have realized, but didn't realized it really cause
> issue.

I mess with dynamic resizing of VMs, that's why I usually take a closer 
look at all things that do stuff based on the initial VM size; yes, 
there is still a lot other such things out there.

It also bugged me for quite a bit that we don't have a sane way to 
achieve what we're doing here upstream. It somewhat feels like "this 
doesn't belong in the kernel and is user policy" but then, the existing 
kernel support is suboptimal.

Maybe reserving some "maybe too big but okayish to boot the system in a 
sane environment -- e.g., X% of system RAM and at least Y" size first 
and shrinking it later as triggered by user space early (where we do 
seem to have a way to pre-calculate things now) might actually be a good 
direction to look into.

-- 
Thanks,

David / dhildenb



  reply	other threads:[~2021-05-10 11:01 UTC|newest]

Thread overview: 119+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-07  1:01 incoming Andrew Morton
2021-05-07  1:02 ` [patch 01/91] alpha: eliminate old-style function definitions Andrew Morton
2021-05-07  1:02 ` [patch 02/91] alpha: csum_partial_copy.c: add function prototypes from <net/checksum.h> Andrew Morton
2021-05-07  1:02 ` [patch 03/91] fs/proc/generic.c: fix incorrect pde_is_permanent check Andrew Morton
2021-05-07  1:02 ` [patch 04/91] proc: save LOC in __xlate_proc_name() Andrew Morton
2021-05-07  2:24   ` Linus Torvalds
2021-05-07  1:02 ` [patch 05/91] proc: mandate ->proc_lseek in "struct proc_ops" Andrew Morton
2021-05-07  1:02 ` [patch 06/91] proc: delete redundant subset=pid check Andrew Morton
2021-05-07  1:02 ` [patch 07/91] selftests: proc: test subset=pid Andrew Morton
2021-05-07  1:02 ` [patch 08/91] proc/sysctl: fix function name error in comments Andrew Morton
2021-05-07  1:02 ` [patch 09/91] include: remove pagemap.h from blkdev.h Andrew Morton
2021-05-07  1:02 ` [patch 10/91] kernel.h: drop inclusion in bitmap.h Andrew Morton
2021-05-07  1:02 ` [patch 11/91] linux/profile.h: remove unnecessary declaration Andrew Morton
2021-05-07  1:02 ` [patch 12/91] kernel/async.c: fix pr_debug statement Andrew Morton
2021-05-07  1:02 ` [patch 13/91] kernel/cred.c: make init_groups static Andrew Morton
2021-05-07  1:02 ` [patch 14/91] tools: disable -Wno-type-limits Andrew Morton
2021-05-07  1:02 ` [patch 15/91] tools: bitmap: sync function declarations with the kernel Andrew Morton
2021-05-07  1:02 ` [patch 16/91] tools: sync BITMAP_LAST_WORD_MASK() macro " Andrew Morton
2021-05-07  1:02 ` [patch 17/91] arch: rearrange headers inclusion order in asm/bitops for m68k, sh and h8300 Andrew Morton
2021-05-07  1:02 ` [patch 18/91] lib: extend the scope of small_const_nbits() macro Andrew Morton
2021-05-07  1:03 ` [patch 19/91] tools: sync small_const_nbits() macro with the kernel Andrew Morton
2021-05-07  1:03 ` [patch 20/91] lib: inline _find_next_bit() wrappers Andrew Morton
2021-05-07  1:03 ` [patch 21/91] tools: sync find_next_bit implementation Andrew Morton
2021-05-07  1:03 ` [patch 22/91] lib: add fast path for find_next_*_bit() Andrew Morton
2021-05-07  1:03 ` [patch 23/91] lib: add fast path for find_first_*_bit() and find_last_bit() Andrew Morton
2021-05-07  1:03 ` [patch 24/91] tools: sync lib/find_bit implementation Andrew Morton
2021-05-07  1:03 ` [patch 25/91] MAINTAINERS: add entry for the bitmap API Andrew Morton
2021-05-07  1:03 ` [patch 26/91] lib/bch.c: fix a typo in the file bch.c Andrew Morton
2021-05-07  1:03 ` [patch 27/91] lib: fix inconsistent indenting in process_bit1() Andrew Morton
2021-05-07  1:03 ` [patch 28/91] lib/list_sort.c: fix typo in function description Andrew Morton
2021-05-07  1:03 ` [patch 29/91] lib/genalloc.c: fix a typo Andrew Morton
2021-05-07  1:03 ` [patch 30/91] lib: crc8: pointer to data block should be const Andrew Morton
2021-05-07  1:03 ` [patch 31/91] lib: stackdepot: turn depot_lock spinlock to raw_spinlock Andrew Morton
2021-05-07  1:03 ` [patch 32/91] lib/percpu_counter: tame kernel-doc compile warning Andrew Morton
2021-05-07  1:03 ` [patch 33/91] lib/genalloc: add parameter description to fix doc " Andrew Morton
2021-05-07  1:03 ` [patch 34/91] lib: parser: clean up kernel-doc Andrew Morton
2021-05-07  1:03 ` [patch 35/91] include/linux/compat.h: remove unneeded declaration from COMPAT_SYSCALL_DEFINEx() Andrew Morton
2021-05-07  1:03 ` [patch 36/91] checkpatch: warn when missing newline in return sysfs_emit() formats Andrew Morton
2021-05-07  1:03 ` [patch 37/91] checkpatch: exclude four preprocessor sub-expressions from MACRO_ARG_REUSE Andrew Morton
2021-05-07  1:04 ` [patch 38/91] checkpatch: improve ALLOC_ARRAY_ARGS test Andrew Morton
2021-05-07  1:04 ` [patch 39/91] kselftest: introduce new epoll test case Andrew Morton
2021-05-07  1:04 ` [patch 40/91] fs/epoll: restore waking from ep_done_scan() Andrew Morton
2021-05-07  1:04 ` [patch 41/91] isofs: fix fall-through warnings for Clang Andrew Morton
2021-05-07  1:04 ` [patch 42/91] fs/nilfs2: fix misspellings using codespell tool Andrew Morton
2021-05-07  1:04 ` [patch 43/91] nilfs2: fix typos in comments Andrew Morton
2021-05-07  1:04 ` [patch 44/91] hpfs: replace one-element array with flexible-array member Andrew Morton
2021-05-07  1:04 ` [patch 45/91] do_wait: make PIDTYPE_PID case O(1) instead of O(n) Andrew Morton
2021-05-07  1:04 ` [patch 46/91] kernel/fork.c: simplify copy_mm() Andrew Morton
2021-05-07  1:04 ` [patch 47/91] kernel/fork.c: fix typos Andrew Morton
2021-05-07  1:04 ` [patch 48/91] kernel/crash_core: add crashkernel=auto for vmcore creation Andrew Morton
2021-05-07  7:25   ` Linus Torvalds
2021-05-08  3:13     ` Baoquan He
2021-05-08  3:29       ` Baoquan He
2021-05-07  8:16   ` David Hildenbrand
2021-05-08  8:51     ` Baoquan He
2021-05-08  9:22       ` David Hildenbrand
2021-05-10  4:53         ` Baoquan He
2021-05-10  8:32           ` David Hildenbrand
2021-05-10 10:43             ` Baoquan He
2021-05-10 11:01               ` David Hildenbrand [this message]
2021-05-10 11:44                 ` Dave Young
2021-05-10 11:56                   ` David Hildenbrand
2021-05-11 13:36                     ` Baoquan He
2021-05-11 16:31                       ` Mike Rapoport
2021-05-11 17:07                         ` David Hildenbrand
2021-05-12 14:51                           ` Baoquan He
2021-05-12 15:07                             ` David Hildenbrand
2021-05-13  5:04                               ` Baoquan He
2021-05-12 19:03                             ` Kairui Song
2021-05-17  8:22                             ` David Hildenbrand
2021-05-18  8:49                               ` Baoquan He
2021-05-18  8:51                                 ` David Hildenbrand
2021-05-18  9:24                                   ` Dave Young
2021-05-12 14:13                         ` Baoquan He
2021-05-12  7:42                     ` Dave Young
2021-05-07  1:04 ` [patch 49/91] kexec: add kexec reboot string Andrew Morton
2021-05-07  1:04 ` [patch 50/91] kernel: kexec_file: fix error return code of kexec_calculate_store_digests() Andrew Morton
2021-05-07  1:04 ` [patch 51/91] kexec: dump kmessage before machine_kexec Andrew Morton
2021-05-07  1:04 ` [patch 52/91] gcov: combine common code Andrew Morton
2021-05-07  1:04 ` [patch 53/91] gcov: simplify buffer allocation Andrew Morton
2021-05-07  1:04 ` [patch 54/91] gcov: use kvmalloc() Andrew Morton
2021-05-07  1:04 ` [patch 55/91] gcov: clang: drop support for clang-10 and older Andrew Morton
2021-05-07  1:04 ` [patch 56/91] smp: kernel/panic.c - silence warnings Andrew Morton
2021-05-07  1:05 ` [patch 57/91] delayacct: clear right task's flag after blkio completes Andrew Morton
2021-05-07  1:05 ` [patch 58/91] gdb: lx-symbols: store the abspath() Andrew Morton
2021-05-07  1:05 ` [patch 59/91] scripts/gdb: document lx_current is only supported by x86 Andrew Morton
2021-05-07  1:05 ` [patch 60/91] scripts/gdb: add lx_current support for arm64 Andrew Morton
2021-05-07  1:05 ` [patch 61/91] kernel/resource: make walk_system_ram_res() find all busy IORESOURCE_SYSTEM_RAM resources Andrew Morton
2021-05-07  1:05 ` [patch 62/91] kernel/resource: make walk_mem_res() find all busy IORESOURCE_MEM resources Andrew Morton
2021-05-07  1:05 ` [patch 63/91] kernel/resource: remove first_lvl / siblings_only logic Andrew Morton
2021-05-07  1:05 ` [patch 64/91] kernel/resource: allow region_intersects users to hold resource_lock Andrew Morton
2021-05-07  1:05 ` [patch 65/91] kernel/resource: refactor __request_region to allow external locking Andrew Morton
2021-05-07  1:05 ` [patch 66/91] kernel/resource: fix locking in request_free_mem_region Andrew Morton
2021-05-07  1:05 ` [patch 67/91] selftests: remove duplicate include Andrew Morton
2021-05-07  1:05 ` [patch 68/91] kernel/async.c: stop guarding pr_debug() statements Andrew Morton
2021-05-07  1:05 ` [patch 69/91] kernel/async.c: remove async_unregister_domain() Andrew Morton
2021-05-07  1:05 ` [patch 70/91] init/initramfs.c: do unpacking asynchronously Andrew Morton
2021-05-07  1:05 ` [patch 71/91] modules: add CONFIG_MODPROBE_PATH Andrew Morton
2021-05-07  1:05 ` [patch 72/91] ipc/sem.c: mundane typo fixes Andrew Morton
2021-05-07  1:05 ` [patch 73/91] mm: fix some typos and code style problems Andrew Morton
2021-05-07  1:05 ` [patch 74/91] drivers/char: remove /dev/kmem for good Andrew Morton
2021-05-07  1:06 ` [patch 75/91] mm: remove xlate_dev_kmem_ptr() Andrew Morton
2021-05-07  1:06 ` [patch 76/91] mm/vmalloc: remove vwrite() Andrew Morton
2021-05-07  1:06 ` [patch 77/91] arm: print alloc free paths for address in registers Andrew Morton
2021-05-07  1:06 ` [patch 78/91] scripts/spelling.txt: add "overlfow" Andrew Morton
2021-05-07  1:06 ` [patch 79/91] scripts/spelling.txt: add "diabled" typo Andrew Morton
2021-05-07  1:06 ` [patch 80/91] scripts/spelling.txt: add "overflw" Andrew Morton
2021-05-07  1:06 ` [patch 81/91] mm/slab.c: fix spelling mistake "disired" -> "desired" Andrew Morton
2021-05-07  1:06 ` [patch 82/91] include/linux/pgtable.h: few spelling fixes Andrew Morton
2021-05-07  1:06 ` [patch 83/91] kernel/umh.c: fix some spelling mistakes Andrew Morton
2021-05-07  1:06 ` [patch 84/91] kernel/user_namespace.c: fix typos Andrew Morton
2021-05-07  1:06 ` [patch 85/91] kernel/up.c: fix typo Andrew Morton
2021-05-07  1:06 ` [patch 86/91] kernel/sys.c: " Andrew Morton
2021-05-07  1:06 ` [patch 87/91] fs: fat: fix spelling typo of values Andrew Morton
2021-05-07  1:06 ` [patch 88/91] ipc/sem.c: spelling fix Andrew Morton
2021-05-07  1:06 ` [patch 89/91] treewide: remove editor modelines and cruft Andrew Morton
2021-05-07  1:06 ` [patch 90/91] mm: fix typos in comments Andrew Morton
2021-05-07  1:06 ` [patch 91/91] " Andrew Morton
2021-05-07  7:12 ` incoming Linus Torvalds

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