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From: Baoquan He <bhe@redhat.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
	Dave Young <dyoung@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	christian.brauner@ubuntu.com, colin.king@canonical.com,
	corbet@lwn.net, 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,
	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>,
	kasong@redhat.com, piliu@redhat.com
Subject: Re: [patch 48/91] kernel/crash_core: add crashkernel=auto for vmcore creation
Date: Wed, 12 May 2021 22:13:03 +0800	[thread overview]
Message-ID: <20210512141303.GF2834@localhost.localdomain> (raw)
In-Reply-To: <YJqxWy63cokQZ4jQ@kernel.org>

On 05/11/21 at 07:31pm, Mike Rapoport wrote:
> Hi Baoquan,
> 
> On Tue, May 11, 2021 at 09:36:41PM +0800, Baoquan He wrote:
> > On 05/10/21 at 01:56pm, David Hildenbrand wrote:
> > > On 10.05.21 13:44, Dave Young wrote:
> > > > Hi David,
> > > 
> > > Hi Dave,
> > > 
> > > > On 05/10/21 at 01:01pm, David Hildenbrand wrote:
> > > > [snip]
> > > > > 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.
> > > > 
> > > > Hmm, that is also an option we considered before.  Even for your
> > > > suggestion we still need a kernel option to set the default ratio/value.
> > > > and the ratio/value should be another patch which expands crashkernel
> > > > syntax.
> > > 
> > > Right.
> > > 
> > > > 
> > > > Actually the kconfig help text in this patch is indeed misleading, it is
> > > > not introducing crashkernel=a:b... and no need to explain about the
> > > > crashkernel syntax, the config option is actually just some interface we
> > > > can add any valid crashkernel settings to be used by default. So current
> > > > patch help text describes the default value of crash auto str, instead
> > > > of describes what crash auto str is.
> > > 
> > > Right. And I would much rather prefer either
> > > 
> > > a) handling "auto" completely in the kernel, not just setting some
> > > questionable default at compile time
> > 
> > Thanks for the suggestions.
> > 
> > If the way adding default value into kernel config is disliked,
> > this a) option looks good. We can get value with x% of system RAM, but
> > clamp it with CRASH_KERNEL_MIN/MAX. The CRASH_KERNEL_MIN/MAX may need be
> > defined with a default value for different ARCHes. It's very close to
> > our current implementation, and handling 'auto' in kernel.
> > 
> > And kernel config provided so that people can tune the MIN/MAX value,
> > but no need to post patch to do the tuning each time if have to?
>  
> Maybe I'm missing something, but the whole point is to avoid kernel
> configuration option at all. If the crashkernel=auto works good for 99% of
> the cases, there is no need to provide build time configuration along with
> it. There are plenty of ways users can control crashkernel reservations
> with the existing 2-4 (depending on architecture) command line options.
> 
> Simply hard coding a reasonable defaults (e.g.
> "1G-64G:128M,64G-1T:256M,1T-:512M"), and using these defaults when
> crashkernel=auto is set would cover the same 99% of users you referred to.

Thanks for looking into this, Mike.

The crashkernel=auto works well for 99% of systems with a prerequisite
that values of 'auto' corresponds to a certain kernel, e.g distros kernel.
Say so because the kernel configs of a distros kernel decides the kernel
size, and also the initrd size. A generic default value for
crashkernel=auto doesn't make much sense when we make it into distros.
That's why we want to add the default value into kernel config originally.
Then asking for a minimal size with a kernel config tunable as the second
best when handle 'auto' in kernel as David's option a) suggested.

Here it's a little not clear to me about why kernel config has to be
avoided. We have this kind of tunable, e.g CONFIG_CMA_SIZE_SEL_MBYTES.

> 
> If we can resize the reservation later during boot this will also address
> David's concern about the wasted memory.

We can't resize the reservation, we can only shrink currently.

> 
> You mentioned that amount of memory that is required for crash kernel
> reservation depends on the devices present on the system. Is is possible to
> detect how much memory is required at late stages of boot?

It may be doable to detect at late stage of boot, need investigation, now we
are working to do after system bootup. The thing is the detection is
very coarse-grained. We count all loaded kernel modules in. But in kdump
kernel, only very necessary modules is added in our distros. e.g if we
dump through network, NIC modules are collected. otherwise we filter it out
to reduce memory usage in kdump kernel. For most of normal systems with
dozens of devices, memory required by device driver in kdump kernel is
limited. On VM guests, it's even much less since only very necessary
devices are added, e.g disk/NIC/serial.

So, I said 99% of systems can be covered by default value, it's based on
a certain kernel with fixed kernel configs, mainly related to distros.
Adding a permanent default value in upstream kernel doesn't make much
sense, if no tunable provided for distros to adjust.

> 
> > > b) passing it explicitly in via the cmdline
> > > 
> > > > 
> > > > And crashkernel=auto makes this more flexibly. We can tune the values
> > > > easily when upgrading.  But if we pass a fixed value in userspace we
> > > > can not know if the value is set by distribution automatically or by user
> > > > manually thus we can not blindly update it.
> > > 
> > > I think there are two different cases:
> > > 
> > > 
> > > 1. kernel space updates the value later during boot. "crashkernel=auto"
> > > really does the right thing, meaning
> > > 
> > > a) allocate something reasonable and safe during early boot
> > > b) update the allocation during late boot when we know what kind of system
> > > we're running on
> > > 
> > > Then, we indeed care about "crashkernel=auto" in the kernel and I think it
> > > would be a nice thing to have. The only question is on how to make that a
> > > little configurable, depending on different thingies we might want to run in
> > > the crashkernel (assuming someone doesn't want kdump).
> > > 
> > > 
> > > 2. user space updates the value later during boot
> > > 
> > > IMHO we don't really car who decided on the value as we do the update from
> > > user space. If an admin messes with crashkernel=, the admin can also mess
> > > with kdump not doing any overwrites (e.g., make that configurable, or detect
> > > the overwrite in kdump somehow).
> > > 
> > > -- 
> > > Thanks,
> > > 
> > > David / dhildenb
> > > 
> > 
> > 
> 
> -- 
> Sincerely yours,
> Mike.
> 


  parent reply	other threads:[~2021-05-12 14:14 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
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 [this message]
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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