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Tue, 11 May 2021 13:36:44 +0000 (UTC) Date: Tue, 11 May 2021 21:36:41 +0800 From: Baoquan He To: David Hildenbrand Cc: Dave Young , Andrew Morton , 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, 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 , kasong@redhat.com Subject: Re: [patch 48/91] kernel/crash_core: add crashkernel=auto for vmcore creation Message-ID: <20210511133641.GE2834@localhost.localdomain> References: <20210507010432.IN24PudKT%akpm@linux-foundation.org> <889c6b90-7335-71ce-c955-3596e6ac7c5a@redhat.com> <20210508085133.GA2946@localhost.localdomain> <2d0f53d9-51ca-da57-95a3-583dc81f35ef@redhat.com> <20210510045338.GB2946@localhost.localdomain> <4a544493-0622-ac6d-f14b-fb338e33b25e@redhat.com> <20210510104359.GC2946@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Rspamd-Queue-Id: 4602FE000123 Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="WK/8sEG6"; dmarc=pass (policy=none) header.from=redhat.com; spf=none (imf21.hostedemail.com: domain of bhe@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=bhe@redhat.com X-Rspamd-Server: rspam03 X-Stat-Signature: brcp8kfnyso8y1onjihfiqy9zh8gzbbo Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf21; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=170.10.133.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620740214-625672 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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? > 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 >