Date: Thu, 9 Aug 2018 21:25:13 -0500 From: Kyle Evans <kevans@freebsd.org> To: Bruce Evans <brde@optusnet.com.au> Cc: src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r337518 - in head/sys: kern sys Message-ID: <CACNAnaFkMhWVSQEm0uyKLa7V8gh9JZFd9Q7HHNox%2BZHa6pspYw@mail.gmail.com> In-Reply-To: <20180810115339.X1652@besplex.bde.org> References: <201808090132.w791WAQm057199@repo.freebsd.org> <20180810081913.P1108@besplex.bde.org> <CACNAnaF%2BEHEfv2%2BaqAkx-JDxaKWmS7uLefHLqjO7Te8h_30Mew@mail.gmail.com> <20180810105239.J1482@besplex.bde.org> <20180810115339.X1652@besplex.bde.org>
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On Thu, Aug 9, 2018 at 9:12 PM, Bruce Evans <brde@optusnet.com.au> wrote: > On Fri, 10 Aug 2018, Bruce Evans wrote: > >> On Thu, 9 Aug 2018, Kyle Evans wrote: >> ... >>> >>> When you say you call msgbufinit() earlier-- before the static >>> environment is even setup, or at least after that? >> >> >> The above is with a simplified version that first calls msgbufinit() >> after the static env is set up. In other working versions, msgbufinit() >> is called long before the static env is set up. In a commented out >> version, a small MI msgbuf is statically initalized so that some msgbuf >> is always mapped, but msgbufmapped still starts as 0 and msgbufinit() >> must still be called to synchronize. Some synchronization is needed >> since the msgbuf uses broken mutex locking and mutexes use curthread and >> curthread is not initialized early. The simplified versions delay the >> msgbufinit() calls until mutexes are initialized enough, and this >> accidentally moves them after other initializations. >> >> So the tunable is unavailable in the best version :-(. The small initial >> msgbuf could even start with the statically initialized buffer containing >> the tag, and must if it is to be ready to go without a synchronization >> call. > > > The tag can be added later fairly easily, and this order corresponds to > the problem being solved (that we don't know where the data for the new boot > starts iff there is old data): > > - start with a small statically initialized msgbuf. No need for a tag; the > new data starts at the start of the buffer > - write new data to this buffer > - reinit is not really initialization, but supplies data from the previous > boot. Construct the new buffer as the concatenation of the old data, a > tag to delimit the new data, then the new data. > > But I think a variable tag is worse than a fixed tag. With a fixed tag, > it is easy for dmesg, syslogd and users to search for the tag and replace > it by a better one determined using full runtime resources. E.g., use > sed on /var/log/messages to find the last instance of the tag and replace > it by something depending on the current context, while a tunable tag must > be set earlier and then all utilities and methods for finding and modifying > must understand its variations. > I honestly have very low expectations for the tunable's use, but it's a low-cost addition and I find it more useful in a less automated context. One that I've had recently would have been implemented with the following process: 1. Boot -v into installed kernel with normal tag, ---<<BOOT>>-- 2. Briefly review `dmesg`, notice issue with clock driver 3. Install new kernel with changes to said clock driver. Tag it ---<<A20CLKNG>>--, boot -v 4. Review `dmesg` again to observe changes The non-standard tag would certainly be an anomaly, but it gives me the benefit of discerning a standard boot from this one in which I fiddled with the A20 clock driver without having to compile a non-standard boot tag into the kernel. It also gives me a clear indicator as I'm paging through output to quickly at-a-glance realize that I've reached a new (or the correct) boot. I'll likely commit the modified version soon, anyways, and we can hash out any further details after 12 branches. Thanks, Kyle Evanshome | help
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