Messages in this thread | | | Date | Mon, 9 Nov 2009 13:04:45 +0100 | From | Andi Kleen <> | Subject | Re: [PATCH 22/23] sysctl arm: Remove binary sysctl support |
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On Mon, Nov 09, 2009 at 03:45:06AM -0800, Eric W. Biederman wrote: > Andi Kleen <andi@firstfloor.org> writes: > > > ebiederm@xmission.com (Eric W. Biederman) writes: > >> > >> The glibc pthread code that uses sysctl has no problems if sys_sysctl > >> is gone. It both falls back to reading /proc/sys and it just controls > >> an optimization and the code works with either result. Been there, > >> done that. > > > > /proc/sys is much slower than sysctl though. So you made program startup > > slower. > > Not much slower, but slower. I just measured it in a case that favors > sysctl and the ration is about 5:2. Or sysctl is about 2.5x faster. > About 49usec for open/read/close on proc and 19usec for sysctl. > In my emulation it is a bit slower than that.
That's not good.
> > > Also I agree with Arjan that breaking such a common ABI is not > > really a good idea. But I think it's enough to only handle > > common sysctls that are actually used, which are very few. > > Well I haven't broken anything at this point. I am simply edging > us to the point when we are close to being able to forget about > sys_sysctl for good.
I think all-or-nothing that you have right now is a bad trade-off because it breaks an established interface used by lots of code (glibc)
You should have three states
a) all b) common ones used by glibc and perhaps a few others only c) none
I suspect most users would use (b), in fact (c) might be redundant if (b) is cheap enough (which it should be) > As for the rest the common number of sysctls with glibc > 2.8 is > exactly 0. Which makes compiling out sys_sysctl support sane. > Especially since we have been throwing a warning for years if > anyone uses any of the others.
Yes, but people ignore the warning. Perhaps should make it a WARN() and track it with kernelops?
> > > It would be better to simply keep the commonly used binary sysctls > > as emulation around always (commonly = used by glibc and perhaps > > added by user printk feedback) That's very cheap because it's just > > a simple translation and can be done internally cheaper than going > > through the VFS with a bazillion of locks. > > A micro optimization for code that does not exist. That is a bad > trade off.
Hmm? There's plenty of glibc code that uses the binary sysctl.
> Further it is my intention to optimize /proc/sys when I get the > chance now that we don't have all of the old sysctl baggage holding > back the code.
The VFS will always be comparably slow I suspect; I'm not sure you can optimize it that much compared to a fast custom path (especially handling the kernel version should be really fast)
> Ultimately what drives me most is that people are still accidentally > adding binary sysctls, which no one uses or tests. For a recent > example see:
Yes I agree new binary sysctls should just be deprecated right now.
-Andi
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