get_vmalloc_info() and /proc/meminfo insanely expensive

From: Linus Torvalds
Date: Wed Aug 12 2015 - 23:29:39 EST

I just did some profiling of a simple "make test" in the git repo, and
was surprised by the top kernel offender: get_vmalloc_info() showed up
at roughly 4% cpu use.

It turns out that bash ends up reading /proc/meminfo on every single
activation, and "make test" is basically just running a huge
collection of shell scripts. You can verify by just doing

strace -o trace sh -c "echo"

to see what bash does on your system. I suspect it's actually glibc,
because a quick google finds the function "get_phys_pages()" that just
looks at the "MemTotal" line (or possibly get_avphys_pageslooks at the
MemFree" line).

Ok, so bash is insane for caring so deeply that it does this
regardless of anything else. But what else is new - user space does
odd things. It's like a truism.

My gut feel for this is that we should just rate-limit this and cache
the vmalloc information for a fraction of a second or something. Maybe
we could expose total memory sizes in some more efficient format, but
it's not like existing binaries will magically de-crapify themselves,
so just speeding up meminfo sounds like a good thing.

Maybe we could even cache the whole seqfile buffer - Al? How painful
would something like that be? Although from the profiles, it's really
just the vmalloc info gathering that shows up as actually wasting CPU

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