Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

Mike Galbraith
2008-09-17T13:11:51Z

On Wed, 2008-09-17 at 14:49 +0200, Ingo Molnar wrote:
> * Mike Galbraith <efault-Mmb7MZpHnFY@public.gmane.org> wrote:
>
> > > It looks like a potentially bogus bisection result, but _maybe_ it
> > > has relevance: changes the size of "struct security_operations",
> > > which could have alignment and layout effects on all sorts of kernel
> > > variables, kmalloc sizes, etc.
> >
> > This may well be a mythical creature infestation for all I know ;-),
> > but it's address is somewhere in the 2069f45..847106f block, 316
> > commits, none of which look like they should be the least bit
> > interesting to netperf. I reverted this particular commit in 27.git,
> > got the expected result. Looks like I'll keep poking at it, can't
> > seem to resist. Grr.
>
> are you sure it's 2069f45..847106f? Filtering out the
> likely-uninteresting commits:

Yeah, as sure as I can be. I've built both (et al) kernels several
times, and it has repeated every time. Would be nice if someone would
try to confirm/deny though. For my little quad, I do..

#!/bin/sh

echo 0 > /proc/sys/kernel/sched_wakeup_granularity_ns

netserver -p 12865
netserver -p 12866
netserver -p 12867
netserver -p 12868

netperf -p 12865 -t TCP_RR -l 60 -H 127.0.0.1 -T 0,0 -- -r 1,1&
netperf -p 12866 -t TCP_RR -l 60 -H 127.0.0.1 -T 1,1 -- -r 1,1&
netperf -p 12867 -t TCP_RR -l 60 -H 127.0.0.1 -T 2,2 -- -r 1,1&
netperf -p 12868 -t TCP_RR -l 60 -H 127.0.0.1 -T 3,3 -- -r 1,1&

wait
killall netserver

> git log --pretty=format:"%h: %s" 2069f45..847106f | grep -viE \
> 'block|alsa|pcmcia|sound|Merge|iosched|blk|DAC960|scsi|s390|paride|pktcdvd|filter|cdrom|drm'
>
> gives us:
>
> 7daf705: Start using the new '%pS' infrastructure to print symbols
> 6f0f0fd: security: remove register_security hook
> 93cbace: security: remove dummy module fix
> 5915eb5: security: remove dummy module
> b478a9f: security: remove unused sb_get_mnt_opts hook
> 32502b8: splice: fix generic_file_splice_read() race with page invalidation
> 8b3d356: ramfs: enable splice write
> a144ff0: xen: Avoid allocations causing swap activity on the resume path
>
> which really only leaves that security commit your bisection fingered.
> Which _slightly_ raises its likelyhood of being implicated. Structure
> size changes can move two formerly far-apart netperf-relevant symbols on
> the same cacheline, which can start cache ping-pong-ing badly.

I sure hope it's something like ping-pong, it's driving me NUTS.

-Mike