Re: [ANNOUNCE] LinSched for v3.3-rc7

[Michael Wang <wangyun@linux.vnet.ibm.com>]

On 03/15/2012 12:08 PM, Dhaval Giani wrote:

> [Adding abhishek to the cc]
> 
> On Wed, Mar 14, 2012 at 8:58 PM, Paul Turner <pjt@google.com> wrote:
>> Hi All,
>>
>> [ Take 2, gmail tried to a non text/plain component into the last email .. ]
>>
>> Quick start version:
>>
>> Available under linsched-alpha at:
>>  git://git.kernel.org/pub/scm/linux/kernel/git/pjt/linsched.git  .linsched
>>
>> NOTE: The branch history is still subject to some revision as I am
>> still re-partitioning some of the patches.  Once this is complete, I
>> will promote linsched-alpha into a linsched branch at which point it
>> will no longer be subject to history re-writes.
>>
>> After checking out the code:
>> cd tools/linsched
>> make
>> cd tests
>> ./run_tests.sh basic_tests
>> << then try changing some scheduler parameters, e.g. sched_latency,
>> and repeating >>
>>
>> (Note:  The basic_tests are unit-tests, these are calibrated to the
>> current scheduler tunables and should strictly be considered sanity
>> tests.  Please see the mcarlo-sim work for a more useful testing
>> environment.)
>>
>> Extended version:
>>
>> First of all, apologies in the delay to posting this -- I know there's
>> been a lot of interest.  We made the choice to first rebase to v3.3
>> since there were fairly extensive changes, especially within the
>> scheduler, that meant we had the opportunity to significantly clean up
>> some of the LinSched code.  (For example, previously we were
>> processing kernel/sched* using awk as a Makefile step so that we could
>> extract the necessary structure information without modifying
>> sched.c!)  While the code benefited greatly from this, there were
>> several other changes that required fairly extensive modification in
>> this process (and in the meanwhile the v3.1 version became less
>> representative due to the extent of the above changes); which pushed
>> things out much further than I would have liked.  I suppose the moral
>> of the story is always release early, and often.
>>
>> That said, I'm relatively happy with the current state of integration,
>> there's certainly some specific areas that can still be greatly
>> improved (in particular, the main simulator loop has not had as much
>> attention paid as the LinSched<>Kernel interactions and there's a long
>> list of TODOs that could be improved there), but things are now mated
>> fairly cleanly through the use of a new LinSched architecture.  This
>> is a total re-write of almost all LinSched<>Kernel interactions versus
>> the previous (2.6.35) version, and has allowed us to now carry almost
>> zero modifications against the kernel source.  It's both possible to
>> develop/test in place, as well as being patch compatible.  The
>> remaining touch-points now total just 20 lines!  Half of these are
>> likely mergable, with the other 10 lines being more LinSched specific
>> at this point in time, I've broken these down below:
>>
>> The total damage:
>>  include/linux/init.h      |    6 ++++++   (linsched ugliness,
>> unfortunately necessary until we boot-strap proper initcall support)
>>  include/linux/rcupdate.h  |    3 +++    (only necessary to allow -O0
>> compilation which is extremely handy for analyzing the scheduler using
>> gdb)
>>  kernel/pid.c              |    4 ++++        (linsched ugliness,
>> these can go eventually)
>>  kernel/sched/fair.c       |    2 +-          (this is just the
>> promotion of 1 structure and function from static state which weren't
>> published in the sched/ re-factoring that we need from within the
>> simulator)
>>  kernel/sched/stats.c      |    2 +-
>>  kernel/time/timekeeping.c |    3 ++-    (this fixes a time-dilation
>> error due to rounding when our clock-source has ns-resolution, e.g.
>> shift==1)


The edit in timekeeping:

xtime.tv_nsec = ((s64)timekeeper.xtime_nsec + (1ULL << timekeeper.shift)
- 1) >> timekeeper.shift;

Looks better then the old code which blindly add 1ns for the lost in
rounding, is it possible to commit this change to mainline?

Regards,
Michael Wang

>>  6 files changed, 17 insertions(+), 3 deletions(-)
>>
>> Summarized changes vs 2.6.35 (previous version):
>>
>> - The original LinSched (understandably) simplified many of the kernel
>> interactions in order to make simulation easier.  Unfortunately, this
>> has serious side-effects on the accuracy of simulation.  We've now
>> introduced a large portion of this state, including: irq and soft-irq
>> contexts (we now perform periodic load-balance out of SCHED_SOFTIRQ
>> for example), support for active load-balancing, correctly modeled
>> nohz interactions, ipi and stop-task support.
>>
>> - Support for record and replay of application scheduling via perf.
>> This is not yet well integrated, but under tests exist the tools to
>> record an applications behavior using perf sched record, and then play
>> it back in the simulator.
>>
>> - Load-balancer scoring.  This one is a very promising new avenue for
>> load-balancer testing.  We analyzed several workloads and found that
>> they could be well-modeled using a log-normal distribution.
>> Parameterizing these models then allows us to construct a large (500)
>> test-case set of randomly generated workloads that behave similarly.
>> By integrating the variance between the current load-balance and an
>> offline computed (currently greedy first-fit) balance we're able to
>> automatically identify and score an approximation of our distance from
>> an ideal load-balance.  Historically, such scores are very difficult
>> to interpret, however, that's where our ability to generate a large
>> set of test-cases above comes in.  This allows us to exploit a nice
>> property, it's much easier to design a scoring function that diverges
>> (in this case the variance) than a nice stable one that converges.  We
>> can then catch regressions in load-balancer quality by measuring the
>> divergence in this set of scoring functions across our set of
>> test-cases.  This particular feature needs a large set of
>> documentation in itself (todo), but to get started with playing with
>> it see Makefile.mcarlo-sims in tools/linsched/tests.  In particular to
>> evaluate the entire set across a variety of topologies the following
>> command can be issued:
>>  make -j <num_cpus * 2 > -f Makefile.mcarlo-sims
>> (The included 'diff-mcarlo-500' tool can then be used to make
>> comparisons across result sets.)
>>
>> - Validation versus real hardware.  Under tests/validation we've
>> included a tool for replaying and recording the above simulations on a
>> live-machine.  These can then be compared to simulated runs using the
>> tools above to ensure that LinSched is modelling your architecture
>> reasonably appropriately.  We did some reasonably extensive
>> comparisons versus several x86 topologies in the v3.1 code using this;
>> it's a fundamentally hard problem -- in particular there's much more
>> clock drift between events on real hardware, but the results showed
>> the included topologies to be a reasonable simulacrum under LinSched.
>>
>> What's to come?
>> - More documentation, especially about the use of the new
>> load-balancer scoring tools.
>> - The history is very coarse right now as a result of going through a
>> rebase cement-mixer.  I'd like to incrementally refactor some of the
>> larger commits; once this is done I will promote linsched-alpha to a
>> stable linsched branch that won't be subject to history re-writes.
>> - KBuild integration.  We currently build everything out of the
>> tools/linsched makefiles.  One of the immediate TODOs involves
>> re-working the arch/linsched half of this to work with kbuild so that
>> its less hacky/fragile.
>> - Writing up some of the existing TODOs as starting points for anyone
>> who wants to get involved.
>>
>> I'd also like to take a moment to specially recognize the effort of
>> the following contributors, all of whom were involved extensively in
>> the work above.  Things have come a long way since the 5000 lines of
>> "#ifdef LINSCHED", the current status would not be possible without
>> them.
>>  Ben Segall, Dhaval Giani, Ranjit Manomohan, Nikhil Rao, and Abhishek
>> Srivastava
>>
>> Thanks!
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