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    SubjectRe: [RFT][patch 17/18] sched: use jump labels to reduce overhead when bandwidth control is inactive
    On Thu, Jul 21, 2011 at 06:38:01PM -0700, Paul Turner wrote:
    > On Thu, Jul 21, 2011 at 6:17 PM, Jason Baron <jbaron@redhat.com> wrote:
    > > On Thu, Jul 21, 2011 at 05:57:31PM -0700, Paul Turner wrote:
    > >> On Thu, Jul 21, 2011 at 5:32 PM, Jason Baron <jbaron@redhat.com> wrote:
    > >> > rth@redhat.com
    > >> > Bcc:
    > >> > Subject: Re: [RFT][patch 17/18] sched: use jump labels to reduce overhead
    > >> >  when bandwidth control is inactive
    > >> > Reply-To:
    > >> > In-Reply-To: <20110721184758.403388616@google.com>
    > >> >
    > >> > On Thu, Jul 21, 2011 at 09:43:42AM -0700, Paul Turner wrote:
    > >> >> So I'm seeing some strange costs associated with jump_labels; while on paper
    > >> >> the branches and instructions retired improves (as expected) we're taking an
    > >> >> unexpected hit in IPC.
    > >> >>
    > >> >> [From the initial mail we have workloads:
    > >> >>   mkdir -p /cgroup/cpu/test
    > >> >>   echo $$ > /dev/cgroup/cpu/test (only cpu,cpuacct mounted)
    > >> >>   (W1) taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "for ((i=0;i<5;i++)); do $(dirname $0)/pipe-test 20000; done"
    > >> >>   (W2)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;true"
    > >> >>   (W3)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;"
    > >> >> ]
    > >> >>
    > >> >> To make some of the figures more clear:
    > >> >>
    > >> >> Legend:
    > >> >> !BWC = tip + bwc, BWC compiled out
    > >> >> BWC = tip + bwc
    > >> >> BWC_JL = tip + bwc + jump label (this patch)
    > >> >>
    > >> >>
    > >> >> Now, comparing under W1 we see:
    > >> >> W1: BWC vs BWC_JL
    > >> >>                             instructions            cycles                  branches              elapsed
    > >> >> ---------------------------------------------------------------------------------------------------------------------
    > >> >> clovertown [BWC]            845934117               974222228               152715407             0.419014188 [baseline]
    > >> >> +unconstrained              857963815 (+1.42)      1007152750 (+3.38)       153140328 (+0.28)     0.433186926 (+3.38)  [rel]
    > >> >> +10000000000/1000:          876937753 (+2.55)      1033978705 (+5.65)       160038434 (+3.59)     0.443638365 (+5.66)  [rel]
    > >> >> +10000000000/1000000:       880276838 (+3.08)      1036176245 (+6.13)       160683878 (+4.15)     0.444577244 (+6.14)  [rel]
    > >> >>
    > >> >> barcelona [BWC]             820573353               748178486               148161233             0.342122850 [baseline]
    > >> >> +unconstrained              817011602 (-0.43)       759838181 (+1.56)       145951513 (-1.49)     0.347462571 (+1.56)  [rel]
    > >> >> +10000000000/1000:          830109086 (+0.26)       770451537 (+1.67)       151228902 (+1.08)     0.350824677 (+1.65)  [rel]
    > >> >> +10000000000/1000000:       830196206 (+0.30)       770704213 (+2.27)       151250413 (+1.12)     0.350962182 (+2.28)  [rel]
    > >> >>
    > >> >> westmere [BWC]              802533191               694415157               146071233             0.194428018 [baseline]
    > >> >> +unconstrained              799057936 (-0.43)       751384496 (+8.20)       143875513 (-1.50)     0.211182620 (+8.62)  [rel]
    > >> >> +10000000000/1000:          812033785 (+0.27)       761469084 (+8.51)       149134146 (+1.09)     0.212149229 (+8.28)  [rel]
    > >> >> +10000000000/1000000:       811912834 (+0.27)       757842988 (+7.45)       149113291 (+1.09)     0.211364804 (+7.30)  [rel]
    > >> >> e.g. Barcelona issues ~0.43% less instructions, for a total of 817011602, in
    > >> >> the unconstrained case with BWC.
    > >> >>
    > >> >>
    > >> >> Where "unconstrained, 10000000000/1000, 10000000000/10000" are the on
    > >> >> measurements for BWC_JL, with (%d) being the relative difference to their
    > >> >> BWC counterparts.
    > >> >>
    > >> >> W1: BWC vs BWC_JL is very similar.
    > >> >>       BWC vs BWC_JL
    > >> >> clovertown [BWC]            985732031              1283113452               175621212             1.375905653
    > >> >> +unconstrained              979242938 (-0.66)      1288971141 (+0.46)       172122546 (-1.99)     1.389795165 (+1.01)  [rel]
    > >> >> +10000000000/1000:          999886468 (+0.33)      1296597143 (+1.13)       180554004 (+1.62)     1.392576770 (+1.18)  [rel]
    > >> >> +10000000000/1000000:       999034223 (+0.11)      1293925500 (+0.57)       180413829 (+1.39)     1.391041338 (+0.94)  [rel]
    > >> >>
    > >> >> barcelona [BWC]             982139920              1078757792               175417574             1.069537049
    > >> >> +unconstrained              965443672 (-1.70)      1075377223 (-0.31)       170215844 (-2.97)     1.045595065 (-2.24)  [rel]
    > >> >> +10000000000/1000:          989104943 (+0.05)      1100836668 (+0.52)       178837754 (+1.22)     1.058730316 (-1.77)  [rel]
    > >> >> +10000000000/1000000:       987627489 (-0.32)      1095843758 (-0.17)       178567411 (+0.84)     1.056100899 (-2.28)  [rel]
    > >> >>
    > >> >> westmere [BWC]              918633403               896047900               166496917             0.754629182
    > >> >> +unconstrained              914740541 (-0.42)       903906801 (+0.88)       163652848 (-1.71)     0.758050332 (+0.45)  [rel]
    > >> >> +10000000000/1000:          927517377 (-0.41)       952579771 (+5.67)       170173060 (+0.75)     0.771193786 (+2.43)  [rel]
    > >> >> +10000000000/1000000:       914676985 (-0.89)       936106277 (+3.81)       167683288 (+0.22)     0.764973632 (+1.38)  [rel]
    > >> >>
    > >> >> Now this is rather odd, almost across the board we're seeing the expected
    > >> >> drops in instructions and branches, yet we appear to be paying a heavy IPC
    > >> >> price.  The fact that wall-time has scaled equivalently with cycles roughly
    > >> >> rules out the cycles counter being off.
    > >> >>
    > >
    > > if i understand your results, for barcelona you did see an improvement
    > > in cycles and eslapsed time with jump labels for unconstrained?
    > >
    >
    > Under W2, yes.
    >
    > >> >> We are seeing the expected behavior in the bandwidth enabled case;
    > >> >> specifically the <jl=jmp><ret><cond><ret> blocks are taking an extra branch
    > >> >> and instruction which shows up on all the numbers above.
    > >> >>
    > >> >> With respect to compiler mangling the text is essentially unchanged in size.
    > >> >> One lurking suspicion is whether the inserted nops have perturbed some of the
    > >> >> jmp/branch alignments?
    > >
    > > hmmmm....not sure, I'm adding Richard Henderson, to the 'cc list, who
    > > worked on the 'asm goto' in gcc.
    > >
    > >> >>
    > >> >>     text    data     bss     dec     hex filename
    > >> >>  7277206 2827256 2125824 12230286         ba9e8e vmlinux.jump_label
    > >> >>  7276886 2826744 2125824 12229454         ba9b4e vmlinux.no_jump_label
    > >> >>
    > >
    > > the other thing here is that vmlinux.jump_label includes the extra
    > > kernel/jump_label.o file, so you can sort of subtract the text size of
    > > that file to do a fair comparison.
    >
    > Even without doing that it's only a 1.00004% change in text size.
    >
    > I was just making the inference that if it's gcc mangling it's likely
    > in the layout/alignment.
    >
    > >
    > > Also, I would have expected the data section to have increased more with
    > > jump labels enabled. Are tracepoints disabled (a current user of jump
    > > labels).
    >
    > Yeah -- Tracing is enabled so the BWC build should have labels
    > already; this likely accounts for the small increase noted above.
    >
    > >
    > >> >>  I have checked to make sure that the right instructions are being patched in
    > >> >>  at run-time.  I've also pulled a fully patched jump_label out of the kernel
    > >> >>  into a userspace test (and benchmarked it directly under perf).  The results
    > >> >>  here are also exactly as expected.
    > >> >>
    > >> >> e.g.
    > >> >>  Performance counter stats for './jump_test':
    > >> >>      1,500,839,002 instructions, 300,147,081 branches 702,468,404 cycles
    > >> >> Performance counter stats for './jump_test 1':
    > >> >>      2,001,014,609 instructions, 400,177,192 branches 901,758,219 cycles
    > >> >>
    > >
    > > what no-op did you use in userspace? I wouldn't think the no-op choice
    > > would make any difference though...At compile time we use a 'jmp 0', and
    > > then at boot we dynamically patch the 'jmp 0' with the no-op we think works
    > > best...
    > >
    >
    > Sorry -- what I meant here is I pulled the run-time chosen "best" nop
    > out of /proc/kcore and tested a
    > tight loop about a <JL><RET><COND><RET> sequence (e.g.
    > cfs_rq_throttled()) with JL being the nop and jmp respectively.
    >
    > Specifically for Westmere this ends up being K8_NOP5 -- 0x666666D0
    >
    > > thanks,
    > >
    > > -Jason
    > >
    > >> >> Overall if we can fix the IPC the benefit in the globally unconstrained case
    > >> >> looks really good.
    > >> >>
    > >> >> Any thoughts Jason?
    > >> >>
    > >> >
    > >> > Do you have CONFIG_CC_OPTIMIZE_FOR_SIZE set? I know that when
    > >> > CONFIG_CC_OPTIMIZE_FOR_SIZE is not set, the compiler can make the code
    > >> > more optimal.
    > >> >
    > >>
    > >> Ah I should have mentioned that was one of the holes I stared down:
    > >>
    > >> Builds were -O2 (gcc-4.6.1) and
    > >> $  zcat /proc/config.gz | grep CONFIG_CC_OPTIMIZE_FOR_SIZE
    > >> # CONFIG_CC_OPTIMIZE_FOR_SIZE is not set
    > >>
    > >> Same kernel image across all platforms.
    > >>
    > >>

    Hi Paul,

    Ok, I think I finally tracked this down. It may seem a bit crazy, but
    when we are getting down to cycle counting like this, it seems that the
    link order in the kernel/Makefile can make difference. I had the
    jump_label.o listed after the core files, whereas all the code in
    jump_label.o is really slow path code (used when toggling branch
    values). As follows:


    --- a/kernel/Makefile
    +++ b/kernel/Makefile
    @@ -10,7 +10,7 @@ obj-y = sched.o fork.o exec_domain.o panic.o printk.o \
    kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
    hrtimer.o rwsem.o nsproxy.o srcu.o semaphore.o \
    notifier.o ksysfs.o pm_qos_params.o sched_clock.o cred.o \
    - async.o range.o jump_label.o
    + async.o range.o
    obj-y += groups.o

    ifdef CONFIG_FUNCTION_TRACER
    @@ -107,6 +107,7 @@ obj-$(CONFIG_PERF_EVENTS) += events/
    obj-$(CONFIG_USER_RETURN_NOTIFIER) += user-return-notifier.o
    obj-$(CONFIG_PADATA) += padata.o
    obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
    +obj-$(CONFIG_JUMP_LABEL) += jump_label.o

    ifneq ($(CONFIG_SCHED_OMIT_FRAME_POINTER),y)
    # According to Alan Modra <alan@linuxcare.com.au>, the -fno-omit-frame-pointer is

    I've tested the patch using a single 'static_branch()' in the getppid() path,
    and basically running tight loops of calls to getppid(). Before, the
    patch, I was seeing results similar to what you reported, after the
    patch, things improved for all metrics. Here are my results for the
    branch disabled case:

    With jump labels turned on (CONFIG_JUMP_LABEL), branch disabled:

    Performance counter stats for 'bash -c /tmp/timing;true' (50 runs):

    3,969,510,217 instructions # 0.864 IPC ( +-0.000% )
    4,592,334,954 cycles ( +- 0.046% )
    751,634,470 branches ( +- 0.000% )

    1.722635797 seconds time elapsed ( +- 0.046% )

    Jump labels turned off (CONFIG_JUMP_LABEL not set), branch disabled:

    Performance counter stats for 'bash -c /tmp/timing;true' (50 runs):

    4,009,611,846 instructions # 0.867 IPC ( +-0.000% )
    4,622,210,580 cycles ( +- 0.012% )
    771,662,904 branches ( +- 0.000% )

    1.734341454 seconds time elapsed ( +- 0.022% )


    So all of the measured metrics improved in the jump labels case b/w
    0.5% - 2.5%.

    I'm curious to see what you find with this patch.

    Thanks,

    -Jason


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