Re: [RFT][patch 17/18] sched: use jump labels to reduce overhead whenbandwidth control is inactive

From: Paul Turner
Date: Thu Jul 21 2011 - 21:38:42 EST

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In reply to: Jason Baron: "Re: [RFT][patch 17/18] sched: use jump labels to reduce overheadwhen bandwidth control is inactive"
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On Thu, Jul 21, 2011 at 6:17 PM, Jason Baron <jbaron@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>> > rth@xxxxxxxxxx
>> > 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@xxxxxxxxxx>
>> >
>> > 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.
>>
>>
>>
>>
>>
>>
>> > thanks,
>> >
>> > -Jason
>> >
>> --
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Next message: Benjamin Herrenschmidt: "Re: [RFC/PATCH] mm/futex: Fix futex writes on archs with SWtracking of dirty & young"
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In reply to: Jason Baron: "Re: [RFT][patch 17/18] sched: use jump labels to reduce overheadwhen bandwidth control is inactive"
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